Schulz-Flory Distribution Research Articles

Advancements and Challenges in the Synthesis of Oxymethylene Ethers (OMEs) as Sustainable Transportation Fuels.

The urgent need for sustainable alternatives to fossil fuels in the transportation sector is driving research into novel energy carriers that can meet the high energy density requirements of heavy-duty vehicles without exacerbating the climate change. This concept article examines the synthesis, mechanisms, and challenges associated with oxymethylene ethers (OMEs), a promising class of synthetic fuels potentially derived from carbon dioxide and hydrogen. We highlight the importance of OMEs in the transition towards non-fossil energy sources due to their compatibility with the existing Diesel infrastructure and their cleaner combustion profile. The synthesis mechanisms, including the Schulz-Flory distribution and its implications for OME chain length specificity, and the role of various catalysts and starting materials are discussed in depth. Despite advancements in the field, significant challenges remain, such as overcoming the Schulz-Flory distribution, efficiently managing water as an undesirable byproduct, and improving the overall energy efficiency of the OME synthesis. Addressing these challenges is crucial for OMEs to become a viable alternative fuel, contributing to the reduction of greenhouse gas emissions and the transition to a sustainable energy future in the transportation sector.

Insights into Fischer–Tropsch catalysis: current perspectives, mechanisms, and emerging trends in energy research

Fischer–Tropsch (FT) synthesis is an important module for the production of clean and sustainable fuels and chemicals, making it a topic of considerable interest in energy research. This mini-review covers the current literature on FT catalysis and offers insights into the primary products, the nuances of the FT reaction, and the product distribution, with particular attention to the Anderson–Schulz–Flory distribution (ASFD) and known deviations from this fundamental concept. Conventional FT catalysts, particularly Fe- and Co-based catalysis systems, are reviewed, highlighting their central role and the influence of water and water–gas shift (WGS) activity on their catalytic behavior. Various mechanisms of catalyst deactivation are also investigated, and the high methanation activity of Co-based catalysts is illustrated. To make this complex field accessible to a broader audience, we explain conjectured reaction mechanisms, namely, the carbide mechanism and CO insertion. We discuss the complex formation of a wide range of products, including olefins, kerosenes, branched hydrocarbons, and by-products such as alcohols and oxygenates. The article goes beyond the traditional scope of FT catalysis by addressing topics of current interest, including the direct hydrogenation of CO2 for power-to-X applications and the use of bifunctional catalysts to produce tailored FT products, most notably for the production of sustainable aviation fuel (SAF). This mini-review provides a holistic overview of the evolving landscape of FT catalysts and is aimed at both experienced researchers and those new to the field while covering current and emerging trends in this important area of energy research.

Computer-aided ligand screening to develop polymer-free catalyst in Cr-catalyzed ethylene oligomerization

Preventing the formation of undesirable polymer in ethylene oligomerization is a major challenge since the presence of these insoluble materials can cause severe reactor fouling. Based on previous experimental data, the concomitant formation of these solid compounds can be divided in two types. Detailed discussion in combination with mathematical analysis revealed that the low-molecular weight waxes (low-Mw waxes) are most likely the tail products of the Schulz-Flory distribution while high-molecular weight polyethylene (high-Mw PE) may be catalyzed by multiple active sites independent from the oligomerization cycle. Controlling the Schulz-Flory coefficient (K value) to remain in a lower range is crucial in achieving plentiful C4–C8 components and inhibiting polymer formation at the same time. Attempting to predict the K value, the energetic disparities of key transition states which control 1-butene selectivity have been extracted as a chemical descriptor to establish a general model. Promising ligands derived from computer-aided screening are expected to avoid the inherent waxes formation. Plausible strategies for the elimination of external polymerization sites are also suggested. The combination of them may provide a chance to develop more polymer-free catalysts to meet the requirements of industrial manufacturing.

Unravelling the chain growth mechanism in Cr/NNN-catalysed ethylene oligomerization

Density functional theory (DFT) methods were employed to explore the inherent chain growth mechanism targeting a special and controversial Cr/NNN ethylene oligomerization system.

