Experimental and Theoretical Studies into the Formation of C₄–C₆ Products in Partially Chlorinated Hydrocarbon Pyrolysis Systems: A Probabilistic Approach to Congener-Specific Yield Predictions

This work presents a study of the pyrolytic formation of vinylacetylene and benzene congeners formed from chlorinated hydrocarbon precursors, a complex, multipath polymerization system formed in a monomer-rich environment. (Co-)­pyrolyses of dichloro- and trichloroethylene yield a rich array of products, and assuming a single dominant underlying growth mechanism, this (on comparing expected and observed products) allows a number of potentially competing channels to C₄ and C₆ products to be ruled out. Poor congener/isomer descriptions rule out even-carbon radical routes, and the absence of C₃ and C₅ products rule out odd-carbon processes. Vinylidenes appear unable to describe the increased reactivity of acetylenes with chlorination noted in our experiments, leaving molecular acetylene dimerization processes and, in C₆ systems, the closely related Diels–Alder cyclization as the likely reaction mechanism. The feasibility of these routes is further supported by ab initio calculations. However, some of the most persuasive evidence is provided by congener-specific yield predictions enabled by the construction of a probability tree analogue of kinetic modeling. This approach is relatively quick to construct, provides surprisingly accurate predictions, and may be a very useful tool in screening for important reaction channels in poorly understood congener- or isomer-rich reaction systems

Molecular Mechanisms in the Pyrolysis of Unsaturated Chlorinated Hydrocarbons: Formation of Benzene Rings. 1. Quantum Chemical Studies

Analogues of important aromatic growth mechanisms in hydrocarbon pyrolysis and combustion systems are extended to chlorinated systems. We consider the addition of C₂Cl₂ to both C₄Cl₃ and C₄Cl₅ radicals at the M06-2X/6-311+G­(3df,3p)//B3LYP/6-31G­(d) level of theory, and we demonstrate that these reaction systems have much in common with those of nonchlorinated species. In particular, we find that these radicals appear to lead preferentially to fulvenes, and not to the observed aromatic products, as is found in nonchlorinated systems. We have therefore also considered nonradical C₄/C₂ channels by way of Diels–Alder cyclization of C₄Cl₄/C₂Cl₂ and C₄H₂Cl₂/C₂HCl pairs to describe aromatic formation. While the latter pair readily leads to the formation of partially chlorinated benzenes, the fully chlorinated congeners are sterically prohibited from ring closing directly; this leads to a series of novel rearrangement processes which predict the formation of hexachloro-1,5-diene-3-yne, in addition to hexachlorobenzene, in good agreement with experiment. This suggests, for the first time, that facile nonradical routes to aromatic formation are operative in partially and fully chlorinated pyrolysis and combustion systems

High Temperature Chemistry of Chlorinated Acenaphthylene. 3C Bay Acetylene Additions and Annealing by Five-Membered Ring Shifts

Experimental and theoretical results concerning the growth and isomerization of chlorinated acenaphthylene, C₁₂H₈, during the pyrolysis of chlorohydrocarbons are presented here. A fullerene subunit, C₁₂H₈, is a useful system to investigate regarding C₆₀ formation. However, direct experimental observation of isomerization and annealing processes in particular are difficult to confirm due to the high symmetry of the parent molecule. Chlorination lowers the symmetry, essentially labeling carbon atoms, allowing growth and isomerization to be followed directly. Pyrolysis of dichloro- and trichloroethylene, and their copyrolyses with trichlorobenzenes, provides an efficient and general source of chlorinated acenaphthylenes in a range of degrees of chlorination and over a number of unique congeners. Analysis of congener yields as a function of reagents employed, guided by DFT/B3LYP/6-311G­(d,p) level calculations, strongly suggests that C₂ addition across three-carbon bays in naphthalene is a major driver of growth. Additionally, extremely facile five-membered ring shifts are operative, with chlorine promoting isomerization. Theoretical study of C₁₆H₁₀- and C₁₈H₁₀-based congeners indicate that this is a general phenomenon, and with chlorine also favoring internal cyclopentafused rings in addition to increased isomerization rates, this suggests halogen moieties may be an important feature for efficient fullerene growth

Role of Hydrogen Abstraction Acetylene Addition Mechanisms in the Formation of Chlorinated Naphthalenes. 2. Kinetic Modeling and the Detailed Mechanism of Ring Closure

The dominant formation mechanisms of chlorinated phenylacetylenes, naphthalenes, and phenylvinylacetylenes in relatively low pressure and temperature (∼40 Torr and 1000 K) pyrolysis systems are explored. Mechanism elucidation is achieved through a combination of theoretical and experimental techniques, the former employing a novel simplification of kinetic modeling which utilizes rate constants in a probabilistic framework. Contemporary formation schemes of the compounds of interest generally require successive additions of acetylene to phenyl radicals. As such, infrared laser powered homogeneous pyrolyses of dichloro- or trichloroethylene were perturbed with 1,2,4- or 1,2,3-trichlorobenzene. The resulting changes in product identities were compared with the major products expected from conventional pathways, aided by the results of our previous computational work. This analysis suggests that a Bittner–Howard growth mechanism, with a novel amendment to the conventional scheme made just prior to ring closure, describes the major products well. Expected products from a number of other potentially operative channels are shown to be incongruent with experiment, further supporting the role of Bittner–Howard channels as the unique pathway to naphthalene growth. A simple quantitative analysis which performs very well is achieved by considering the reaction scheme as a probability tree, with relative rate constants being cast as branching probabilities. This analysis describes all chlorinated phenylacetylene, naphthalene, and phenylvinylacetylene congeners. The scheme is then tested in a more general system, i.e., not enforcing a hydrogen abstraction/acetylene addition mechanism, by pyrolyzing mixtures of di- and trichloroethylene without the addition of an aromatic precursor. The model indicates that these mechanisms are still likely to be operative

