Oxidative Coupling of Methane: A Microkinetic Model Accounting for Intraparticle Surface-Intermediates Concentration Profiles

A microkinetic model for oxidative coupling of methane (OCM) has been developed that comprises a reaction network of 39 gas-phase and 26 catalytic elementary steps. It has been implemented in a heterogeneous reactor model that explicitly accounts for the interactions between gas phase and surface species. Concentration gradients arising from mass transport limitations are found to develop inside the catalyst pellet for all intermediates (i.e., surface and gas-phase) even under an intrinsic kinetics regime for the molecules and clearly affect the C₂ selectivity. Special attention has been devoted to the reduction of the number of adjustable parameters in the model and the a priori determination of thermodynamic as well as kinetic parameters. A contribution analysis is conducted in order to elucidate the complex reaction pathways in OCM that lead to the desired products. Apart from the methyl radicals that couple to an extent of almost 70% in the void space between the pellets, the catalyst pellet accounts for the majority of molecules and radicals conversion, which are produced on the surface and further interact either in the catalyst pores or with other surface species. Almost 95% of CH₄ consumption and more than half of the C₂H₆ production take place inside the catalyst pellet. A similar analysis is applied to understand the effect of various textural properties of catalysts on the performance of OCM, for example, increasing the catalyst porosity is found beneficial for the C₂ yield, as long as a sufficient CH₄ activation takes place, so that the coupling pathway is promoted over the heterogeneous oxidation of CH₃<b>·</b>

DFT Investigation into Alumina ALD Growth Inhibition on Hydroxylated Amorphous Silica Surface

Alumina (Al₂O₃), a suitable replacement for silica (SiO₂) as gate oxide in metal oxide semiconductor field effect transistors (MOSFET), is deposited on the amorphous silica layer of the semiconductor substrate by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water as precursors. A computationally efficient model for the hydroxylated amorphous silica surface is obtained by means of molecular dynamics and is used to investigate the reason behind the observed growth inhibition during alumina ALD. The reactions of TMA are investigated by periodic DFT calculations on surfaces with hydroxyl coverage of 3.38 OH nm^–2 and 5.07 OH nm^–2. The formation of SiCH₃ surface species is found to be possible only on the less hydroxylated surface during the first TMA half-cycle, while the subsequent reaction of water with the SiCH₃ surface species is found to be highly activated (<i>E</i>_a = 196 kJ mol^–1). Since these SiCH₃ surface species are rather unreactive toward water, fewer hydroxyls are regenerated during this first water half-cycle, resulting in the observed initial growth inhibition. Moreover, alumina growth can continue over the aluminum surface species, trapping the unreactive SiCH₃ species at the interface between deposited alumina and silica. Such carbon impurities at the interface should be avoided nonetheless, since they can create undesirable tunneling currents in MOSFETs

Assessment of a Gas–Solid Vortex Reactor for SO₂/NO_<i>x</i> Adsorption from Flue Gas

The feasibility of performing the SO₂/NO_<i>x</i> adsorption process in a gas–solid vortex reactor (GSVR) is examined and compared with the more traditional riser technology. The multiphase reacting flow is modeled using the Eulerian–Eulerian two-fluid model. Models of nonreacting flows were validated using data from a bench-scale experimental setup. The GSVR has the potential to significantly improved heat/mass transfer between phases, as compared to more conventional fluidization technologies. Process intensification opportunities are investigated. The model predicts continuous removal efficiencies greater than 99% for SO₂ and approximately 80% for NO_<i>x</i>. The gas–solid slip velocity and convective mass transfer coefficient for the riser were 0.2–0.5 and 0.06–0.12 m/s, respectively, whereas the values for the GSVR were 6–7 and 1.0–1.1 m/s, respectively. This order of magnitude increase in the external mass transfer coefficient highlights the potential intensification opportunities provided by the GSVR

Combined Comprehensive Two-Dimensional Gas Chromatography Analysis of Polyaromatic Hydrocarbons/Polyaromatic Sulfur-Containing Hydrocarbons (PAH/PASH) in Complex Matrices