The Molecular Weight Distribution of Occluded Hydrocarbon Gases in the Khibiny Nepheline–Syenite Massif (Kola Peninsula, NW Russia) in Relation to the Problem of Their Origin

The origin of hydrogen–hydrocarbon gases present in the rocks of the Khibiny massif in unusually high concentrations has been the subject of many years of discussion. To assess the role of potential mechanisms and relative time of formation of gases occluded in inclusions in minerals, the molecular weight distribution of C1–C5 alkanes in the main rock types of the Khibiny massif was studied. For this purpose, the occluded gases were extracted from rocks by mechanical grinding and their composition was analyzed on a gas chromatograph. It is established that the molecular weight distribution of occluded hydrocarbon gases in the Khibiny massif corresponds to the classical Anderson–Schulz–Flory distribution. In addition, the slopes of the linear relationships are relatively steep. This indicates a predominantly abiogenic origin of the occluded gases of the Khibiny massif. At the same time, a small proportion of biogenic hydrocarbons is present and is associated with the influence of meteoric waters. It was also found that in the Khibiny massif, the proportion of relatively high-temperature gases decreases towards the Main foidolite Ring in the following sequence: foyaite and khibinite–trachytoid khibinite–rischorrite and lyavochorrite–foidolites and apatite–nepheline ores. In the same sequence, an increase in the proportion of heavy hydrocarbons and the increasing role of oxidation and condensation reactions in the transformation of hydrocarbons occurs.

A Novel Method for Dynamic Molecular Weight Distribution Determination in Organometallic Catalyzed Olefin Polymerizations

In this study, a mathematical model for the time evolution of molecular weight distribution (MWD) was developed. This temporal model is based on the well-known Ziegler–Natta polymerization mechanism and reaction kinetics by the parametric solving of related differential equations. However, due to the generality of the reactions involved, the model can be extended to the other type of catalysts, such as metallocenes, Phillips, etc. The superiority of this model lies in providing the possibility of a more precise prediction over the active sites and kinetic parameters using a simple mathematical equation, which leads to improved reactor design in large-scale production. The model uses a function to develop a methodology for MWD calculations. In this way, the transient response is limited to the first few minutes of the reaction; however, it is important as it demonstrates the establishment of the final MWD. According to the results, almost for practical conditions with negligible transfer resistances, the time dependency of the MWD has a transient interval, depending on the kinetic constants of polymerization reactions. Increasing the time to infinity results in an increase in MW and a widening in MWD, which confirms the experimental plots well. In short, the main advantage of our proposed model over the previous ones is its ability to predict the MWD even before the completion of the polymerization reaction. The results of the present model match well with those of the well-known Schulz–Flory distribution, which only predicts the final molecular weight distribution, thus confirming that the model is reliable and generalizable.

Apparent kinetics and prospective operation on synthesizing Poly(oxymethylene) dimethyl ethers over shaped zeolite BETA in Fixed-bed reactor

Poly(oxymethylene) dimethyl ethers (PODEn=3-5), as promising diesel additive, were synthesized by reaction of 1,3,5-Trioxan (TOX) and methylal (PODE1) on the catalyst of zeolite BETA in a fixed-bed reactor and kinetically researched under reaction temperature from 308 to 358 K, initial mole ratio 3∙NTOX/NPODE1 from 0.8 to 1.2, space time from 0.33 to 20.78 min⋅kg⋅l−1 and water content from 0 to 2 wt%. A series of rate laws were formulated for quantifying not only their reaction rates but also the role of water. The obtained activation energies of propagation and decomposition reactions of PODEn (n = 3 ∼ 8) are 22.89 kJ⋅mol−1 and 27.46 kJ⋅mol−1 respectively, and their pre-exponential factors depend on the “n” in PODEn. Through optimizing the Schulz-Flory distribution of PODEn based on the kinetics, the maximal selectivity of TOX to PODE3-5 of 57.80% will achieve under anhydrous conditions, while this will become 54.60% under 5 wt% water in TOX.

Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz–Flory distribution of α-olefins.

Effects of water and methanol on synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and paraformaldehyde

Water and methanol are two unavoidable by-products for synthesis of polyoxymethylene dimethyl ethers (PODEn) from dimethoxymethane (DMM) and paraformaldehyde (PF), but their effects have not been clearly studied. As DMM decomposes with water and is formed with methanol, the decomposition ratio of DMM is a key quantity to describe the effects of water and methanol. Using this quantity, a simple and accurate method was proposed for calculating CH2O conversion and a complete model of Schulz-Flory (SF) distribution was established in the presence of water/methanol, both of which were validated experimentally. Then the effects of water and methanol were investigated in detail. The presence of water and methanol had complicated effects on equilibrium of the reaction, and both significantly decreased the yield of PODE3-5. These results showed that both water and methanol should be removed from the recycling stream in an industrial plant to enhance the efficiency of the reactor.

