Isomerization Selectivity Research Articles

Stimuli‐Mediated Structural Interchange Between Pd6 and Pd12 Architectures: Selective Recognition of E‐Stilbene by the Pd6 Architecture and its Photoprotection

AbstractThe dynamic behaviour of metal‐ligand bonding cultivates stimuli‐mediated structural transformations in self‐assembled molecular architectures. The propensity of synthetically designed self‐assembled systems to interchange between higher‐order architectures is increased multi‐fold when the building blocks have higher conformational degrees of freedom. Herein, we report a new ligand, (2,7‐bis(di(pyridin‐4‐yl)amino)‐9H‐fluoren‐9‐one) (L), which, upon self‐assembly with a cis‐[(ethylene‐1,2‐diamine)Pd(NO3)2] acceptor (M), resulted in the formation of a M6L3 trifacial barrel (C1) in water. Interestingly, during crystallization, a rare M12L6 triangular orthobicupola architecture (C2) was generated along with C1. C2 could also be generated in solution via the application of several stimuli. C1 in aqueous media could stabilize one trans‐stilbene (tS) or cis‐stilbene (cS) molecule in its cavity, with a selectivity for the former from their mixture. Moreover, C1 acted as an effective host to prevent the otherwise facile photoisomerization of tS to cS inside its hydrophobic cavity under UV irradiation. Conversely, the visible‐light‐induced reverse isomerization of encapsulated cS to encapsulated tS could be achieved readily due to the better stabilization of tS within the cavity of C1 and its transparency to visible light. A multi‐functional system was therefore designed, which at the same time is stimuli‐responsive, shows isomer selectivity, and photo‐protects trans‐stilbene.

Diastereomeric pure pyrazolyl-indolyl dihydrofurans: Unveiling isomeric selectivity in antibacterial action via in vitro and in silico insights

Developing pure diastereoisomeric molecular hybrids for the selective inhibition of bacterial growth opened new avenues for combating the ever-increasing microbial resistance. Considering this, a series of diastereoisomeric pure pyrazolyl-dihydrofurans (7a-7y) were synthesized and characterized using NMR, LCMS, and X-ray crystallography. DFT based method was used to explore the configurational stability of cis over trans isomeric form. Considering 7a and 8a as representative isomeric forms with same structural framework, the difference in their bio-efficacy against bacterial and fungal strains was assessed using serial dilution method. The relatively high inhibition of bacterial growth by the cis isomeric form (7a) (MIC = 1.562 µg/mL), amoxicillin (MIC = 3.125 µg/mL) inspired us to broaden the substrate scope for synthesizing a series of pure diastereoisomeric cis forms as selective anti-bacterial agents. However, both the isomers displayed antifungal activity less than the standard drug (Fluconazole) employed in the study. All the reactions proceeded smoothly and yielded a diverse array of dihydrofuran derivatives. The developed synthetics were found to be non-cytotoxic against mouse fibroblast cells and didn’t affect the seed germination of Brassica nigra seeds when treated at a concentration of 1 mg/mL. The experimentally determined in vitro results were further validated using in silico molecular docking and dynamics studies.

Two different rims functionalized pillar[5]arene stationary phase for capillary gas chromatography

Herein, we synthesized and used a novel pillar[5]arene with two functionalizable rims (AP5A-C10-OAc) as stationary phase in gas chromatography (GC). The AP5A-C10-OAc column (0.25 mm, i.d.) showed a high column efficiency of 4107 plates/m and medium polarity. The AP5A-C10-OAc stationary phase possesses unique amphiphilic structure, including a π-electron rich cavity and two functionalized arms composed of five nonpolar alkyl chains and five polar ester chains, and showed good separation performance for diverse types of analytes and isomers (apolar/polar, aliphatic/aromatic). Particularly, the AP5A-C10-OAc phase showed high selectivity for the challenging isomers of chloronitrobenzenes, bromonitrobenzenes, benzenediols, and iodoanilines with distinct advantages if compared with the commercial polysiloxane column.

