Harold H. Schobert

Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based insights and outlook

This article is a review of the current knowledge of the chemical physics of carbon dioxide (CO2) conversion to fuels using light energy and water (CO2 photoreduction) on titania (TiO2)-based catalysts and Ti-species in porous materials. Fairly comprehensive literature reviews of CO2 photoreduction are available already. However, this article is focused on CO2 photoreduction on Ti-based catalysts, and incorporates fundamental aspects of CO2 photoreduction, knowledge from surface science studies of TiO2 and the surface chemistry of CO2. Firstly, the current state of development of this field is briefly reviewed, followed by a description of and insights from surface state and surface site approaches. Using examples such as metal-doping of TiO2, dye-sensitization, oxygen vacancies in TiO2 and isolated-Ti centers in microporous/mesoporous materials, the utility of these approaches to understand photoinduced reactions involved in CO2activation is examined. Finally, challenges and prospects for further development of this field are presented. Enhanced understanding of the CO2 : TiO2 system, with a combination of computational and experimental studies is required to develop catalysts exhibiting higher activity towards CO2 photoreduction.

Chemicals and materials from coal in the 21st century

Coal may become more important both as an energy source and as the source of organic chemical feedstock in the 21st century. The demonstrated coal reserves in the world are enough for consumption for over 215 years at the 1998 level, while the known oil reserves are only about 39 times of the worlds consumption level in 1998 and the known natural gas reserves are about 63 times of the worlds consumption level in 1998. Coal has several positive attributes when considered as a feedstock for aromatic chemicals, specialty chemicals, and carbon-based materials. Substantial progress in advanced polymer materials, incorporating aromatic and polyaromatic units in their main chains, has created new opportunities for developing value-added or specialty organic chemicals from coal and tars from coal carbonization for coke making. The decline of the coal tar industry diminishes traditional sources of these chemicals. The new coal chemistry for chemicals and materials from coal may involve direct and indirect coal conversion strategies as well as the co-production approach. Needs for environmental-protection applications have also expanded market demand for carbon materials. Current status and future directions are discussed in this review.

Opportunities for developing specialty chemicals and advanced materials from coals

The main objective of this paper is to explore the potentials and possible ways to develop high-value chemicals and materials from coals and coal liquids. Recently it has become clear that more extensive use of fossil fuels, especially coal, may be constrained not only by economics, but also by environmental considerations such as SOx and NOx emissions and global warming. Therefore, new concepts are required, and significant advances are essential for the effective utilization of coals in the next century. Both from economic and environmental viewpoints, developing high-value chemicals and materials from coals and coal liquids should lead to more efficient and environmentally safe utilization of the valuable carbonaceous resources. It is important to explore the routes and methods for developing specialty chemicals, which are difficult to obtain or not readily available from petroleum, advanced polymeric engineering materials, and high-performance carbon materials. Recent years have seen significant progress in the development and application of new, industrially important aromatic engineering plastics, thermoplastic materials, liquid crystalline polymers, and membrane materials. Many of the monomers for these materials can be prepared from one- to four-ring aromatics such as alkylated benzenes, naphthalene, biphenyl, anthracene, phenanthrene, pyrene, phenol, and carbazole. Especially important are 2,6-dialkylnaphthalenes, 4,4′-dialkylbiphenyls, and 1,4-dialkylbenzenes. The large-volume application of aromatic high-performance polymers depends on lowering their cost, which in turn is largely determined by the cost of the aromatic monomers. By developing the critical aromatic chemicals from coals, coal-to-chemicals research could contribute significantly to high-technology development. Potential large-volume markets for materials from coal can be stimulated by developing high-performance carbon materials such as carbon fibers and graphites, and by developing ways to make advanced adsorbents for environmental applications such as air and water purification.

Non-fuel uses of coals and synthesis of chemicals and materials

Abstract This paper provides an account of our analysis of future needs for non-fuel uses of fossil fuels, particularly coal, and a discussion of possible new routes for developing chemicals and materials from coal. An overview of energy supply and demand in the world and the existing non-fuel uses of fossil fuels in the US is given first. The amount of energy used for non-fuel purpose is small compared with the amount of energy consumed by end users. The non-fuel uses of fossil fuels—particularly coal—may become more important in the future. The demonstrated coal reserves in the world are enough for consumption for over 220 years at the 1992 level, while the oil reserves are only about 40 times the worlds consumption level in 1992. Coal may become more important both as an energy source and as the source of chemical feedstocks in the 21st century. However, traditional non-fuel uses of coals (coke ovens and the coal tars) are diminishing rapidly. We will discuss possible new processes for making both bulk and specialty chemicals, polymers and carbon materials from coals and liquids from coal liquefaction. Specific examples will be provided from work in progress in our laboratory, including conversion of coals and coal liquids to specialty chemicals, polymer materials, activated carbons, graphitic carbons, and electrode materials.

