Alkyl Side Research Articles

Controlling the Molecular Self-Assembly into Nanofibers of Amphiphilic Zinc(II) Salophen Complexes

The synthesis, characterization, and aggregation properties in the solid state of a series of amphiphilic Zn(salophen) Schiff-base complexes are presented, through a combined FE-SEM/XRD approach. It is found that these complexes self-assemble into nanofibers depending on the solvent used to prepare the solutions. Thus, fibrous aggregates are obtained from solutions of weak and volatile Lewis base solvents, either by drop-casting or by complete solvent evaporation, whereas, in the case of noncoordinating solvents, where oligomeric aggregates are already present in solution, no formation of nanofibers is observed. The length of side alkyl groups and their degree of interdigitation lead to a 2D columnar square structure in the case of the complex with the short 4-ethyloxy substituents, whereas complexes having longer 4-alkyloxy chains are characterized by a lamellar structure. Bundles of twisted nanofibers are formed by further interactions and interdigitation of the outside alkyl groups of each nanofiber. A simple model, which describes the mechanism of formation and structure of these nanofibers, is presented.

Analysis of Petroleum Aromatics by Laser-Induced Acoustic Desorption/Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry

Laser-induced acoustic desorption coupled with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (LIAD/SVUVPI-MS) is employed to analyze aromatics prepared under different conditions from Lungu atmospheric residue (LGAR), i.e., the primary aromatics separated directly from LGAR, and the secondary aromatics after hydrogenation of LGAR and its resins. The mass spectra of the primary aromatics present a bimodal normal distribution in the range of 200–900 Da, in which the relative intensity of the two peaks changes significantly with the SVUV photon energies (9.0, 11.0, and 14.0 eV), indicating that at least two categories of compounds with different ionization energies (IEs) are included, i.e., polycyclic aromatics (IEs < 10.0 eV) in the mass range of 400–900 Da, and aliphatic and alicyclic compounds (IEs close to 11.0 eV) in 200–400 Da. Also detected in the aromatics are metalloporphyrins. Furthermore, the mass spectra of the secondary aromatics separated from LGAR and its resins at different hydrogenation temperatures (390, 400, 410, and 420 °C) are also recorded. The results indicate that the hydrogenation process, especially at higher temperatures, results in removal of alkyl-side and bridge chains in the aromatics, and the secondary aromatics from LGAR resins contain more alkyl side and bridge chains and metal compounds than those from LGAR.

Liquid crystal alignment properties of poly(styrenesulphonate)/alkyltrimethylammonium complexes

We synthesised a series of poly(4-styrenesulphonate)/alkyltrimethylammonium (PSS-#Cx, # = 12, 14 and 16; x = 80, 60, 40 and 20) complexes, where # is the number of carbon atoms in the alkyl groups in alkyltrimethylammonium bromide, and x is the molar content (%) of alkyltrimethylammonium moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a longer alkyl side group and a higher molar content of alkyl side group showed homeotropic LC alignment behaviour with a pretilt angle of about 90°. The homeotropic LC alignment behaviour was well correlated with the surface energy of the polymer films. Homeotropic LC alignment was observed when the surface energy values of the polymer were smaller than about 44.87 mJ/m2.

Photorefractive effects in polymer dissolved liquid crystal composites dopes with fullerene derivatives

We investigated the photorefractive performance of the polymer dissolved liquid crystalline composite (PDLCC), in which liquid crystalline polymer and low-molar-mass liquid crystal are miscible without phase separation, doped with three kinds of fullerene derivatives with different length of alkyl groups. The photorefractive performance was improved for the photorefractive PDLCC doped with fullerene derivatives with long length of alkyl groups. The photorefractive grating formation originates in the cooperative reorientation of the liquid crystalline director and the space charge field was estimated using the elastic continuum theory and the field for the PDLCC doped with the functionalized fullerene with longer alkyl side groups was larger than that for the PDLCC doped with conventional fullerene C60.