Stimuli-Mediated Structural Interchange Between Pd6 and Pd12 Architectures: Selective Recognition of E-Stilbene by the Pd6 Architecture and its Photoprotection.

The dynamic behaviour of metal-ligand bonding cultivates stimuli-mediated structural transformations in self-assembled molecular architectures. The propensity of synthetically designed self-assembled systems to interchange between higher-order architectures is increased multi-fold when the building blocks have higher conformational degrees of freedom. Herein, we report a new ligand, (2,7-bis(di(pyridin-4-yl)amino)-9H-fluoren-9-one) (L), which, upon self-assembly with a cis-[(ethylene-1,2-diamine)Pd(NO3)2] acceptor (M), resulted in the formation of a M6L3 trifacial barrel (C1) in water. Interestingly, during crystallization, a rare M12L6 triangular orthobicupola architecture (C2) was generated along with C1. C2 could also be generated in solution via the application of several stimuli. C1 in aqueous media could stabilize one trans-stilbene (tS) or cis-stilbene (cS) molecule in its cavity, with a selectivity for the former from their mixture. Moreover, C1 acted as an effective host to prevent the otherwise facile photoisomerization of tS to cS inside its hydrophobic cavity under UV irradiation. Conversely, the visible-light-induced reverse isomerization of encapsulated cS to encapsulated tS could be achieved readily due to the better stabilization of tS within the cavity of C1 and its transparency to visible light. A multi-functional system was therefore designed, which at the same time is stimuli-responsive, shows isomer selectivity, and photo-protects trans-stilbene.

Preparation of two amphiphilic dendritic small molecule gelators based on poly (aryl ether) modified silica-based chromatographic stationary phases and molecular shape recognition for shape-restricted isomers

Geometric isomers tend to have similar polarities and differ only in molecular shape. Vigorously developing new stationary phases to meet the requirements for the separation of isomers that have similar physicochemical properties is still an urgent topic in separation science. Poly (arylene ether)-based dendrimers are known for their multifunctional branched peripheral structures and high self-assembly properties. In this paper, two amphiphilic dendritic organic small molecule gelling agents based on poly (aryl ether), PAE-ANT and PAE-PA, were prepared and conjugated to the silica surface. SiO2@PAE-ANT and SiO2@PAE-PA were used as HPLC stationary phases for the separation of non-polar shape-restricted isomers. Both stationary phases have very high molecular shape selectivity for isomers such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), tocopherols and carotenoids. Separation of cis-trans geometric isomers such as diethylstilbestrol and polar compounds such as monosubstituted benzenes and anilines can also be achieved. These two columns offer more flexible selectivity and higher separation performance than commercial C18 and phenyl columns. There is a difference in molecular shape selectivity between the two stationary phases for the same analyte test probes. SiO2@PAE-ANT showed slightly better linear selectivity for non-polar shape-restricted isomers compared to SiO2@PAE-PA with Janus-type PAE-PA bonding phase. This separation behavior may be attributed to the ordered spatial structure formed by the gel factor on the surface of the stationary phase and the combined effect of multiple weak interaction centers (hydrophobic, hydrophilic, hydrogen bonding and π-π interactions). It was also possible to separate nucleoside and nucleobase strongly polar compounds well in the HILIC mode, suggesting that hydrophilic groups in PAE-ANT and PAE-PA are involved in the interactions, reflecting their amphiphilic nature.The results show that the ordered gelation of dendritic organic small molecule gelators on the SiO2 surface, along with multiple carbonyl-π, π-π and other interactions, play a crucial role in the separating shape-restricted isomers. The integrated and ordered functional groups serve as the primary driving force behind the exceptionally high molecular shape selectivity of SiO2@PAE-ANT and SiO2@PAE-PA phases. Alterations in the structure of dendritic organic small molecule gelators can impact both molecular orientation and recognition ability, while changes in the type of functional groups influences the separation mechanism of shape-restricted isomers.

