Shape Selectivity Effects Research Articles

Understanding shape selectivity effects of hydroisomerization using a reaction equilibrium model.

We study important aspects of shape selectivity effects of zeolites for hydroisomerization of linear alkanes, which produces a myriad of isomers, particularly for long chain hydrocarbons. To investigate the conditions for achieving an optimal yield of branched hydrocarbons, it is important to understand the role of chemical equilibrium in these reversible reactions. We conduct an extensive analysis of shape selectivity effects of different zeolites for the hydroisomerization of C7 and C8 isomers at chemical reaction equilibrium conditions. The reaction ensemble Monte Carlo method, coupled with grand-canonical Monte Carlo simulations, is commonly used for computing reaction equilibrium of heterogeneous reactions. The computational demands become prohibitive for a large number of reactions. We used a faster alternative in which reaction equilibrium is obtained by imposing chemical equilibrium in the gas phase and phase equilibrium between the gas phase components and the adsorbed phase counterparts. This effectively mimics the chemical equilibrium distribution in the adsorbed phase. Using Henry's law at infinite dilution and mixture adsorption isotherm models at elevated pressures, we calculate the adsorbed loadings in the zeolites. This study shows that zeolites with cage or channel-like structures exhibit significant differences in selectivity for alkane isomers. We also observe a minimal impact of pressure on the gas-phase equilibrium of these reactions at typical experimental reaction temperatures 400-700K. This study marks initial strides in understanding the reaction product distribution for long-chain alkanes.

Beta zeolite as an efficient catalyst for the synthesis of diphenolic acid (DPA) from renewable levulinic acid

The solvent-free production of diphenolic acid (DPA) from levulinic acid (LA) and phenol is studied using readily accessible commercial acid zeolites like Beta, ZSM-5 and USY. Acid zeolites are cost-effective catalysts, and they are herein benchmarked against the sulfonic acid resins Amberlyst-15 and Nafion®, and sulfonic acid-functionalized SBA-15 silicas. Beta zeolite with a moderate aluminum content (H-Beta 19, Si/Al=23) presents the best catalytic performance, owing to the right combination of the shape selectivity effect conferred by the BEA structure, and the adequate balance of acidity (Al content and speciation). The optimization of the reaction conditions is tackled by the response surface methodology using as optimization factors the temperature, the PhOH:LA molar ratio, and the catalyst loading. Thus, under the optimized reaction conditions (12 mmol LA, 140 ᵒC, 0.30 g catalyst loading, PhOH:LA = 6:1 mol), over 70% yield to DPA with LA conversion around 77% is obtained after 72 h. Despite the catalyst shows a progressive activity decay in successive uses because of fouling, removal of the formed organic deposits by calcination in air allows restoring the starting catalytic performance.

Highly Selective Production of Aromatic Hydrocarbons by CO2 Hydrogenation over Fe-Zn Oxide + H-ZSM-5 Composite Catalyst

Abstract Composite catalysts consisting of Fe-Zn oxides and H-ZSM-5 were very effective for the selective production of aromatic hydrocarbons by CO2 hydrogenation. The ratios of aromatic hydrocarbons in all hydrocarbons were beyond 80 C-mol% using these catalysts. When the weight ratio of Fe-Zn oxide and H-ZSM-5 was optimized, the ratio of aromatic hydrocarbons reached more than 90 C-mol%. The Fe-Zn oxides acted as methanol synthesis catalyst in the composite catalyst, forming aromatic hydrocarbons by methanol-to-hydrocarbons reaction over H-ZSM-5. Two aromatic hydrocarbons, p-xylene and 1,2,4-trimethylbenzene, were mainly obtained by the shape selectivity effect of H-ZSM-5. The composite catalyst deactivated by coke formation could be regenerated by air flow treatment.