Reaction kinetics study of ethylene oligomerization into linear olefins over carbon-supported cobalt catalysts

Ethylene oligomerization over carbon-supported cobalt catalysts at 200 °C selectively produces linear olefins (>80% linearity up to C12 olefins at < 48% conversion) and linear alpha olefins (LAOs) (>30% LAO isomer distribution up to C8 olefins at < 10% conversion). Increasing the catalyst pretreatment temperature from 230 to 560 °C in argon prior to a reaction reduces cobalt oxide to cobalt metal and increases the catalytic activity. Cobalt metal supported on carbon deactivates during the reaction, likely due to higher rate of polyethylene formation which decreases the catalytic activity. The product distribution follows a Schulz-Flory distribution. The reaction is first order with respect to ethylene partial pressure, suggesting a mechanism where ethylene reacts with adsorbed olefins on a surface that is highly covered with olefins.

On the Role of Metal-Containing Imidazolium-Based Ionic Liquid Catalysts in the Formation of Tailored Polystyrene

In this work, we synthesized ionic liquid (IL) catalysts based on 1-butyl-3-methylimidazolium chloride (BMI·Cl), incorporating various types of metal chlorides (as the anions) and in different molar ratios, for application in the styrene bulk cationic polymerization. The polymerizations proceeded with high reproducibility, and by employing some IL catalysts, high conversions were achieved in short times. The reactivity of these catalysts depended on the concentration of the metal chloride in the IL. The synthesized polymers presented different molar mass ranges (2.8 kg·mol–1 ≤ M̅n ≤ 556 kg·mol–1), molar-mass dispersities, and glass transition temperatures (Tg), depending on the catalytic system employed. Structural analysis by the 1H NMR revealed that the end group of the polymer chain is a methyne group attached to a chlorine and some chains have the terminal indanyl group. The Schulz–Flory distributions indicated the existence of three to four catalytic species in the most active catalysts for the styrene polymerization. The polymerization was investigated by ESI(+)-MS(/MS), which showed the IL effect in the formation and stabilization of cationic intermediates.

Fischer–Tropsch Synthesis over a Cobalt Catalyst Supported on Titania-Doped Silicon Carbide

A cobalt Fischer–Tropsch synthesis catalyst supported on titania-doped silicon carbide is synthesized and comprehensively studied by a number of physicochemical means. The dynamics of the behavior and deactivation of the catalyst was studied after its stable operation for 1500 h during continuous experimental resource testing with a periodic change in the syngas flow rate and pressure and the process temperature. This catalyst has two types of active sites, on which products with different Schulz–Flory distribution parameters form. Depending on the process regime, the effective formation of soft waxes or more high-molecular-weight products is possible.

Synthesis of poly(oxymethylene) dimethyl ethers from methylal and trioxane over acidic ion exchange resins: A kinetic study

Poly(oxymethylene) dimethyl ethers (POMDME) are fuel additives that can reduce the emissions associated with diesel fuels considerably. POMDME have been synthetized from methylal and trioxane, catalyzed by the acidic ion exchange resin Amberlyst 15. A batch autoclave reactor has been used to carry out experiments conducted up to equilibrium conditions. The resulting POMDME distribution is modelled using the Schulz-Flory distribution theory, which confirms a sequential chain growth with insertion of one formaldehyde molecule at each reaction step. The kinetic experiments were carried out in an isothermal continuous fixed-bed reactor. The influence of temperature (45–80 °C) and space-time (1.6–3.2 kg h/m3) on trioxane conversion and product selectivity was determined. A kinetic model of the reaction has been proposed and validated with the experimental results. It has been demonstrated that trioxane decomposition to formaldehyde is the kinetically limiting step of the reaction scheme, with an activation energy of 112.2 kJ/mol. POMDME chain propagation reactions are in equilibrium under the conditions studied.

Core–Shell Metal Zeolite Composite Catalysts for In Situ Processing of Fischer–Tropsch Hydrocarbons to Gasoline Type Fuels