Impact of the material hydroxylation on the possibility of pentane isomers separation by UiO-66 (Zr) MOF: A combined 2H NMR and MD study

The mobility of the pentane isomers in the hydroxylated UiO-66 metal-organic framework has been characterized. 2H NMR spectroscopy was applied to measure the jump rate of alkanes guest molecules between adjacent cages and estimate the diffusivity. It is inferred that the difference in diffusion coefficients defines the kinetic separation selectivity, which is higher for the linear alkanes. The adsorption of pentane isomers in UiO-66 has been modeled with molecular dynamics (MD) simulation. The adsorbed quantity of isopentane is higher than that for n-pentane, providing the possibility of separation with selectivity α ≈ 8 in stationary conditions. The impact of the UiO-66 MOF hydroxylation state on the mobility of pentane isomers has been characterized by comparison with results obtained for the dehydroxylated UiO-66 material. The hydroxylated state of UiO-66 MOF has 6-time higher separation selectivity for pentane isomers compared to its dehydroxylated state (αhyd ≈ 76). MD calculations show that hydroxylated UiO-66 MOF is more efficient, as the separation selectivity is 4 times lower in the dehydroxylated material. The UiO-66 hydroxylation effect is compared for the mobility of C4 and C5 alkanes. The optimal conditions for C4/C5 alkanes kinetic separation by UiO-66 MOF are established.

External surface Al-rich ZSM-48 zeolite with high efficient n-dodecane hydroisomerization activity

ZSM-48 zeolites with exterior Al-rich (Z-48-Sx) are synthesized by the recrystallization method. The crystal structure of Z-48-Sx with the Al-rich outer layer is verified by analyzing the crystallization process of Z-48-Sx and the EDS of the ultrathin section. The diffusion properties of Z-48-S3 and Z-48-C are investigated by the zero-length column (ZLC) method, and the synthesized ZSM-48 zeolite with an Al-rich external layer effectively improves the diffusion ability. The effective diffusion constants of Z-48-S3 are 53 %–102 % higher than those of the conventional ZSM-48 (Z-48-C). Meanwhile, the values of turnover frequency (TOF) of Z-48-S3 are increased by 14 %–63 % at similar n-dodecane conversions compared with Z-48-C, which is closely related to the diffusion characteristics. In addition, the depth of hydroisomerization (Iiso) of Z-48-S3 is decreased by 65.3 %, and the isomerization selectivity is increased by 52.5 % at 340 °C compared with that of Z-48-C. Hence, the ZSM-48 molecular sieves synthesized by the recrystallization hydrothermal method are beneficial for controlling the depth of hydroisomerization of the catalyst and reducing the occurrence of the cracking reaction. Meanwhile, the isomer distribution does not change significantly compared with that of the conventional ZSM-48, and the 5-methylundecane is still the main isomerization product.

Improved hydrophobic subtraction model of reversed-phase liquid chromatography selectivity based on a large dataset with a focus on isomer selectivity