The effect of controllable thin film crystal growth on the aggregation of a novel high panchromaticity squaraine viable for organic solar cells

A description of the synthesis procedure is provided for a new squaraine molecule, DiPSQ(OH)2. This molecule is an interesting target molecule for organic photovoltaics (OPV) because of its unusually broad panchromaticity, suggesting a tighter molecular packing than would be expected, given its bulky alkyl side groups. UV–vis–NIR absorption data provides evidence for the presence of both H- and J-aggregates. Thin film X-ray diffraction (TFXRD) data supports the presence of one dominant crystal structure. Further interpretation of TFXRD results and complementary Atomic Force Microscopy (AFM) characterization for pure and blended samples provides a comprehensive explanation of how DiPSQ(OH)2 aggregates are affected by annealing and crystal growth. The described aggregation properties of donor–acceptor blends for solar cells are strongly dependent on annealing. Influencing phase separation, local crystallinity, and macroscopic crystallization, these physical properties are known to significantly impact charge dissociation, exciton diffusion rates, and bulk charge transport in devices. The strategy of systematically varying annealing times has resulted in improved bulk heterojunction crystallinity and regions of increased phase separation, both necessary to achieve the long term goal of morphological control for commercially viable NIR-active OPV devices.

The effect of alkyl side groups on the secondary structure and crystallization of poly(ethylene glycol)-block-polypeptide copolymers

The solid state and melt nanoscale structures of a series of poly(ethylene glycol)-block-poly(γ-alkyl-l-glutamate)s (PEG-b-PALG) copolymers bearing different alkyl side groups, i.e., n-butyl, n-hexyl, n-octyl, and n-dodecyl, have been investigated in the present work. An interesting effect of alkyl side groups on the secondary conformation of polypeptide block, crystallization of PEG segments and morphology of the copolymers was observed. With increasing the length of alkyl side groups or raising the temperature, the predominant secondary structure gradually changed from α-helix to β-sheet conformation. Differential scanning calorimetry, wide-angle X-ray diffraction analysis and polarized optical microscopy indicated that the crystallization and morphology of PEG segments were markedly restricted by the polypeptide block. Therefore, the alkyl side groups not only exhibited significant influence on the conformation of polypeptide block, but also affected the crystallization of PEG segments. The secondary structure, crystallization and spherulite morphology of these synthetic biomaterials, which were significantly influenced by alkyl side groups, may play an important role in their physicochemical properties, such as self-assembly, as well as in some biomedical applications.

PH- and thermo-responsive poly(N-isopropylacrylamide-co-acrylic acid derivative) copolymers and hydrogels with LCST dependent on pH and alkyl side groups.

A series of pH- and temperature-responsive poly(N-isopropylacrylamide-co-acrylic acid derivative) (P(NIPAM-co-AAD)) copolymers and hydrogels were prepared. The lower critical solution temperatures (LCSTs) of the copolymers exhibited a dependence on both pH and the hydrophobicity of the AAD unit. The influence of pH and temperature on the equilibrium swelling ratio of the hydrogels was investigated. The hydrogels displayed a unique thermo-induced swelling-deswelling transition that can be self-regulated to occur at above or below the physiological temperature in response to the environmental pH. Scanning electron microscopic (SEM) analysis revealed porous sponge-like microstructures of the hydrogels. Insulin was loaded into the hydrogels as a model protein, and the in vitro release profiles indicated that the loaded protein could be protected within the hydrogels in an acidic environment and selectively released in neutral medium. MTT assay proved that both the copolymers and hydrogels are nontoxic. After oral administration of the insulin-loaded hydrogels to streptozotocin-induced diabetic rats at 60 IU per kg, the fasting plasma glucose level was reduced continuously to 72.1% within 6 h. The bioavailability of hydrogel-encapsulated insulin via the oral administration to healthy rabbits reached 5.24%, which is much higher than that of pure insulin solution given orally. These results showed that the smart copolymers and hydrogels may hold great promise for pH-triggered drug delivery systems.

Nanophase Separation and Exotic Dynamic Behavior in Comb-Like Polymers

We show the potential of neutron scattering combined with isotopic labeling to unravel structural and dynamical properties of “comb-like” polymers. Results on homopolymers with alkyl side groups of varying length, namely on the families of poly(n-alkyl methacrylates) (PnMAs) and poly(alkylene oxides) (PAOs) are summarized. Diffraction experiments and fully atomistic molecular dynamics simulations evidence the formation of self-assembled nanodomains of the side groups surrounded by the main chains, independently of the nature of the latter. Neutron spin echo, time of flight and backscattering techniques were employed to investigate the collective dynamics relating inter- and intra-nanodomain correlations as well as the incoherent scattering function of hydrogens. The results are combined with other experimental techniques-dielectric spectroscopy and differential scanning calorimetry- to put into a context the dynamical processes identified. Plasticization and confinement effects arise in PnMAs, but not in ...