Fischer–Tropsch synthesis has two main challenges related to direct production of gasoline fuels. First, the chain length distribution of the products follows a broad and unselective Anderson–Schulz–Flory distribution. Second, mostly linear hydrocarbons are formed in the Fischer–Tropsch reaction, thus requiring isomerization while manufacturing gasoline fuels. The present paper addresses a synthetic strategy for the preparation of hierarchical metal and zeolite nanocomposite catalysts for direct synthesis of iso-paraffins from syngas. The nanocomposites are synthesized in three steps. In the first step, the parent (core) zeolite is etched with an ammonium fluoride solution. The etching creates small mesopores inside the zeolite crystals. In the second step, the Ru nanoparticles prepared using water-in-oil microemulsion are deposited in the mesopores of the zeolite. In the third step, a zeolite shell of MFI-type zeolites (silicalite-1 or ZSM-5) is grown on the parent zeolite crystals coating both the etched surface and metallic nanoparticles. Thus, the metal nanoparticles become entirely encapsulated inside the zeolite matrix. Most important parameters such as ruthenium content, zeolite mesoporosity, and more particularly, the acidity of the catalyst shell, which affect the catalytic performance of the synthesized nanocomposite materials in low-temperature Fischer–Tropsch synthesis were identified in this work. The higher relative amount of iso-paraffins was observed on the catalysts containing a shell of ZSM-5. The proximity between metal and acid sites in the zeolite shell of the nanocomposite catalysts is a crucial parameter for the design of efficient metal zeolite bifunctional catalysts for selective synthesis of gasoline-type fuels via Fischer–Tropsch synthesis, while the acidity of the catalyst core has only a limited impact on the catalytic performance.

Design of a core-shell catalyst: an effective strategy for suppressing side reactions in syngas for direct selective conversion to light olefins.

An elegant catalyst is designed via the encapsulation of metallic oxide Zn–Cr inside of zeolite SAPO34 as a core–shell structure (Zn–Cr@SAPO) to realize the coupling of methanol-synthesis and methanol-to-olefin reactions. It can not only break through the limitation of the Anderson–Schulz–Flory distribution but can also overcome the disadvantages of physical mixture catalysts, such as excessive CO2 formation. The confinement effect, hierarchical structure and extremely short distance between the two active components result in the Zn–Cr@SAPO capsule catalyst having better mass transfer and diffusion with a boosted synergistic effect. Due to the difference between the adsorption energies of the Zn–Cr metallic oxide/SAPO zeolite physical mixture and capsule catalysts, the produced water and light olefins are easily removed from the Zn–Cr@SAPO capsule catalyst after formation, suppressing the side reactions. The light olefin space time yield (STY) of the capsule catalyst is more than twice that of the typical physical mixture catalyst. The designed capsule catalyst has superior potential for scale-up in industrial applications while simultaneously extending the capabilities of hybrid catalysts for other tandem catalysis reactions through this strategy.

Carbon and hydrogen isotopes of the wet gases produced by gamma-ray-induced polymerisation of methane: Insights into radiogenic mechanism and natural gas formation

The 60Co gamma ray radiolysis of methane was undertaken at room temperature and at a low proportion of methane conversion to observe initial polymerisation reactions in the formation of C2─C5 saturated hydrocarbon gases. The gamma ray radiolysis product follows a Schulz-Flory distribution model for the straight-chained C2+ alkane gases consistent with chain propagation by addition of C1 intermediates. The high relative concentration of branched isomers suggests the total product distribution is likely controlled by propagation reactions involving mainly radical intermediates. In particular, the pentane isomers show a relative dominance of the branched isomers with sub-equal amounts of iso-pentane and neo-pentane. Carbon isotopes of the C2─C5 wet gases are all depleted in 13C relative to δ13C methane reactant with δ13C C2 < δ13C C3 and carbon isotope reversals in the C3─C5 gases while only ethane and neo-pentane are depleted in 2H compared to δ2H of the starting methane. As such, these molecular and isotopic features show similar characteristics to abiogenic wet gases found in terrestrial settings.

Investigations on an efficient and environmentally benign poly(oxymethylene) dimethyl ether (OME3-5) fuel synthesis

Poly(oxymethylene dimethyl ethers) (OMEn, CH3O˗(CH2O)n˗CH3) with the appropriate chain length n = 3–5 are discussed as alternative diesel fuels due to their applicability for direct blending with diesel fuel in slightly modified diesel engines. This group of oxygenates have regained much interest as they significantly reduce soot formation during combustion, which is reflected in their large-scale pilot plant production. This work focuses on a favorable route for an efficient synthesis of OME3-5 from dimethoxymethane and trioxane at ambient pressure under mild reaction conditions. A detailed study was conducted on the catalytic performance of both methanesulfonic acid and the commercial solid acid Deloxan® as environmentally benign catalysts and promising replacements for sulfuric acid. For this, special focus on the influences of different reaction parameters (e.g. temperature, molar educt ratio, reaction time and pressure) was investigated herein. Hence, methanesulfonic acid (3.5 wt%) and Deloxan® (1.7 wt%) were found to be suitable catalysts for the anhydrous synthesis of OME with an OME3-5 selectivity of 32.7 wt% and 30.0 wt%, respectively. Furthermore, synthesis of OMEs catalyzed by either methanesulfonic acid or Deloxan® was found to follow Schulz-Flory distribution denoting a sequential OME chain-propagation.