Reversed-phase (RP) liquid chromatography is an important tool for the characterization of materials and products in the pharmaceutical industry. Method development is still challenging in this application space, particularly when dealing with closely-related compounds. Models of chromatographic selectivity are useful for predicting which columns out of the hundreds that are available are likely to have very similar, or different, selectivity for the application at hand. The hydrophobic subtraction model (HSM1) has been widely employed for this purpose; the column database for this model currently stands at 750 columns. In previous work we explored a refinement of the original HSM1 (HSM2) and found that increasing the size of the dataset used to train the model dramatically reduced the number of gross errors in predictions of selectivity made using the model. In this paper we describe further work in this direction (HSM3), this time based on a much larger solute set (1014 solute/stationary phase combinations) containing selectivities for compounds covering a broader range of physicochemical properties compared to HSM1. The molecular weight range was doubled, and the range of the logarithm of the octanol/water partition coefficients was increased slightly. The number of active pharmaceutical ingredients and related synthetic intermediates and impurities was increased from four to 28, and ten pairs of closely related structures (e.g., geometric and cis-/trans- isomers) were included. The HSM3 model is based on retention measurements for 75 compounds using 13 RP stationary phases and a mobile phase of 40/60 acetonitrile/25 mM ammonium formate buffer at pH 3.2. This data-driven model produced predictions of ln α (chromatographic selectivity using ethylbenzene as the reference compound) with average absolute errors of approximately 0.033, which corresponds to errors in α of about 3 %. In some cases, the prediction of the trans-/cis- selectivities for positional and geometric isomers was relatively accurate, and the driving forces for the observed selectivity could be inferred by examination of the relative magnitudes of the terms in the HSM3 model. For some geometric isomer pairs the interactions mainly responsible for the observed selectivities could not be rationalized due to large uncertainties for particular terms in the model. This suggests that more work is needed in the future to explore other HSM-type models and continue expanding the training dataset in order to continue improving the predictive accuracy of these models. Additionally, we release with this paper a much larger data set (43,329 total retention measurements) at multiple mobile phase compositions, to enable other researchers to pursue their own lines of inquiry related to RP selectivity.

Isomerization of maltose to maltulose under microwave heating using uncalcined scallop shell powder

Uncalcined scallop shell powder, consisting mainly of calcium carbonate, and an aqueous maltose solution were placed in a pressure-resistant vessel and heated in a domestic microwave oven to isomerize maltose to maltulose. When a maltose solution (50 mL) was heated at a power of 700 W, a high maltulose yield of approximately 30 % was achieved in as short a time as 105 s. As the reaction progressed, the shell powder dissolved and neutralized acidic by-products to suppress the decrease in pH, keeping the pH above 7, where the isomerization by Lobry de Bruyn-Alberda van Ekenstein transformation proceeded. The average selectivity for isomerization of maltose to maltulose under various reaction conditions was 0.822. This would be because the shell powder kept the pH of the reaction solution in the range of 7–9 and suppressed the progression of side reactions. The coloration of the reaction solution increased rapidly when the yield of maltulose approached its maximum value. This indicated that if the reaction were to be stopped when the yield of maltulose was close to its maximum value, a high yield of maltulose solution with low coloration could be obtained.

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.

Synergistic Catalytic Performance of Pt-Au Bimetallic Catalysts on High-Crystallinity ZSM-23 Zeolite for Hexadecane Hydroisomerization: Metal-Acid Balance and Enhanced Isomerization Selectivity.

Highly crystalline ZSM-23 zeolite, exhibiting a distinctive dumbbell morphology, was synthesized via a hydrothermal method. Bifunctional catalysts, comprising single metals (Pt or Au) and bimetals (Pt-Au), were successfully prepared by using a positional precipitation method. The hydroisomerization of hexadecane served as a model reaction to assess the catalytic performance arising from the synergistic effects of bimetallic active sites. In comparison to single-metal catalysts, 0.3Au0.7Pt/ZSM-23 demonstrated increased n-C16 conversion, while 0.5Au0.5Pt/ZSM-23 exhibited enhanced i-C16 selectivity, achieving the highest i-C16 yield. The bimetallic catalyst not only finely tuned the metal site activity through bimetallic synergy but also achieved a superior balance between metal and acid catalysis, resulting in improved catalytic performance in the n-C16 hydroisomerization. The Pt-Au bimetallic catalyst approached the ideal requirements for a hydroisomerization catalyst, achieving a harmonious balance of metal and acid catalysis.