Self-condensation of levulinic acid into bio-jet fuel precursors over acid zeolites: Elucidating the role of nature, strength and density of acid sites

The production of jet-fuel precursors from the aldol-dimerization of levulinic acid (LA) over acid zeolites is presented. Under solventless conditions, high LA conversion with selectivities to LA dimers > 90% are achieved. Chemisorption and spectroscopic analyses of the materials have revealed a cooperative effect between strong Brønsted (BS) and strong Lewis (LS) acid sites, which favors the selective formation of dimers. BEA structure is the most efficient owing to shape selectivity effect. H-Beta 19, having an optimum Brønsted to Lewis acid sites ratio and the adequate balance of BS/LS acid sites, displayed the best catalytic performance in terms of activity and selectivity to LA dimers. Under optimized reaction conditions H-Beta 19 achieved 79% LA conversion and > 98% selectivity. An analysis on the stability showed good reusability in consecutive reaction cycles. The small loss of activity, ascribed to the formation of organic deposits, can be reverted by calcination in air.

Synthesis of a phosphomolybdic acid/nanocrystalline titanium silicalite-1 catalyst in the presence of hydrogen peroxide for effective adsorption-oxidative desulfurization

The ODS efficiency is in the order thiophene > dibenzothiophene > benzothiophene and may be attributed to the combined effect of HPMo and shape selectivity over Nano-TS-1.

Gas-phase isomerisation of m-xylene on isoreticular zeolites with tuneable porosity

Nanoporous crystalline aluminosilicates, zeolites, are synthesised by solvothermal method producing three-dimensional (3D) crystals. An alternative, more controllable approach of zeolite synthesis - ADOR – can be used for preparation of isoreticular zeolites with tuneable porosity. Zeolites porosity allows their wide use as isomerisation catalysts due to their shape selectivity effects. Selective isomerisation of m-xylene towards p-xylene is an industrially important reaction due to a high demand for the latter as a substrate for terephthalic acid production. In this work, we investigated the influence of pore size (shape selectivity effect) on the isomerization of m-xylene using a system of isoreticular Al-containing zeolites. These materials, prepared by ADOR approach (UTL, IPC-7, IPC-2, IPC-6, and IPC-4) had different layers connectivity, and therefore various channel systems. We tracked the influence of the pore systems of ADOR zeolites (8- up to 14-ring channels) on the catalytic performance in gas-phase m-xylene isomerisation. We investigated the crystallinity and interlayer distances, phase purity, textural properties of prepared materials, their crystals morphology, and aluminium content. M-xylene isomerisation was carried out in a fixed-bed reactor at 350 °C. ADOR catalysts were compared with standard ZSM-5 zeolite. We show the dependence of zeolite porosity on the performance in isomerisation of m- to p-xylene. Smaller pore zeolites: IPC-4 (8- and 10-ring channels) and IPC-6 (8-, 10- and 12-ring channels) exhibited the lowest conversions, while the highest conversion and p-xylene yields were observed for IPC-2 (10- and 12-ring channels). Presence of extra-large, 14-ring channels (IPC-7 and UTL) resulted in the drop of selectivity due to the xylene disproportionation.

Design and application of photocatalysts using porous materials

ABSTRACT Porous materials have attracted considerable attention in materials science, chemistry and multidisciplinary areas of research due to their diverse structures, tailorable functionalities, large surface areas, remarkable optical transparency, confinement and shape selectivity effects. The precise architecture of porous photocatalytic materials has been extensively reported by the hybridization of semiconducting nanoparticles (NPs), light-responsive metal complexes and plasmonic metal NPs with silica-based micro-, meso- and macro-porous nanostructures. This review targets to cover state-of-the-art accomplishments in the surface engineering chemistry of zeolites, mesoporous silica, and metal-organic frameworks (MOFs) as excellent host materials for solar to chemical energy conversion and environmental remediation. The specific advantages and disadvantages of representative porous materials in photocatalysis have also been elaborated and summarized. Finally, key advances and prospects are presented to overcome the current challenges in photocatalysis and to inspire further studies in this rapidly evolving research field.

Linear and nonlinear optical response of g-C3N4-based quantum dots* *Project supported by the National Key R&D Program of China (Grant No. 2017YFA0303600), the National Natural Science Foundation of China (Grant No. 11974253), and Science Speciality Program of Sichuan University (Grant No. 2020SCUNL210).