Rate-selected growth of ultrapure semiconducting carbon nanotube arrays

Carbon nanotubes (CNTs) are promising candidates for smart electronic devices. However, it is challenging to mediate their bandgap or chirality from a vapor-liquid-solid growth process. Here, we demonstrate rate-selected semiconducting CNT arrays based on interlocking between the atomic assembly rate and bandgap of CNTs. Rate analysis confirms the Schulz-Flory distribution which leads to various decay rates as length increases in metallic and semiconducting CNTs. Quantitatively, a nearly ten-fold faster decay rate of metallic CNTs leads to a spontaneous purification of the predicted 99.9999% semiconducting CNTs at a length of 154 mm, and the longest CNT can be 650 mm through an optimized reactor. Transistors fabricated on them deliver a high current of 14 μA μm−1 with on/off ratio around 108 and mobility over 4000 cm2 V−1 s−1. Our rate-selected strategy offers more freedom to control the CNT purity in-situ and offers a robust methodology to synthesize perfectly assembled nanotubes over a long scale.

(Trans)acetalization Reactions for the Synthesis of Oligomeric Oxymethylene Dialkyl Ethers Catalyzed by Zeolite BEA25

Oligomeric oxymethylene dimethyl ethers (DMEs; OMDMEn) are acetals of the type CH3O(CH2O)nCH3 with a high oxygen content and without carbon–carbon bonds in the molecular structure. Therefore, formation of soot and NOx emissions is largely suppressed during combustion. Oligomers with n = 3–5 exhibit properties similar to conventional diesel fuel and this stimulated extensive research in this field. Reactions of OMDME1, the corresponding ethyl derivative OMDEE1 and dimethyl ether (DME) with trioxane have been carried out employing zeolite BEA25 as acidic catalyst. Thus, oligomer mixtures with n = 1–5 are obtained and very small amounts of higher oligomers are also formed. OMDMEn- or OMDEEn-forming reactions from the starting materials take place as well as subsequent transacetalization reactions which lead usually to a product distribution according to a Schulz–Flory distribution. Several transacetalization reactions have been studied, e.g. reactions of OMDME1 with OMDME3 and OMDEE1 with OMDEE3, which lead to the typical mixtures with n = 1–5. Reactions of OMDME1 with OMDEE1 yielded ethoxymethoxymethane, i.e. the acetal with mixed end groups. This allows for the tuning of oligomer mixtures and adjustment of properties according to the required demands of the respective application. Regarding DME as starting compound, the transacetalization reaction with OMDME3 yielded only small amounts of OMDME1–5 while the reaction of DME with trioxane yielded considerable amounts of the oligomers. The reaction is comparatively slow enabling a kinetic control of the oligomer distribution which exhibits an elevated selectivity for the desired OMDME3–5 fraction in the beginning of the reaction.

Ethylene oligomerization studies utilizing nickel complexes bearing pyridine-imine ligands

Two pyridine-imine ligands (6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine (L1) and (6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine (L2) were synthesized in good yields. Reactions with L1 and L2 with Ni salts afforded nickel complexes NiCl2{(6E,7E)-N1,N4-bis((pyridin-2-yl)methylene)benzene-1,4-diamine}NiCl2 (C1) and NiCl2{(6E,13E)-N1,N4-bis(1-(pyridin-2-yl)ethylidene)benzene-1,4-diamine}NiCl2 (C2). The compounds were characterized through NMR (1H NMR and 13C NMR), IR, UV–Vis, MS and elemental analysis. Upon activation with a co-catalyst, C1 and C2 both displayed good catalytic activity (0.63 × 104–16.66 × 104 g/mol Ni h and 0.11 × 105–23.70 × 105 g/mol Ni h, respectively) in ethylene oligomerization experiments, the oligomer products followed the classic Schulz-Flory distribution with mostly 1-C4= oligomers. The catalytic performance was obviously affected by the reaction parameters including reaction temperature, reaction pressure, Al/Ni molar ratio, reaction time and co-catalyst type. The experimental data suggested that C1 and C2 generally had quite different catalytic performance under the same reaction conditions indicating the catalyst structure had a significant influence on the catalytic performance. The density functional theory (DFT) calculation was utilized to optimize the geometry structure for exploring the relationship between structure and catalytic properties. Combined with a proposed mechanism, reaction intermediates and frontier molecular orbitals were simulated for further studying the intermediates structure and energy variation in ethylene oligomerization process.