Insights into the selectivity of polar stationary phases based on quantitative retention mechanism assessment in hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) offers different selectivity than reversed-phase liquid chromatography (RPLC). However, our knowledge of the driving force for selectivity is limited and there is a need for a better understanding of the selectivity in HILIC. Quantitative assessment of retention mechanisms makes it possible to investigate selectivity based on understanding the underlying retention mechanisms. In this study, selected model compounds from the Ikegami selectivity tests were evaluated on different polar stationary phases. The study results revealed significant insights into the selectivity in HILIC. First, hydroxy and methylene selectivity is driven by hydrophilic partitioning; but surface adsorption for 2-deoxyuridine or 5-methyluridine reduces the selectivity factor. Furthermore, the retention of 2-deoxyuridine or 5-methyluridine by surface adsorption in combination with the phase ratio explain the difference in hydroxy or methylene selectivity observed among different stationary phases. Investigations on xanthine positional isomers (1-methylxanthine/3-methylxanthine, theophylline/theobromine) indicate that isomeric selectivity is controlled by surface adsorption; however, hydrophilic partitioning may contribute to resolution by enhancing overall retention. In addition, two pairs of nucleoside isomers (adenosine/vidarabine, 2′-deoxy and 3′-deoxyguanosine) provide an example that isomeric selectivity can also be controlled by hydrophilic partitioning if their partitioning coefficients are significantly different in HILIC. Although more data is needed, the current study provides a mechanistic based understanding of the selectivity in HILIC and potentially a new way to design selectivity tests.

Synergistic expression of ZIF-67 and amphiphilic polymer hydrogel in multi-mode liquid chromatographic separation

In this work, zeolitic imidazolate framework-67 (ZIF-67) with excellent stability synthesized at room temperature was used together with amphiphilic polymer hydrogel to synthesize a captivating mixed-mode silica stationary phase for the first time. The amphiphilic polymer hydrogel prepared by weakly basic cationic and acidic anionic monomers that was modified on the surface of ZIF-67@SiO2 by a simple physical coating method not only improved the separation performance, but also enhanced the applicability of ZIF-67@SiO2 in chromatographic analysis. Meanwhile, the porous ZIF-67 effectively alleviated the problem that polymer hydrogel was easy to swell. To sum up, the amphiphilic polymer hydrogel and ZIF-67 showed excellent synergistic effect in improving the separation ability and the applicability of stationary phase. The experimental results demonstrated that the ZIF-67/hydrogel@SiO2 stationary phase could be applied in various chromatographic separation modes, demonstrating superior resolution and separation selectivity for polycyclic aromatic hydrocarbons, steroid hormones, nucleosides, B-vitamins, sulfonamides, benzoic acids and dinitrobenzene isomers. In this study, the synergistic expression of ZIF-67 and polymer hydrogel provided a new idea for the development of silica stationary phase co-modified by metal-organic framework and hydrogel material. Concurrently, the successful preparation of ZIF-67/hydrogel@SiO2 composite revealed a new possibility for utilizing ZIF-67 as the functional material of stationary phase, expanding the application potential of ZIF-67 in the field of chromatography.

Isomerization performance of n-hexane in hydrogen atmosphere over the multistage porous composite NixPy-MCM-41/MOR catalyst

With the increasingly strict environmental protection requirements and the continuous improvement for the performance of clean fuel, it is imperative to develop technology for producing high octane gasoline blending components. Alkane isomerization was one of the important ways due to it contain little olefin and aromatic components which are harmful to the environment. In this study, the multistage porous composite NixPy-MCM-41/MOR catalysts were synthesized in the hydrogen atmosphere, the conversion rate and selectivity of n-hexane hydrogen isomerization with different conditions was tested in a fixed bed reactor. The results shown that the loading volume of active components is 4.0 wt%, nickel-phosphorus ratio of 0.5, reaction temperature of 250 °C, reaction pressure 3.0 MPa and volume space velocity is 2.0 h−1 with the hydrogen condition showed good catalytic activity for the n-hexane isomerization. The conversion rate of n-hexane is 73.0 % and the selectivity of 84.0 %, respectively. The isomerization stability of the Ni2P-MCM-41/MOR catalyst was investigated. After the long period test, the catalytic activity of the catalyst could keep good. Based on the study of reaction effect with different test conditions in hydrogen atmosphere, the mechanism of alkane isomerization for Ni2P-MCM-41/MOR catalyst was proposed.

