Intercrystalline Mesoporosity Research Articles

Metal-Organic Frameworks-Based Immobilized Enzyme Microreactors Integrated with Capillary Electrochromatography for High-Efficiency Enzyme Assay.

Enzyme assays are important for studying enzyme-mediated biochemical reactions and for clinical diagnosis and drug development. The technique of an immobilized enzyme microreactor (IMER) integrated with capillary electrophoresis (CE) has been frequently utilized in online enzyme assays. However, the traditional approaches for IMER-CE enzyme analysis have some defects such as low loading capacity and poor stability. Herein, metal-organic frameworks (MOFs), which have enormous potential in the fields of enzyme immobilization and capillary electrochromatographic (CEC) separation, were first explored as novel support materials with good enzyme immobilization performance and stationary phases with excellent separation abilities to construct an integrated MOFs-IMER-CEC microanalysis system for a high-efficiency online enzyme assay. As a proof-of-concept demonstration, acetylcholinesterase (AChE) was immobilized on a densely packed UiO-66-NH2 nanocrystal coating on a capillary inner surface with abundant intercrystalline mesoporosity and was employed as a highly effective and robust IMER for CEC-integrated online enzyme analysis. The excellent separation performance of the UiO-66-NH2-modified capillary was verified by high-efficiency separation of three types of neutral, acidic, and basic compounds. The Michaelis-Menten constant and enzyme inhibition kinetics of UiO-66-NH2-IMER were systematically assessed, exhibiting distinct advantages such as remarkably increased enzyme loadability, superior affinity for substrates, and greatly improved stability and repeatability compared to CE-integrated IMERs prepared by the traditional covalent bonding method. Furthermore, the developed method was successfully utilized for detecting organophosphorus pesticides in leguminous vegetable samples, demonstrating its strong practicality. The study not only proposed a novel support material and construction strategy for a high-performance microchannel-based IMER but also can be widely used in bioanalysis and biosensing research.

Surfactant-induced hierarchically porous MOF-based catalysts prepared under sustainable conditions and their ability to remove bisphenol A from aqueous solutions

Metal-Organic Frameworks (MOFs) are extensively used in different applications, including heterogeneous catalysis given the fact that practically any center can be designed on demand and is accessible for reactants. Just like any other microporous catalysts, the limited diffusion of reactants and products is an endemic drawback also in MOFs. To face it, the use of nanocrystalline MOFs is a proven alternative, which in addition is often accompanied by improvements in sustainability of synthesis procedure (room temperature, water as solvent, etc.). This work goes further adding surfactant-induced mesoporosity to the nanocrystallinity of these materials. As a proof of concept, the nanocrystalline NH2-MIL-53(Al) and CTA+ (cetyltrimethylammonium) were selected as the sustainably-prepared nanocrystalline MOF and a cationic surfactant, respectively. The MOF system is formed by massive and practically instantaneous precipitation entrapping the surfactant, which generates mesopores in the aggregates. The simple washing with ethanol at room temperature leads to a nanocrystalline NH2-MIL-53(Al) material containing outstanding intercrystalline mesoporosity. The so-obtaining hierarchically-porous NH2-MIL-53(Al) is able to effectively remove bisphenol A (BPA) from aqueous solution (more than 90% of a 25 ppm BPA aqueous solution in just 10 min at room temperature), duplicating the performance given by its counterpart nanocrystalline NH2-MIL-53(Al) prepared in the absence of surfactant.

Synthesis and formation mechanism analysis of meso‐microporous ZSM‐5 with controllable mesoporous volume

AbstractBACKGROUNDZSM‐5 molecular sieve has an important application value in industrial adsorption. Due to the existence of micropores, it is hard for larger molecules to enter the ZSM‐5 molecular sieve. Meso‐microporous MFI zeolite was prepared by hydrothermal crystallization regarding the organosilane as the structure‐directing agent. Then the samples were characterized by X‐ray diffraction, infrared spectroscopy and so on. The effect of organosilane on mesoporous volume and the mechanism of mesoporous formation were further investigated. Finally the performance of adsorption for volatile organic compounds (VOCs) on meso‐microporous ZSM‐5 were analyzed by static adsorption and dynamic adsorption experiments.RESULTSThe meso‐microporous ZSM‐5, which were prepared with the organosilanes, contained not only large and uniform mesopores but also certain amounts of micropores. The results of N2 physisorption showed that the meso‐microporous ZSM‐5 obtained by propyl trimethoxy silane had the highest specific surface area (438.19 m2 g−1) and mesoporous volume (0.15 cm3 g−1). When the molar ratio of SiO2: octyl trimethoxy silane was 1: 0.1, the specific surface area (508.03 m2 g−1) and mesoporous volume (0.23 cm3 g−1) of the O‐ZSM‐5 were maximum. Compared with traditional ZSM‐5, the toluene adsorption performance of meso‐microporous ZSM‐5 had been effectively improved.CONCLUSIONThis research focused on offering the simple and high efficiency meso‐ microporous ZSM‐5 synthetic routes. The newly intercrystalline mesoporosity in ZSM‐5 zeolite showed a controllable mesoporous channel, which was better than traditional ZSM‐5. The prepared molecular sieve was a promising adsorption material for the volatile organic compounds. © 2021 Society of Chemical Industry (SCI).