Study for Pyrolysis of Ethylbenzene and Xylene under Supercritical Pressure

Under the supercritical pressure, the pyrolysis reactions of ethylbenzene and xylene were investigated by using the continuous flow device. At the pressure of 4 MPa and the different temperatures of 700, 750 and 780 ℃, the pyrolysis gaseous products were analyzed by online gas chromatography for ethylbenzene and xylene, while their liquid products were quantitatively analyzed by gas chromatography-mass spectrometry (GC-MS). It is found that the gaseous products of ethyl- benzene decomposition are similar to those of xylene, while the liquid products of ethylbenzene and xylene decomposition are quite different, of which the major products are aromatic hydrocarbons. The experiment reveals that the higher the pyro- lysis temperature is, the higher the conversion ratio will be for ethylbenzene and xylene. On the other hand, the conversion ratio of ethylbenzene is higher than that of xylene at the same temperature in experiment. From the experimental observation, we conclude that the pure aromatic hydrocarbon does not cause serious coking during the cracking process, owing to the low pyrolysis. Theoretical calculations are performed to obtain the bond energies for the different C—C or C—H bond types in side alkyl groups of ethylbenzene and xylene, by using density functional theory (DFT) method at the BHandHLYP/6-31+ G(d, p) level. The bond energies calculated in this study agree well with those available from literature. It is found that the weakest bond is the C—C bond in the ethyl group of ethylbenzene, with a bond energy of 313.1 kJ/mol. This value is much smaller than the smallest bond energy of 393.2 kJ/mol of the C—H bond in the substituted methyl group in xylene. The weakness of the C—C bond in ethylbenzene thus leads to the fact that the pyrolysis of ethylbenzene is much easier than that of xylene. The theoretical results predict a higher conversion ratio of ethylbenzene than that of xylene. So the calculation gives a good explanation for the experimental phenomenon. Finally, the study in this work shows a new and important insight to the coking mechanism for the hydrocarbon fuels. Keywords ethylbenzene; xylene; pyrolysis; supercritical; coke; bond energy

Molecular modeling investigation of the fundamental structural parameters of polymers of intrinsic microporosity for the design of tailor-made ultra-permeable and highly selective gas separation membranes

Molecular dynamics techniques have been used to investigate the pronounced permeability and selectivity of polymers of intrinsic microporosity (PIM). While helium exhibited greater permeability coefficients due to higher diffusivity resulting from its small molecular size, the higher permeability coefficients of carbon dioxide were attributed to the favorable values of the enthalpy of mixing of the gas molecules with the polymeric chains. The simulation results also showed that the fractional free volume and the Connolly surface of the PIMs had greater values resulting from the inability of the rigid and contorted structures of the PIMs to pack space efficiently and thus creating interconnected nanochannels throughout the matrix of the polymers of microporosity. Replacing the heteroatoms of the PIM with carbon atoms resulted in a major decrease in the fractional free volume with a corresponding decline in the diffusion coefficients whereas replacing the site of contortion with a flexible methylene group did not have much effect. Attaching alkyl side groups to the polymeric chains was shown to reduce the free volume as a result of the side chains filling the voids of the micropores. The results were employed in the design of tailor-made membranes for use in the separation of natural gas components.

Packing Dependent Electronic Coupling in Single Poly(3-hexylthiophene) H- and J-Aggregate Nanofibers

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.3 at 4 K. Temperature-dependent PL line shape parameters (i.e., 0-0 energies and 0-0/0-1 intensity ratios) undergo an abrupt change in the range of ~110-130 K suggesting a change in NF chain packing. Pressure-dependent PL lifetimes also show increased contributions from an instrument-limited decay component which is attributed to greater torsional disorder of the P3HT backbone upon decreasing NF volume. It is proposed that the P3HT alkyl side groups change their packing arrangement from a type I to type II configuration causing a decrease in J-aggregate character (lower intrachain order) in both temperature- and pressure-dependent PL spectra. Chain packing dependent exciton and polaron relaxation and recombination dynamics in NF aggregates are next studied using transient absorption spectroscopy (TAS). TAS data reveal faster polaron recombination dynamics in H-type P3HT NFs indicative of interchain delocalization whereas J-type NFs exhibit delayed recombination suggesting that polarons (in addition to excitons) are more delocalized along individual chains. Both time-resolved and steady-state spectra confirm that excitons and polarons in J-type NFs are predominantly intrachain in nature that can acquire interchain character with small structural (chain packing) perturbations.