Confinement Effects in Catalysis: Steering the Product Selectivity in Cannabidiol Isomerization by the Resorcinarene Supramolecular Capsule

AbstractThe supramolecular hydrogen‐bonded self‐assembled resorcinarene hexameric capsule promotes the ring closing isomerization of cannabidiol through encapsulation of the intermediate carbocation species formed by protonation with HCl. The confinement effect imparted by the capsule steers product selectivity towards the uncommon products Δ8‐iso‐THC, Δ4(8)‐iso‐THC, together with the hydrated 8‐Hydroxy‐iso‐THC.

Study on characteristics of toluene/chlorobenzene nitrification in different microreactors

Aromatic nitrification with mixed acid is an important industrial reaction with fast, strong-exothermic, and high-risk reaction characteristics. To investigate the promotion for the preparation of nitrotoluene (MNT) and nitrochlorobenzene (NCB), three microreactors with different structural features, namely, heart-shaped, diamond-shaped, and zigzag-shaped microreactors, are designed and fabricated. Firstly, the effects of temperature, residence time, and molar ratio on toluene (TOL) and chlorobenzene (CB) nitrification are investigated, and orthogonal experiments are further conducted to determine the optimum reaction conditions. Results showed that channel structures and the residence time τ have negligible effects on the isomers selectivity of the three microreactors, and higher reaction temperatures and mole ratios would lead to the substantial occurrence of side reactions and byproducts in the zigzag microreactor. The heart-shaped microreactor has the highest value of 91.49 % for the MNT's optimal yield, followed by the diamond-shaped and zigzag-shaped microreactors (89.91 % and 89.81 %, respectively). The heart-shaped microreactor provides the maximum amount of NCB preparation (93.95 %), while the yields of the diamond-shaped and zigzag-shaped microreactors are 91.51 % and 88.25 %, respectively. This work provides the experimental basis for the efficient preparation and industrial scale-up of aromatic nitrification in microreactors.

Selective sorting of hexane isomers by anion-functionalized metal-organic frameworks with optimal energy regulation

Extensive efforts have been made to improve the separation selectivity of hydrocarbon isomers with nearly distinguishable boiling points; however, how to balance the high regeneration energy consumption remains a daunting challenge. Here we describe the efficient separation of hexane isomers by constructing and exploiting the rotational freedom of organic linkers and inorganic SnF62− anions within adaptive frameworks, and reveal the nature of flexible host-guest interactions to maximize the gas-framework interactions while achieving potential energy storage. This approach enables the discrimination of hexane isomers according to the degree of branching along with high capacity and record mono-/di-branched selectivity (6.97), di-branched isomers selectivity (22.16), and upgrades the gasoline to a maximum RON (Research Octane Number) of 105. Benefitting from the energy regulation of the flexible pore space, the material can be easily regenerated only through a simple vacuum treatment for 15 minutes at 25 °C with no temperature fluctuation, saving almost 45% energy compared to the commercialized zeolite 5 A. This approach could potentially revolutionize the whole scenario of alkane isomer separation processes.

Synthesis of bimetallic Ni-Pt/SAPO-11 composite and the catalytic application in n-heptane hydroisomerization

The monometallic Pt/SAPO-11 and bimetallic Ni-Pt/SAPO-11 catalysts were synthesized with Instant Exactness Synthesis (IES) method and characterized by XRD, nitrogen adsorption–desorption, SEM, TEM, FT-IR, Py-IR, NH3-TPD and XPS, and their properties were evaluated for the n-heptane hydroisomerization. The catalysts had spherical structure and moderate acid strength, and exhibited good n-heptane conversion and catalytic selectivity toward n-heptane isomers. Typically, Ni-Pt/SAPO-11 exhibited an isomer selectivity of 80.9% with a conversion of 69.9% at 310 °C, which was obviously better than Pt/SAPO-11 (isomer selectivity of 75.1% with a conversion of 60.6% at 310 °C). Moreover, the Ni-Pt/SAPO-11 catalyst displayed higher percentage of i-heptane (79%) and lower percentage of n-heptane (6%) than Pt/SAPO-11. The continuous test of 360 min indicated Ni-Pt/SAPO-11 catalyst had good catalytic stability. Bimetallic Ni-Pt alloy loaded on SAPO-11 had significantly effect on catalytic properties of n-heptane hydroisomerization.