Shape-selective isopropylation of naphthalene over mordenite catalysts: Computational analysis using MOPAC

Abstract In our experimental work on shape-selective isopropylation of naphthalene, the selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN) and the ratio of 2,6-DIPN to 2,7-DIPN were increased by mordenite catalyst dealumination. However, it was not clear whether the differentiation between the two isomers was caused by their differences in molecular dimensions or in electronic properties. In this work we performed a computational analysis of the molecular dimensions and frontier electron density f r ( E ) using MOPAC program for naphthalene, isopropyl- and diisopropylnaphthalene. The f r ( E ) value for electrophilic substitution reaction represents the density of electrons in the highest occupied molecular orbital (HOMO). According to the frontier molecular orbital theory, the most reactive position (the carbon atom on which electrophilic attack occurs most likely) has the highest frontier electron density. The calculation shows that 2,6-DIPN has a slightly smaller critical diameter. More importantly, position 6 in 2-IPN has higher f r ( E ) value than that of position 7 in 2-IPN. This suggests that during 2-IPN isopropylation inside a mordenite channel, the formation of 2,6-DIPN is electronically more favored than that of 2,7-DIPN.

Molecular simulation on hydrodesulfurization of thiophenic compounds over MoS2 using ZINDO

In order to develop a fundamental understanding of the HDS mechanism of thiophenic compounds over molybdenum disulfide (MoS2), a molecular simulation of the hydrodesulfurization (HDS) of thiophenic compounds over MoS2 has been performed using Zerners Intermediate Neglect of Differential Overlap (ZINDO) program. On the basis of the calculated edge structure, stoichiometry of MoS2, shape of the crystal, and the size corresponding to real MoS2 particles, a single-slab cluster, Mo27S54, has been proposed for modeling the highly dispersed MoS2. The proposed cluster is a regular hexagon with (1010) and (3030) edge planes only. According to the calculated electronic properties of the surface, the coordinately unsaturated MoIV in the (3030) plane is expected to be the active site for hydrogenation of thiophenic and aromatic compounds. The most stable adsorption configuration of thiophene on the MoIV is a flat adsorption configuration via the η5-bound coordination, whereas the most stable adsorption configuration of tetrahydrothiophene (THT) on the MoIV is a tilted adsorption configuration via the S-bound coordination. HDS mechanism of thiophene through the hydrogenation pathway over the (3030) plane of MoS2 is discussed according to quantum chemical insights in combination with experimental results from the literature.

Semi-empirical studies on electronic structures of a boron-doped graphene layer — implications on the oxidation mechanism

Abstract The electronic structures of pure and boron-doped graphene layers have been investigated using the semi-empirical Molecular Orbital Package (MOPAC) and large clusters of carbon atoms. It is shown that boron-doping on the edge and internal lattice sites of the graphene layer produces very different effects on the electronic structure around the edges. It is found that the substitutional boron atoms on the edges dramatically alter the density distribution of high energy electrons along the edges and the substitutional boron atoms in the deep internal lattice sites do not produce any significant effect on the density distribution along the edges. Based on the results obtained, a model is proposed for describing the oxidation process in boron-doped graphite. The mechanism of oxidation inhibition due to boron-doping of a graphene layer is chemical inhibition via the reduction of electron density with high energy at surface sites, and consequently, a reduction in the total number of active sites for gasification of the carbon.