Multicompartment thermoresponsive gels: does the length of the hydrophobic side group matter?

Multicompartment thermoresponsive gels are novel materials with fascinating self-assembly and interesting applications. The aim of this study was to investigate for the first time the effect of the length of the alkyl side group of a hydrophobic monomer on the thermoresponsive and self-assembly behaviour of terpolymers. Specifically twelve well-defined terpolymers based on the hydrophilic monomers 2-(dimethylamino)ethyl methacrylate (DMAEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA), and on the hydrophobic monomer ethyl-, n-butyl or n-hexyl methacrylate (EtMA, BuMA or HMA) of varying architectures (ABC, ACB, BAC and statistical) were synthesised using Group Transfer Polymerisation. The A, B and C blocks were based on PEGMA, the alkyl containing methacrylate monomer, and DMAEMA, respectively. The molecular weights (MWs) and compositions of the polymers were kept the same. The polymers and their precursors were characterised in terms of their MWs, MW distributions and compositions. Aqueous solutions of the polymers were studied by turbidimetry, hydrogen ion titration, light scattering and rheology to determine their cloud points, pKas, hydrodynamic diameters and thermoresponsive behaviour and investigate the effect of the architecture and the hydrophobic alkyl side group of the terpolymers. It was found that the pKas and the Tgs were mostly affected by the hydrophobicity of the side groups and not by the architecture, while the cloud points and the sol–gel transition of the polymers were affected by both the length of the alkyl side group and the polymer architecture. Interestingly the sharpest sol–gel transitions and stable multicompartment hydrogels were observed for the ABC triblock copolymers with the short alkyl-side groups even though the sol–gel transition occurred at higher temperatures.

Side-Chain-Dependent Helical Conformation of Amylose Alkylcarbamates: Amylose Tris(ethylcarbamate) and Amylose Tris(n-hexylcarbamate)

Eight amylose tris(ethylcarbamate) (ATEC) samples ranging in the weight-average molar mass M(w) from 1.0 × 10(4) to 1.1 × 10(6) g mol(-1) and five amylose tris(n-hexylcarbamate) (ATHC) samples of which M(w) varies from 4.9 × 10(4) to 2.2 × 10(6) g mol(-1) have been prepared from enzymatically synthesized amylose samples having narrow dispersity indices and no branching. Small-angle angle X-ray scattering (SAXS), light scattering, viscometry, and infrared (IR) absorption measurements were carried out for their dilute solutions, that is, ATEC in tetrahydrofuran (THF), 2-methoxyethanol (2ME), methanol (MeOH), and ATHC in THF and 1-propanol (1PrOH) to determine M(w), particle scattering functions, intrinsic viscosities, and IR spectra. SAXS and viscosity measurements were also made on ATEC in d- and l-ethyl lactates. The data were analyzed in terms of the wormlike cylinder model to estimate the helix pitch (or contour length) per residue h and the Kuhn segment length λ(-1) (stiffness parameter, twice the persistence length). Both ATEC and ATHC have large λ(-1) in THF, that is, 33 and 75 nm, respectively, and smaller λ(-1) were obtained in alcohols, indicating that they have rigid helical conformation stabilized by intramolecular hydrogen bonds in THF. On the contrary, the helical structure estimated from the h value significantly depends on the alkyl side groups in a complex fashion, that is, h = 0.36 nm for ATEC, h = 0.29 nm for ATHC, and h = 0.26 nm for amylose tris(n-butylcarbamate) (ATBC). This is likely related to the bulkiness of side groups packed inside the amylosic helices. The solvent dependence of h, λ(-1), and the fraction f(hyd) of intramolecular hydrogen bonds for ATEC can be explained by a current model as is the case with ATBC [ Terao , K. ; Macromolecules 2010 , 43 , 1061 ], in which each contour point along the chain takes loose helical and rigid helical sequences independently.