A new gas chromatographic method has been developed that is able to quantify polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic sulfur-containing hydrocarbons (PASH) up to four rings. The method combines the power of both flame ionization detection (FID) and sulfur chemiluminescence detection (SCD) in series on a single comprehensive two-dimensional gas chromatography (GC × GC) system and provides mass fractions of compounds separated by carbon number <i>n</i> (C_<i>n</i>H_<i>x</i>S_<i>y</i>) and class. In addition to PAH and PASH separation, the method is extended toward nonaromatic and monoaromatic (sulfur-containing) compounds (paraffins, naphthenes, monoaromatics, thiols, sulfides, disulfides, and thiophenes). The 95% confidence interval is doubled when a single injection technique is used instead of a more-accurate double injection technique. A flexible correction procedure that combines the advantages of the two-dimensional separation of GC × GC and its ability to easily define overlapping groups between the FID and the SCD chromatograms is applied. The method is validated using theoretical reference mixtures and is applied on three commercial gas oils with sulfur content from 0.16 wt % up to 1.34 wt %. The repeatability is good, with an average of 3.4%, which is in the same range as the much more expensive Fourier transform ion cyclotron resonance–mass spectroscopy (FTICR-MS) technique

Isobutene Protonation in H‑FAU, H‑MOR, H‑ZSM-5, and H‑ZSM-22

Kinetics and thermodynamics of isobutene protonation in H-FAU, H-MOR, H-ZSM-5, and H-ZSM-22 have been studied in a temperature range of 300–800 K, combining PW91-D//PW91 periodic density functional theory calculations with statistical thermodynamics. At temperatures relevant for industrial zeolite-catalyzed processes (500–800 K), the <i>tert</i>-butyl carbenium ion is more stable than the <i>tert</i>-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. Entropy contributions govern the standard Gibbs free energy stability of the chemisorbed intermediates. Due to the absence of a C–O covalent bond, formation of the <i>tert</i>-butyl carbenium ion is accompanied by a lower entropy loss and, consequently, has a higher stability than the <i>tert</i>-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. At 800 K, the protonation toward <i>tert</i>-butoxy in H-FAU, H-MOR, and H-ZSM-5 and to the <i>tert</i>-butyl carbenium ion in H-ZSM-22 is 5 to 7 orders of magnitude faster than the protonation toward isobutoxy. Among the four zeolites, the lowest activation energy is found in H-ZSM-22

Coking Resistance of Specialized Coil Materials during Steam Cracking of Sulfur-Free Naphtha

The reactor material strongly affects coke formation during steam cracking of hydrocarbons. Therefore, in the past decade several specialized reactor materials have been developed that have proven to be efficient in reducing coke formation for ethane steam cracking. However, their beneficial anticoking properties are questioned when heavier feedstocks such as naphtha are cracked. Therefore, the effect of the composition of the reactor material has been investigated for ethane and naphtha cracking in an electrobalance setup under industrially relevant conditions. A significant reduction of coke formation is obtained for specialized alloys compared to typical Fe–Cr–Ni heat resistant steels when a sulfur-free naphtha is cracked. A thin layer of alumina on the surface along with manganese chromite provides the highest resistance to coking, as was demonstrated by the SEM and EDX analyses. The decrease in coking rate translates in a run length increase of 50% for a typical naphtha furnace equipped with reactors made out of an Al-enhanced alloy instead of typically applied heat resistant steel

Influence of the Reactor Material Composition on Coke Formation during Ethane Steam Cracking

An experimental study of the coking tendency of nine different materials was carried out in a quartz electrobalance setup with a jet stirred reactor (JSR) under industrially relevant ethane steam cracking conditions: <i>T</i>_material = 1159 K, <i>P</i>_tot = 0.1 MPa, χ_ethane = 73%, dilution δ = 0.33 kg_H2O/kg_HC. A strong influence of the composition of the materials on the coking rate as a function of time on-stream was observed. The initial coking rate varied from 5 × 10^–4 g·m^–2·s^–1 to 27 × 10^–4 g·m^–2·s^–1, while the asymptotic coking rate changed in the range of 2 × 10^–4 g·m^–2·s^–1 to 6 × 10^–4 g·m^–2·s^–1. SEM and EDX analyses of coked and uncoked coupons revealed that the composition of the oxide layer in contact with the cracked gas, formed after the initial preoxidation or decoking, has an important influence on the amount of coke deposited. Materials that formed a thin Al₂O₃ layer on the coupon surface showed a higher coking resistance. A uniform surface composition and a high resistance to spalling and fractures are other important characteristics of good materials