Sustainable M-MOF-74 (M = Cu, Co, Zn) prepared in methanol as heterogeneous catalysts in the synthesis of benzaldehyde from styrene oxidation

Metal-Organic Framework (MOF) materials are promising heterogeneous catalysts in different areas including Fine Chemistry, mainly if they possess open metal sites and if they are nanocrystalline. In this work, a new sustainable methodology to obtain nanocrystalline M-MOF-74 materials at room temperature, without any energy input and in methanol as the unique solvent, is described. Amongst the seven divalent metal tested, pure MOF-74 phase was achieved in the case of Co, Cu and Zn (and maybe Ni), but not in the case of Mg, Mn and Cd. The formation of these MOFs is taken place as soon as the metal source (metal acetate) and the organic linker 2,5-dihydroxyterehpthalic acid are put together. The so-prepared Co- and Cu-MOF-74 (but not Zn-MOF-74) resulted to be nanocrystalline and having high external surface area and intercrystalline mesoporosity. They were tested as heterogeneous catalysts in the synthesis of benzaldehyde from styrene using tert-butylhydroperoxide as an oxidant. Different experimental parameters like reactants ratio, amount of catalysts or reaction temperature were optimized. Cu-MOF-74 gave the highest benzaldehyde yield and interestingly it maintained intact its structure after reaction, unlike the same catalyst used in some other catalytic processes under similar relatively mild conditions.

Versatile post-synthetic Sn incorporation into hierarchically porous BEA zeolite for glucose isomerization in 1-butanol

Abstract Glucose isomerization in water has been widely investigated using Sn-BEA zeolites as Lewis acid catalysts. Recently, it was reported that the use of 1-butanol as a solvent can be advantageous in terms of enhanced fructose yields and easy product separation. In this study, we investigated glucose isomerization both in water and 1-butanol over a series of Sn-BEA zeolites including one directly crystallized in a F− medium and the others prepared by post-synthetic Sn incorporation. The directly crystallized Sn-BEA showed high fructose yield (34%) in water because of its defect-free hydrophobic nature, which suppressed the inhibition of Lewis acid sites by water. However, in 1-butanol, it showed the lowest fructose yield and fastest deactivation among the catalysts, because of its extra-large zeolite crystallites (ca. 15 μm) causing mass transfer limitation and undesired side reactions. The Sn-BEA catalysts prepared by post-synthetic Sn incorporation showed limited catalytic performance in water because of hydroxyl defects. However, they showed superior performances in 1-butanol because of the much smaller crystallites and enhanced mass transfer. In particular, the hierarchical Sn-BEA having 10–20 nm crystallites and significant intercrystalline mesoporosity showed a very large fructose yield (55%) that are difficult to achieve in typical aqueous-phase glucose isomerization (

Ni and Cu ion-exchanged nanostructured mesoporous zeolite: A noble metal free, efficient, and durable electrocatalyst for alkaline methanol oxidation reaction

The direct methanol fuel cell (DMFC) is considered as one of the promising power hubs to ensure the future global energy demand. The development of an efficient and cost-effective electrocatalyst for methanol electrooxidation (MEO) is one of the keys to make DMFC viable for practical application. Herein, an effort has been made to design a low-cost, noble metal free MEO catalyst. The electrocatalyst is synthesized by a facile ion-exchange of mesoporous zeolite ZSM-5 (Zeolite Socony Mobil 5) with non-precious Ni2+ and Cu2+ ions. The detailed electrocatalytic investigations reveal that the optimum contents of Ni2+ and Cu2+ ions in ion-exchanged mesoporous ZSM-5 (Ni2Cu1-MesoZ) are required to exhibit the best electrocatalytic activity (current density and onset potential). Moreover, the MEO capacity of the developed catalyst is found to be comparatively higher than the state-of-art Pt (20%)/C catalyst. The present electrocatalyst retains the catalytic current density even after 1000 cycles of a test run without any micro-structural change. The high surface area, Brönsted acidity, abundant surface hydroxyl groups, inter-crystalline mesoporosity, effective & optimum incorporation of Ni2+/Cu2+ species in the zeolite matrix provide highly dispersed electroactive sites and fast analyte diffusion resulting in a noteworthy MEO kinetics. The CO-stripping analysis also indicates that Ni2Cu1-MesoZ has a high tolerance to CO poisoning which is one of the prime reasons for high MEO activity. The underlying results demonstrate that the impressive electrocatalytic activity and high stability of Ni2+-Cu2+ ion-exchanged mesoporous ZSM-5 would be an interesting electrode material for the commercial methanol fuel cell application. Most importantly, the present study will open up a new avenue in the MEO electrocatalyst development due to its ease of synthesis, scale-up, fabrication, stability/durability, and efficient electrocatalytic activity for alkaline direct methanol fuel cell.