Graphite carbon nitride (g-C3N4) attracts wide-ranging research interest due to its extraordinary physicochemical properties and promising applications ranging from heterogeneous catalysis to fuel cells. In this work, we design different g-C3N4-based quantum dots (gCNQDs), carry out a systematic study of optical properties, and elucidate the shape selectivity, composite nanostructure, and outfield effect. In particular, composites of gCNQDs and metal nanochains present excellent optical response, making it applicable to bioimaging, nano-plasma devices, and metalloenzyme in infrared light related fields. Besides, QDs which original bridging nitrogen atoms are replaced by amino (–NH2), hydroxyl (–OH), and methyl (–CH3) functional groups respectively, have excellent spectral selectivity in the deep ultraviolet region. More interestingly, in the study of the laser interaction with materials, the gCNQDs exhibit extremely high stability and light corrosion resistance. Phase transition from insulation to metal is observed under the critical condition of about 5 eV intensity or 337 nm wavelength. All provided theoretical support for designs and applications in g-C3N4 quantum devices.

Imidazolium-type ionic liquid-assisted formation of the MFI zeolite loaded with metal nanoparticles for hydrogenation reactions

Zeolites loaded with metal nanoparticles (NPs) stand out for their distinct activity and selectivity as heterogeneous catalysts. However, the poor stability of metal species limits their applications. Herein, we report a novel synthetic strategy for stabilizing metal nanoparticles inside the MFI zeolite. For the first time, alkoxysilane functional imidazolium-type ionic liquids (ImILs) were used as assistant agents to protect metal precursors (Pt and Pd) against precipitation during the hydrothermal synthesis of the MFI zeolite. The positively charged imidazolium groups in ImILs interact with the negatively charged metal precursors (PtCl42−, PdCl42−etc.), while the alkoxysilane groups participate in zeolite crystallization. Scanning transmission electron microscopy images indicate that most of the Pt and Pd nanoparticles (average diameters of approximately 1.0 nm and 1.7 nm, respectively) are confined in channels or intersections of the MFI zeolite. The shape-selectivity effect on nitroarene hydrogenation over Pt@MFI_ImILs confirmed the successful encapsulation of metal NPs into the MFI matrix. The conversion of the small molecule 4-nitrotoluene is >99%, whereas the bulky 1,3-dimethyl-5-nitrobenzene shows 5.8% conversion. Simultaneously, the Pd@MFI_ImILs catalyst affords a higher reaction rate (25 mmol/s/gMe) than the impregnated Pd@MFI_Imp catalyst (9 mmol/s/gMe) in propene hydrogenation, which is ascribed to the uniform distribution of Pd nanoparticles. The ImIL-assisted synthesis strategy can be therefore successfully used to confine small metal nanoparticles in the MFI zeolite while maintaining its high catalytic activity and shape-selectivity.

Shape selectivity effects in the hydroconversion of perhydrophenanthrene over bifunctional catalysts

The zeolite pore structure dictates the formation of isomers, which in turn influences the preferred ring opening products and the distribution of cracking products.

Zirconium–Cerium Oxides Supported on SBA-15 as Catalyst for Shape-Selective Synthesis of Lactic Acid from Glycerol

The conversion of an oleochemical waste i.e. glycerol into lactic acid (LA) using bifunctional catalyst is receiving intensive research attention. The major concern is to achieve a suitable combination of the non-precious active metals with specific roles to achieve a fast and selective reaction. Catalyst with a Ce-to-Zr ratio of 1:2 and supported on SBA-15 at total active metal loadings between 10 and 40 wt% were prepared via a two-sequential-step post-impregnation method. The catalysts were then characterized using nitrogen adsorption–desorption and SEM analyses to elucidate their properties. They were then employed in selective glycerol oxidation reaction carried out at between 240 and 280 °C for up to 3 h to yield LA. The catalytic performance in terms of glycerol conversion and lactic acid yield was successfully correlated with the physicochemical properties of the catalysts. The highest glycerol conversion of 91.2% of glycerol was obtained using 10 wt% of CeZr/SBA-15 at a catalyst loading of 25 wt% with almost 51.4% of corresponding LA yield. It was attributed to the desired shape selectivity effect in the benzylic rearrangement (Cannizzaro reaction) of pyruvaldehyde to suppress by-product formations.