Reversible conformation-driven order–order transition of peptide-mimic poly(n-alkyl isocyanate) in thin films via selective solvent-annealing

We report for the first time the conformational and structural details of peptide-mimic poly(n-hexyl isocyanate) (PHIC). PHIC is a representative poly(n-alkyl isocyanate)s, which have received significant attention because of their unique stiff chain characteristics and potential applications in various fields. A well-ordered hexagonal close packing structure of PHIC with 83 helical conformation was clearly observed in the nanoscale thin films that were selectively annealed with carbon disulfide (CS2). A well-ordered multi-bilayer structure of the polymer with β-sheet conformation was also clearly formed in the films that were selectively annealed with toluene. In addition, a fully reversible transformation between these two self-assembled structures was demonstrated by consecutive annealings with CS2 and toluene. A family of peptide-mimic polymers — polyisocyanates with linear alkyl side groups — are known to have a stiff helical backbone. These structural features have garnered attention for the development of chiral recognition, optical switches, liquid crystals or degradable materials. Yet the exact conformation the chains adopt, and how they pack in the solid state, remains unclear. Through detailed characterization, Moonhor Ree, Jae-Suk Lee and co-workers in Korea have now confirmed that, in a thin film, the polymer ‘poly(n-hexyl isocyanate)’ forms one of two different, well-ordered structures. The two morphologies were obtained, and reversibly converted into each other, by annealing under different solvent vapours. In the presence of carbon disulfide, helical chains self-assembled into a hexagonal close packing structure, whereas the use of toluene yielded a multi-bilayer lamellar structure consisting of polymer chains in β-sheet conformation. This intriguing transformation arises from the solvent molecules' different affinities with the backbone and side groups of the polymer chain. Peptide-mimic poly(n-hexyl isocyanate) (PHIC) with stiff chain characteristics demonstrated to selectively form a well-ordered hexagonal close packing structure with 83 helical conformation in the nansocale thin films annealed with carbon disulfide. Moreover, this polymer showed to selectively form a well-ordered multi-bilayer structure with β-sheet conformation in the thin films annealed with toluene. These two self-assembled structures were reversibly transformed by consecutive annealing with carbon disulfide and toluene. These chain conformations and self-assembled structures were confirmed by synchrotron grazing incidence X-ray scattering analysis.

Consequences of Varying Adsorption Strength and Adding Steric Hindrance on Self-Assembly of Supramolecular Polymers on Carbon Substrates

We investigated the consequences of changes in adsorption strength and the influence of steric hindrance with respect to ordering of supramolecular polymers on surfaces. The focus is on the kinetics of domain formation and the guidance of this self-assembly process by the substrate. To demonstrate general features, we compared two molecules, both forming supramolecular polymers, bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT), differing only in the architecture of the alkyl side groups, on two substrates. Although highly oriented pyrolytic graphite (HOPG) and epitaxial graphene (EG) grown on silicon carbide have identical chemical composition and nearly the same crystal lattice parameters at the surface, they differ significantly in adsorption strength. Due to its higher polarizability, HOPG adsorbs molecules much more strongly than EG. Nonetheless, even on EG, the formation of supramolecular polymers was guided by the symmetry of the substrate lattice, but at a much slower rate. A...

Self-Assembly of Poly(4-vynil-N-alkylpyridinium) Bromide onto Silica: Effect of Side-Chain Length on Structural Aspects at a Molecular Level

Self-assembly of poly(4-vynil-N-alkyl)pyridinium bromide with alkyl side chains of 2, 5, 7, 10, or 16 carbons from ethanolic solutions onto flat silica surfaces was studied by means of ellipsometry, atomic force microscopy (AFM), contact angle measurements, and sum-frequency generation (SFG) vibrational spectroscopy in the CH3 and CH2 stretch region. Ab initio quantum-chemical calculations on the N-alkylpyridinium side-group with restricted Hartree–Fock (RHF) method and 6-311G (d,p) basis set were done to estimate the charge distribution along the pyridinium ring and the alkyl side-chain. SFG results showed that longer side chains promote the disorientation of the alkyl groups at the surface, corroborating with the contact angle values. AFM images revealed film homogeneity, regardless the alkyl side group. However, after 24 h contact with water, ringlike structures appeared on the film surfaces, when the polycation alkyl side chain had 7 or less carbons, and as the alkyl chain increased to 10 or 16 carbons, the films dewetted because the hydrophobic interactions prevailed over the electrostatic interactions between the pyridinium charged groups and the negatively charged SiO2 surface. Under acid conditions (HCl 0.1 mol·L–1), the film mean thickness values decreased up to 50% of original values when the alkyl side chains were ethyl or pentyl groups due to ion-pair disruption, but for longer groups they remained unchanged. Quantum-chemical optimization and Mulliken electron population showed that (i) from C2 to C15 the positive charge at the headgroup (HG) decreased 0.025, while the charge at combined HG + α-CH2 increased 0.037; and (ii) for C6 or longer, the alkyl side group presents a tilt in the geometry, moving away from the plane. Such effects summed up over the whole polymer chain give support to suggest that when the side chains are longer than 7 carbons, the hydrophobic interaction decreases film stability and increases acid resistance.