2D versus 3D MFI zeolite: The effect of Si/Al ratio on the accessibility of acid sites and catalytic performance

Through the synthesis of 2D MFI zeolite samples of Si/Al ratio ranged from 13 to 74 with inter-crystalline mesoporosity and their reference 3D counterparts, we have systematically studied and revealed the impact of Si/Al ratio on the inter-dependence of core intrinsic properties of structural porosity and acidity. It is apparent that mesopores in the 2D MFI zeolite play a critical role, dictating the accessibility and distribution of specific acid sites. It was found that, compared to their 3D counterparts, the 2D samples possess a three-times larger accessible surface area owing to the mesopores. Although having a slightly lower total number of acid sites, the 2D samples enjoy a higher percentage of accessible strong acid sites and weak Lewis acid sites. Consequently, in three selected liquid phase reactions, which had different acidity demands and molecular diffusion constraints, the 2D samples demonstrated much higher catalytic activities and resistance to deactivation. This study has, for the first time, established the relationship between Si/Al ratio and acidity for the 2D MFI zeolite, thus enabling rational selection of a Si/Al ratio for a targeted application.

Oligomerization of light olefins over ZSM-5 and beta zeolite catalysts by modifying textural properties

The oligomerization of ethene and 1-hexene was performed under 35 bar and 200 °C over NiH- and H-forms of ZSM-5 and beta zeolite catalysts which had similar Si/Al and Ni content, but different textural properties. Among the H-zeolites, H-ZSM-5 and H-beta composed of nanometer scale (ca. 10 nm) sheet-like crystals, as well as intercrystalline mesoporosity (ca. 5 nm), were found to be much more efficient for the oligomerization of 1-hexene to liquid fuel range products higher than C10 (C10 + ). In the oligomerization of ethene, identically prepared ZSM-5 and beta zeolites but with further exchange with Ni ion, also showed higher activity with remarkable C10+ product selectivity (>80%) after a single stage oligomerization, as compared to the corresponding cation form of ZSM-5 and beta zeolites without nanocrystalline and mesoporous morphology (conversion < 50% and C10+ selectivity < 30%). These results demonstrate that both small crystal size and the mesoporosity of the zeolite catalyst are critical factors for improving the diffusion limitation during the oligomerization of light olefins. This improvement was accompanied by a shift in major oligomeric product selectivity toward C10+ hydrocarbons.

In situ and post‐synthesis immobilization of enzymes on nanocrystalline MOF platforms to yield active biocatalysts

AbstractBACKGROUNDVery recently, metal‐organic framework (MOF) materials have been postulated as emerging supports to achieve solid‐state enzyme‐contained biocatalysts. In this work, post‐synthesis and in situ strategies to immobilize β‐glucosidase and laccase on different MOF materials were studied. The MOF‐based supports, i.e. MIL‐53(Al), NH2‐MIL‐53(Al) and Mg‐MOF‐74, were prepared under soft and sustainable conditions (room temperature and pH values compatible with enzymatic activity), allowing development of the in situ strategy.RESULTSIn both post‐synthesis and in situ approaches, the intercrystalline mesoporosity of the MOF‐based support favored the immobilization efficiency or the specific activity. The latter expressed as units per milligram of immobilized enzyme was higher in the post‐synthesis immobilization, whereas the biocatalysts prepared in situ gave much higher enzyme loading (over 85%) and lower enzyme leaching (around 5%). The in situ approach even worked in a non‐aqueous (N,N‐dimethylformamide) media in which the free enzyme was completely inactive. The immobilized enzymes are much larger than the structural pores of the MOFs.CONCLUSIONSEnzymes can be efficiently immobilized on nanocrystalline MOFs prepared under soft and sustainable conditions despite the supports lacking large enough pores to host the enzymes. The in situ approach is very efficient capturing enzymes and preserving some of their activity even under adverse conditions. © 2017 Society of Chemical Industry

Operando Raman-mass spectrometry investigation of hydrogen release by thermolysis of ammonia borane confined in mesoporous materials