Design, Parameterization, and Implementation of Atomic Force Fields for Adsorption in Nanoporous Materials

AbstractMolecular simulations are an excellent tool to study adsorption and diffusion in nanoporous materials. Examples of nanoporous materials are zeolites, carbon nanotubes, clays, metal‐organic frameworks (MOFs), covalent organic frameworks (COFs) and zeolitic imidazolate frameworks (ZIFs). The molecular confinement these materials offer has been exploited in adsorption and catalysis for almost 50 years. Molecular simulations have provided understanding of the underlying shape selectivity, and adsorption and diffusion effects. Much of the reliability of the modeling predictions depends on the accuracy and transferability of the force field. However, flexibility and the chemical and structural diversity of MOFs add significant challenges for engineering force fields that are able to reproduce experimentally observed structural and dynamic properties. Recent developments in design, parameterization, and implementation of force fields for MOFs and zeolites are reviewed.

Some novel porous materials for selective catalytic oxidations

Abstract Porous materials are outstanding catalysts in various reactions using confinement and shape selectivity effects. Unsurprisingly, extensive research has been conducted on zeolites, pillared clays, metal oxides, metal organic frameworks, and mesoporous materials. In particular, selective oxidation of hydrocarbons stands out an important class of reactions, but the ability to avoid over-oxidation remains a challenge. Thus, the present review highlights the latest findings on octahedral and tetrahedral catalytic materials in oxidation reactions, primarily focusing on types of reactive oxygen species and approaches to their investigation, on the incorporation of different heteroatoms (Ti, Sn) into silica matrices for catalytic purposes and corresponding types of reactions and on surface engineering and design of “single-site catalysts”. In addition, composite catalysts for one-pot selective oxidation reactions are reviewed, providing examples from photochemistry including “single-site photocatalysis”, electrocatalysis, and integration of metal complexes with functional solid materials. Ultimately, the end goal of this review is to discuss current state-of-the-art approaches that are being used in the area of selective catalytic oxidations with some novel porous materials. This includes methods that are being used to study mechanistic details as well as new catalysts and new substrates for such oxidations.

Ruthenium silica nanoreactors with varied metal–wall distance for efficient control of hydrocarbon distribution in Fischer–Tropsch synthesis

We report here a new strategy for the control of hydrocarbon selectivity in Fischer–Tropsch (FT) synthesis using silica yolk–shell ruthenium nanoreactors prepared in water/oil (W/O) microemulsions. The sizes of nanoreactors have been varied by changing the microemulsion composition. Nanoreactors prepared with the smallest internal volume exhibited a restriction of hydrocarbon chain length growth during FT synthesis. An increase in the nanoreactor volume resulted in a gradual shift of the hydrocarbon distribution to longer-chain hydrocarbons till a distribution of hydrocarbons reached that observed over the reference catalysts prepared by deposition of non-encapsulated metal nanoparticles on silica supports. The observed remarkable modifications of hydrocarbon selectivity over Ru nanoreactors have been explained by shape selectivity effects on hydrocarbon growth due to decrease in entropy in comparison with infinite growth and increase in the Gibbs energy for growing hydrocarbon chains located inside the limited volumes of nanoreactors.

Highly selective synthesis of large aromatic molecules with nano-zeolite: beyond the shape selectivity effect

We report a combination of the shape selectivity effect of zeolites and external surface alkylation to produce larger molecules.

Comparison of HZSM-5 Zeolite and SAPO (-18 and -34) Based Catalysts for the Production of Light Olefins from DME

The catalytic transformation of dimethyl ether (DME) for the selective production of light olefins has been studied over HZSM-5 zeolite (with SiO2/Al2O3 = 280) and silicoaluminophosphate (SAPO-18 and -34) based catalysts in order to asses the effect of shape selectivity on the catalytic performance. The catalysts have been synthesized by agglomerating the active materials with bentonite (as a binder) and alpha-alumina (as inert filler). The runs have been carried out at a pressure of 1.5 bar in a fixed bed reactor, under the following reaction conditions: temperature, 350 and 400 °C; space time, 1.6 and 0.064 gcatalyst h molC −1; time on stream, 60 min. The results reveal the importance of the severity of the shape selectivity of the catalysts on the reaction indices (conversion, yield and selectivity of olefins, and catalyst stability). The HZSM-5 zeolite based catalyst has the best kinetic behavior, being stable at 350 °C, with a DME conversion of 4.5 % and selectivity to ethylene, propylene and butenes of 27, 50 and 16 %, respectively.