Adsorptive Capacity and Evolution of the Pore Structure of Alumina on Reaction with Gaseous Hydrogen Fluoride

Brunauer–Emmet–Teller (BET) specific surface areas are generally used to gauge the propensity of uptake on adsorbents, with less attention paid to kinetic considerations. We explore the importance of such parameters by modeling the pore size distributions of smelter grade aluminas following HF adsorption, an industrially important process in gas cleaning at aluminum smelters. The pore size distributions of industrially fluorinated aluminas, and those contacted with HF in controlled laboratory trials, are reconstructed from the pore structure of the untreated materials when filtered through different models of adsorption. These studies demonstrate the presence of three distinct families of pores: those with uninhibited HF uptake, kinetically limited porosity, and pores that are surface blocked after negligible scrubbing. The surface areas of the inaccessible and blocked pores will overinflate estimates of the adsorption capacity of the adsorbate. We also demonstrate, contrary to conventional understanding, that porosity changes are attributed not to monolayer uptake but more reasonably to pore length attenuation. The model assumes nothing specific regarding the Al₂O₃–HF system and is therefore likely general to adsorbate/adsorbent phenomena

The Influence of Surface Structure on H₄SiO₄ Oligomerization on Rutile and Amorphous TiO₂ Surfaces: An ATR-IR and Synchrotron XPS Study

Silicic acid (H₄SiO₄) is ubiquitous in natural aquatic systems. Applications of TiO₂ in these systems will be influenced by H₄SiO₄ sorption and oligomerization reactions on the TiO₂ surface, and this can affect many aspects of TiO₂ reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H₄SiO₄ oligomerization by quantifying the extent of H₄SiO₄ sorption and oligomerization on three TiO₂ phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO₂ (TiO_2(am)). The <i>in situ</i> ATR-IR spectra measured over time as 0.2 mM H₄SiO₄ reacted with TiO₂ were quite different on the three TiO₂ phases. The percentage of the surface H₄SiO₄ that was present as oligomers increased over time on all phases, but after 20 h almost all H₄SiO₄ on the R180 surface was oligomeric, while the H₄SiO₄ on TiO_2(am) was predominantly monomeric. The extent of H₄SiO₄ oligomerization on R60 was intermediate. When the TiO₂ phases reacted with 1.5 mM H₄SiO₄ the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO₂ phases; however, after 20 h the percentage of the surface H₄SiO₄ present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO_2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H₄SiO₄ surface coverage (Γ_Si) were measured by XPS over a range of Γ_Si. For any given Γ_Si the Si 2s BE’s were in the order R180 > R60 > TiO_2(am). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H₄SiO₄ oligomerization where linear trimeric silicates are formed by insertion of a solution H₄SiO₄ between suitably orientated adjacent bidentate sorbed monomers. The TiO_2(am) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO_2(am) surface, the regular arrangement of TiO₆ octahedra on the rutile (110) face means that sorbed H₄SiO₄ monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO_2(am). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO_2(am) particle would increase the spatial separation of adjacent trimers

The Influence of Surface Structure on H₄SiO₄ Oligomerization on Rutile and Amorphous TiO₂ Surfaces: An ATR-IR and Synchrotron XPS Study

Silicic acid (H₄SiO₄) is ubiquitous in natural aquatic systems. Applications of TiO₂ in these systems will be influenced by H₄SiO₄ sorption and oligomerization reactions on the TiO₂ surface, and this can affect many aspects of TiO₂ reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H₄SiO₄ oligomerization by quantifying the extent of H₄SiO₄ sorption and oligomerization on three TiO₂ phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO₂ (TiO_2(am)). The <i>in situ</i> ATR-IR spectra measured over time as 0.2 mM H₄SiO₄ reacted with TiO₂ were quite different on the three TiO₂ phases. The percentage of the surface H₄SiO₄ that was present as oligomers increased over time on all phases, but after 20 h almost all H₄SiO₄ on the R180 surface was oligomeric, while the H₄SiO₄ on TiO_2(am) was predominantly monomeric. The extent of H₄SiO₄ oligomerization on R60 was intermediate. When the TiO₂ phases reacted with 1.5 mM H₄SiO₄ the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO₂ phases; however, after 20 h the percentage of the surface H₄SiO₄ present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO_2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H₄SiO₄ surface coverage (Γ_Si) were measured by XPS over a range of Γ_Si. For any given Γ_Si the Si 2s BE’s were in the order R180 > R60 > TiO_2(am). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H₄SiO₄ oligomerization where linear trimeric silicates are formed by insertion of a solution H₄SiO₄ between suitably orientated adjacent bidentate sorbed monomers. The TiO_2(am) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO_2(am) surface, the regular arrangement of TiO₆ octahedra on the rutile (110) face means that sorbed H₄SiO₄ monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO_2(am). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO_2(am) particle would increase the spatial separation of adjacent trimers