Advanced Elemental Characterization during Pt–In Catalyst Formation by Wavelet Transformed X‑ray Absorption Spectroscopy

Complementary to conventional X-ray absorption near edge structure (XANES) and Fourier transformed (FT) extended X-ray absorption fine structure (EXAFS) analysis, the systematic application of wavelet transformed (WT) XAS is shown to disclose the physicochemical mechanisms governing Pt–In catalyst formation. The simultaneous k- and R-space resolution of the WT XAS signal allows for the efficient allocation of the elemental nature to each R-space peak. Because of its elemental discrimination capacity, the technique delivers structural models which can subsequently serve as an input for quantitative FT EXAFS modeling. The advantages and limitations of applying WT XAS are demonstrated (1) before and (2) after calcination to 650 °C of a Pt­(acac)₂ impregnated Mg­(In)­(Al)­O_<i>x</i> support and (3) after subsequent H₂ reduction to 650 °C. Combined XANES, FT, and WT XAS analysis shows that the acac ligands of the Pt precursor decompose during calcination, leading to atomically dispersed Pt⁴⁺ cations on the Mg­(In)­(Al)­O_<i>x</i> support. H₂ reduction treatment eventually results in the formation of 1.5 nm Pt–In alloyed nanoparticles. Widespread use and systematic application of wavelet-based XAS can potentially reveal in greater detail the intricate mechanisms involved in catalysis, chemistry, and related fields

Coking Tendency of 25Cr-35Ni Alloys: Influence of Temperature, Sulfur Addition, and Cyclic Aging

25Cr-35Ni base alloys are the most frequently used materials for steam cracking reactors. The influence of cyclic aging, reactor temperature, and adding sulfur containing compounds before or during cracking on the rate of coke deposition on a classical 25Cr-35Ni alloy is evaluated using a jet stirred reactor equipped with an electrobalance. As expected, the initial and asymptotic coking rate increased with increasing reactor temperature. Scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) analysis indicated that more Ni and Fe is present on the surface at higher cracking temperatures. Presulfidation led to increased coke deposition and decreased CO yields compared to the reference. When a sulfur containing compound was added continuously, coke deposition increased significantly but carbon oxide formation was suppressed. A pronounced amount of coke was measured in the reactor, followed by suppressed generated amounts of carbon oxides downstream. When combined with the continuous addition of sulfur containing compounds, presulfidation has little effect. Depending on the conditions, the effect of aging of the material is different: during the reference run and when only presulfidation was applied, coking rates increased as the material aged. When sulfur containing compounds were added continuously, with our without presulfidation, coking rates decreased as the material aged. This can be related with increased amounts of MnCr₂O₄ and Cr₂O₃ observed by SEM and EDX analysis

Periodic DFT Study of Benzene Adsorption on Pd(100) and Pd(110) at Medium and Saturation Coverage

Benzene adsorption on Pd(100) and Pd(110) has been investigated using periodic density functional theory (DFT) calculations. 4-Fold hollow geometries are preferentially adopted on both surfaces, and due to stronger repulsive interactions on Pd(100) a larger decrease in adsorption energy is calculated from medium to saturation coverage (∼120 kJ mol^–1) compared to Pd(110) (∼15 kJ mol^–1). On Pd(100), a slight energetic preference is calculated at saturation coverage for an adsorbate with two CC bonds parallel to the [011̅] direction. However, an adsorption geometry with alternately two types of benzene adsorbates, rotated azimuthally by 30° relative to one another, cannot be discarded since both geometries are compatible with ultraviolet photoemission spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS) observations. On Pd(110), there is a slight energetic preference for the hollow(0) site relative to the hollow(15) and hollow(30) at saturation coverage, and their calculated electronic features match UPS experiments. For the hollow(30), calculated vibrational features are not compatible with HREELS experiments, indicating that benzene does not populate hollow(30) sites at saturation coverage. Calculated STM images confirm that the experimentally observed two-lobed protrusion separated by a single depression oriented with its direction some 50° from [11̅0] can only correspond to the hollow(15) adsorbate. Inclusion of van der Waals interactions (vdW-DFT) increases adsorption energies by some 50 kJ mol^–1, but the relative ordering of the various adsorption sites remains unaltered as compared to PW91