Nanoreactors: An Efficient Tool To Control the Chain-Length Distribution in Fischer–Tropsch Synthesis

One of the main problems of the Fischer–Tropsch synthesis is the overly broad distribution of the produced hydrocarbons due to the Anderson–Schulz–Flory law. This work is directed toward the solution of this problem by the application of nanoreactors with incorporated metal nanoparticles. Our results show that encapsulation of Co nanoparticles in nanosized porous silica spheres results in higher activity per catalyst weight and stability with a shift of the chain length distribution of hydrocarbons to lower values in comparison with Fischer–Tropsch synthesis over impregnated catalysts. These effects are due to the presence of well-dispersed isolated and stable Co nanoparticles inside of nanoreactors and shape selectivity effects which restrict the chain growth by the walls of nanoreactors. The proposed new strategy can be further extended to synthesis of olefins and alcohols with desired chain-length distribution from syngas by encapsulation of other metals (Fe, Cu, Rh, etc.).

Synergetic effect of H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst to enhance the selective hydrogenation of p-xylene

A H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst that combined para-directed isomerization and shape-selective hydrogenation was synthesized by coating MFI membranes onto the surface of Pt/Al2O3 pellet. The binary p/m xylene mixture was performed as the feed to evaluate the enhanced selective hydrogenation property of this bi-layered core–shell catalyst. In the hydrogenation of xylene isomers, the hydrogenation efficiency and selectivity of p-xylene over the H-ZSM-5/Silicalite-1@Pt/Al2O3 core–shell catalyst exceeded those over the monolayer H-ZSM-5@Pt/Al2O3 and Silicalite-1@Pt/Al2O3 core–shell catalysts, while maintaining the tiny formation of the unexpected hydrogenation products of m- or o-xylene. The effect of reaction temperature was also investigated on the H-ZSM-5/Silicalite-1@Pt/Al2O3 catalyst. The p-xylene hydrogenation conversion and 1,4-dimethylcyclohexane selectivity of as high as 90.8% and 97.2%, respectively, were achieved at 300°C. The isomerization on the H-ZSM-5 shell and the selective hydrogenation on the Silicalite-1@Pt/Al2O3 cooperated concertedly and promoted mutually. This bi-layered core–shell catalyst, with the synergetic effect of equilibrium shift and shape-selectivity, might have great potential to develop into a novel approach for xylenes separation.

Shape selectivity and acidity effects in glycerol acetylation with acetic anhydride: Selective synthesis of triacetin over Y-zeolite and sulfonated mesoporous carbons

100% triacetin selectivity was achieved in glycerol acetylation with acetic anhydride in 20–50min in the presence of catalytic amounts of mesoporous sulfonated carbons and zeolite H-Y. Experimental results obtained over a range of solid acids with varying structure and acidic properties indicate that selectivity to triacetin was influenced mainly by the pore structure and catalyst surface acid site density. The shape selectivity effect over microporous zeolites was verified from the molecular dimensions of glycerol, monoacetin, diacetin and triacetin obtained from DFT calculations; on the contrary in mesoporous catalyst surface acid site density was responsible for determining triacetin selectivity.

Effect of the porous structure of the support on hydrocarbon distribution in the Fischer–Tropsch reaction

Emissions standards are increasingly stringent due mainly to its impact on the environment. Although the diesel engine is an attractive solution for carbon dioxide reduction, a challenge remains to simultaneously control nitrogen oxides and matter particulate emissions to accepted levels. On engine tests, it has been observed that Fischer–Tropsch diesel significantly reduces CO, HC, PAHs and particulate emissions compared to oil derived diesel. However, selectivity control in Fischer Tropsch Synthesis is still a key challenge due the Anderson-Schultz-Flory polymerization mechanism followed by hydrocarbon distribution. In this work we are presenting the first steps towards a new strategy that can tune, in one step, the selectivity to desired products by taking advantage of the shape selectivity properties of SBA-15 mesoporous silica used as support.Co-SBA-15 (30%wt) catalysts with different pore size were prepared by excess solution impregnation. Our results show that pore diameter not only affects the size and reducibility of Co particles but it also significantly influence the liquid products distribution, with the high molecular weight hydrocarbon fraction increasing on large pores, attributed to the existence of a shape selectivity effect induced by the textural properties of the catalyst namely its pore size and pore volume.