Swelling pretreatment of coals for improved catalytic liquefaction

Abstract Two coals, a Texas lignite and a Utah high volatile C bituminous, were used to examine the effects of solvent swelling pretreatment and catalyst impregnation on conversion behaviour at 275 °C, representative of the first, low-temperature stage in a temperature-staged liquefaction reaction. Iron(II) sulphate, iron pentacarbonyl, ammonium tetrathiomolybdate, and molybdenum hexacarbonyl were used as catalyst precursors. Without swelling pretreatment, impregnation of both coals increased conversion, mainly through increased yields of preasphaltenes. Methanol, tetrahydrofuran, tetrabutylammonium hydroxide, and pyridine were used as swelling agents. In the absence of catalyst, swelling the lignite before reaction improves conversion by enhancing oil and gas yields; the effectiveness of the solvents in enhancing conversion is in the same order as their swelling ratios. Swelling with methanol or pyridine has little effect on reaction of the bituminous coal, but both tetrahydrofuran and tetrabutylammonium hydroxide treatments increase conversion as a result of higher preasphaltene yields. The combined effect of catalyst addition and swelling enhances conversion, as much as two-fold, of the lignite and increases yields of all products. On the other hand, little benefit was obtained by combining catalyst addition and swelling for the bituminous coal, though it is possible to effect changes in the relative amounts of the various products. Tetrabutylammonium hydroxide not removed from the coal after pretreatment appears to decompose to the good solvent tributylamine, suggesting the possibility of using swelling agents as ‘solvent precursors’, first swelling the coal and then thermally decomposing to a strong solvent that is emplaced inside the coal. The action of iron pentacarbonyl is sensitive to the reactive gas atmosphere used; in hydrogen it decomposes to an oxide that mainly facilitates hydrogenation of the heavy products to lighter oils, whereas in hydrogen sulphide/hydrogen mixtures the iron pentacarbonyl is sulphided and mainly facilitates depolymerization of the coal to heavy products. This finding suggests the possibility of tailoring the behaviour of the catalyst by appropriate selection of catalyst precursor and reactive atmosphere.

CPMAS 13C NMR and pyrolysis-GC-MS studies of structure and liquefaction reactions of Montana subbituminous coal

Abstract This paper reports on the application of solid-state CPMAS 13 C NMR and flash pyrolysis-GC-MS for characterization of the macromolecular network of a Montana subbituminous coal and its residues from temperature-programmed and non-programmed liquefaction (TPL and N-PL) at final temperatures ranging from 300 to 425°C in H-donor and non-donor solvents. The combined use of 13 C NMR and Py-GC-MS revealed that this coal contains significant quantities of oxygen-bearing structures, corresponding to about 18 O-bound C per 100 C atoms and one O-bound C per every five to six aromatic Cs. The oxygen-bearing components in the coal include catechol-like structures, which seem to disappear from the liquefaction residues above 300°C; carboxyl groups, which almost disappear after 350°C; and phenolic structures, which are most important in the original coal but diminish in concentration with increasing temperature. These results point to the progressive loss of oxygen functional groups and aliphatic-rich species from the macromolecular network of the coal during programmed heat-up under TPL conditions. The higher conversions in TPL runs in H-donor tetralin (relative to the conventional N-PL runs) suggest that the removal of carboxylic and catechol groups from the coal and the capping of the reactive sites by H-transfer from H-donors at low temperatures (≤350°C) have contributed to minimizing the retrogressive crosslinking at higher temperatures. Quantitative calculation of NMR data and mathematical correlation were also attempted in this work. For 24 liquefaction residues derived under significantly different conditions, linear correlations between C-distribution and reaction temperature (≥ 300°C) have been found, which can be expressed by a simple equation, C i = αf i + βT , where f i and C i , represent content of aromatic, aliphatic, or oxygen-bound carbons in the original coal and residue, respectively; T stands for the reaction temperature; α and β are constants.

Comparative performance of impregnated molybdenum-sulphur catalysts in hydrogenation of Spanish lignite

Abstract Hydrogenation of a Spanish lignite of 12% sulphur content was conducted using three molybdenum-containing catalysts impregnated into the lignite: ammonium tetrathiomolybdate, a sulphided ammonium heptamolybdate, and molybdenum disulphide. The conversions to liquids and hydrodesulphurization were investigated for a series of residence times and temperatures. At 275 °C, the ammonium salts provide no greater conversion or sulphur removal than obtained in the absence of catalyst, because these salts have not decomposed to an active catalyst at this temperature. However, lignite impregnated with molybdenum disulphide does experience greater conversion and desulphurization than lignite reacted without catalyst. At 325 °C, the lignite impregnated with the ammonium salts gives conversions and desulphurization substantially superior to that achieved with molybdenum disulphide or without catalyst. This is attributed to the superior dispersion that can be achieved by impregnation using a solution of a soluble salt rather than a slurry of the insoluble disulphide. The best conversions, liquid yield, and desulphurization are achieved using impregnated sulphided ammonium heptamolybdate.