Comparative study of Pt/zeolites for n-hexadecane hydroisomerization: EU-1, ZSM-48, ZSM-23, ZSM-22, and ZSM-12

The hydroisomerization of n-hexadecane was investigated using bifunctional platinum catalysts supported on EU-1, ZSM-48, ZSM-23, ZSM-22, and ZSM-12 zeolites. These one-dimensional microporous zeolites were synthesized at 160 ℃ with an Si/Al molar ratio of 60, and 0.5 wt% Pt was loaded onto them using the impregnation method. The findings indicate that Brønsted acidity predominantly influences the conversion to n-hexadecane over 10 membered ring zeolites. Additionally, ZSM-12, characterized by a 12-ring channel, exhibits preferential activity in this regard. The apparent activation energy for the reactions increased from 81 kJ/mol (Pt/ZSM-23) to 134 kJ/mol (Pt/ZSM-48) across all tested catalysts. Pt/ZSM-22 demonstrated high selectivity towards mono-branched isomers, whereas Pt/ZSM-12 favored the formation of multi-branched isomers. The isomer selectivity trend was Pt/ZSM-22 > Pt/ZSM-12 > Pt/EU-1 > Pt/ZSM-48 > Pt/ZSM-23. Moreover, TG analysis of spent catalysts unveiled that coke formation on these zeolites was chiefly influenced by zeolite channel structure and Brønsted acid content. The average coke selectivity followed this order: Pt/EU-1 > Pt/ZSM-12 > Pt/ZSM-48 > Pt/ZSM-22 > Pt/ZSM-23.

Silica as support and binder in bifunctional catalysts with ultralow Pt loadings for the hydroconversion of n-alkanes

Hydroconversion is a key step in the production of ultraclean fuels from renewable sources. This reaction is carried out using a bifunctional catalyst consisting of a base metal sulfide or a noble metal and a solid acid. Recently, we have shown that for Pt/Al2O3/ZSM-22 catalysts with low Pt loadings (≤0.01 wt%) it is advantageous – to both the activity as well as the isomer selectivity - to emplace the Pt on the zeolite crystallites instead of on the Al2O3 binder. When these low loadings of Pt were on the alumina binder, small clusters or even single atoms were present which were hard to reduce leading to inactivity of the catalysts. Herein, we explore the replacement of alumina by silica, and the performance of catalysts with ultralow Pt loadings on the conversion of longer-chain hydrocarbons. A series of Pt/SiO2/ZSM-22 catalysts with varying Pt weight loadings (0.001, 0.005, 0.01, 0.05, 0.1 and 0.5 wt%) and location (on silica or on ZSM-22) was prepared and characterized using ICP, NH3-TPD, HAADF-STEM and XAS. Their hydroconversion performance was evaluated using n-heptane and n-hexadecane as model feedstocks. As for the Pt/Al2O3/ZSM-22 catalysts systems, for Pt/SiO2/ZSM-22 catalysts with low Pt loadings (≤0.01 wt% for n-heptane conversion) it was beneficial to have the Pt nanoparticles on the ZSM-22 crystals. Hydroconversion of n-hexadecane over Pt/SiO2/ZSM-22 and Pt/Al2O3/ZSM-22 catalysts showed that for feedstocks with a higher molecular weight, higher Pt loadings (≥0.05 wt%) are required for sufficient catalytic performance. For the conversion of n-hexadecane it was beneficial to locate these higher amounts of Pt on the binder.