Impact of Association Phenomena on the Thermodynamic Properties of Modified Polysulfones in Solutions

Theoretical and experimental aspects of the association phenomena generated by hydrogen bonding, dispersive, and electrostatic interactions in ternary systems consisting of a proton-donor solvent (N,N-dimethylformamide or methanol), a proton-acceptor solvent (water), and a proton-acceptor polymer (polysulfones with different alkyl side groups) are investigated. In this context, binary and ternary thermodynamic interaction parameters are corrected on the basis of the different association constants. Numerical values of these constants were evaluated as a function of the system composition, by mathematical simulations. As a result, mathematical simulations allow a good theoretical description of the preferential adsorption coefficients, in agreement with experimental data.

Solvent-to-Polymer Chirality Transfer in Intramolecular Stack Structure

Solvent-to-polymer chirality transfer was examined using conjugated polymer with intramolecular stack structure (IaSS). When achiral poly(diphenylacetylene)s (PDPAs) dissolved in limonene, the solvent chirality was successfully transferred to the side phenyl stack structure, leading to intramolecular axial chirality. The phenyl–phenyl IaSS was under thermodynamic control to readily undergo asymmetric changes in chiral limonene, leading to optical activity in the isotropic structure between the main chain and resonant side phenyl rings. The axial chirality was significantly affected by the chain length and substitution position of the side alkyl groups. The longer alkyl chains and bulkier alkyl group prevented direct intermolecular interactions between the side phenyl rings and the chiral limonene molecules. PDPA with sterically congested, highly stable, and regulated IaSS was not favorable for efficient solvent-to-polymer chirality transfer.

Planar Donor-acceptor Copolymers for Bulk Heterojunction Solar Cells

Two planar conjugated copolymers poly2,8-{5,11-diisooctylindolocarbazole}-4,7-di[2,5-thiophene]-5,6-dioctyloxy-2,1,3-benzothiadiazole(QP-2) and poly2,8-{5,11-dioctylindolocarbazole}-4,7-di[2,5-thiophene]-5,6-dioctyloxy-2,1,3-benzothiadiazole(QP-3) with main chain donor-acceptor alternating structures were synthesized and their photophysical and photovoltaic properties were investigated.Photovoltaic cells based on polymer QP-3 gave a power conversion efficiency(PCE) of 2.59%,an open circuit voltage(VOC) of 0.72 V,a short circuit current density(JSC) of 9.24 mA/cm2,and a fill factor(FF) of 0.38.The results show that the polymer without branched alkyl side group performs better and they have better crystallinity in solid film.

Theoretical insight into the minor role of paring mechanism in the methanol-to-olefins conversion within HSAPO-34 catalyst

Insight into the structure of hydrocarbon pool species and its effect on the catalytic activity and selectivity are urgently required in methanol-to-olefins (MTO) conversion. The fundamental issue is the understanding of its reaction mechanism. Previously, we have elucidated a complete catalytic cycle of side chain hydrocarbon pool mechanism. In this paper, paring hydrocarbon pool mechanism for different methylbenzenes (MBs) in HSAPO-34 zeotype catalyst is comprehensively investigated by periodic density functional theory calculations. The complete catalytic cycle involves a sequence of elementary steps that include methylation, ring contraction, shift of proton or methyl group, elimination of side alkyl groups, and regeneration of MBs. The major bottleneck is identified as the regeneration of MBs from five-membered ring cations. The intermediate cations having five-membered ring structure and the transition states featuring primary carbocations are unstable in the paring route. The overall energy barriers of different MBs depend strongly on the number of methyl groups. By comparing the kinetics of the paring route and the side chain route, we demonstrate that the full paring mechanism exhibits a higher barrier, and which is a minor route in the MTO conversion.