Ammonia borane is a promising hydrogen storage material, but its application is hurdled by the slow hydrogen release kinetics from this compound, among other factors. Nanoconfinement of ammonia borane in scaffolding porous materials improves the hydrogen release kinetics and lowers the decomposition temperature, but the mechanisms through which nanoconfinement promotes these effects are still unclear. In this work, hydrogen thermal desorption from ammonia borane which has been incorporated into various mesoporous carriers has been investigated by means of operando Raman-Mass Spectrometry methodology. This technique allows the study of molecular transformations of ammonia borane during thermolysis. The sample is in a reactor suitable for spectroscopy. The sample is analyzed with Raman spectroscopy during thermolysis and the released molecules desorbing from the sample are taken by the flowing stream out of the operando reactor, an online mass spectrometer analyzes these as they come out. Both measurements are done simultaneously. In order to evaluate the influence of the properties of the host on nanoconfinement, four different scaffolds with different chemical composition and textural properties (pore diameter and volume) have been studied: Cobalt-substituted AlPO4-5 having intercrystalline mesoporosity, a mesoporous gallium oxide and two SBA-15 type materials. Results indicate the presence of crystalline ammonia borane in all impregnated hosts. By thermolysis, all nanoconfined samples begin to evolve hydrogen at a lower temperature than neat ammonia borane, although this effect is much more pronounced on SBA-15 samples. Furthermore, Raman spectra indicate a modification in the mechanism of decomposition in ammonia borane-loaded SBA-15 materials that suggests a chemical interaction between hydride and host.

Mesoporous EU-1 zeolite as a highly active catalyst for ethylbenzene hydroisomerization

The synthesis of a micro-mesoporous EUO zeolite with a significant volume of intercrystalline mesoporosity revealed an outstanding result in the challenging industrial hydroisomerization of ethylbenzene.

Carbon nanotubes as transient inhibitors in steam-assisted crystallization of hierarchical ZSM-5 zeolites

Carbon nanotubes (CNTs) can be well dispersed in an ethanol diluted zeolite precursor and thus encapsulated in the formed dry gel after solvent evaporation. This specially prepared dry gel allows small quantites of CNTs to effectively act as transient inhibitors in the subsequent steam-assisted crystallization of ZSM-5 nanocrystal aggregates with hierarchical porosity. Although finnally separated from zeolite, CNTs can still transiently inhibit the zeolite growth to avoid fusing into a large crystal. The inter-crystalline mesoporosity in hierarchical ZSM-5 is not created by the space occupied by CNTs, but preserved from the mesoporosity formed by nucleation and initial crystallization. Excessive quantities of CNTs are not recommended because their excessive inhibition induces the appearance of amorphous impurity in hierarchical ZSM-5.

Catalytic behavior of alkali-treated Pt/HMOR in n-hexane hydroisomerization

Bifunctional Pt-HMOR catalysts were prepared by incipient wetness impregnation of various desilicated MOR obtained by alkaline treatment using NaOH concentrations ranging from 0.1 to 0.5M. The zeolite structural changes upon modification were investigated by several techniques including powder X-ray diffraction, 27Al and 29Si MAS-NMR spectroscopy, N2 adsorption, pyridine adsorption followed by infrared spectroscopy and the catalytic model reaction of m-xylene transformation. For low alkaline concentration the zeolite acidity is preserved, along with a slight increase of the volume correspondent to the larger micropores due to the removal of extra-framework debris already existent at the parent zeolite. At higher NaOH concentrations there is a significant loss of crystalinity and acidity as well as the formation of mesoporosity. The characterization of the metal function shows similar patterns for Pt-HMOR and Pt-M/0.1 samples, with Pt particles located mainly inside the inner porosity. In contrast, large Pt particles become visible at the intercrystalline mesoporosity of MOR crystals developed during the desilication treatments at severe alkaline conditions. The catalytic results obtained for n-hexane hydroisomerization showed an improved selectivity for dibranched over monobranched isomers for Pt-M/0.1 sample, likely due to the preservation of the support acidity and the slight enlargement of the micropores. This work is a new example in which the mesoporous development does not improve the catalytic efficiency of the zeolites, whereas mild alkaline desilication might be considered as an effective solution to produce customized catalysts with enhanced performance for a given application.

Fast synthesis of nanosized zeolite beta from a low-seeded, low-templated dry gel with a seeding-steam-assisted conversion method

Nanosized zeolite beta with a tuneable crystal morphology and size, rich intercrystalline mesoporosity and high catalytic activity for low-density polyethylene cracking, was fast transformed from a low-seeded, low-templated dry gel by a seeding-steam-assisted conversion method with a small quantity of water; the crystallization of zeolite beta is well induced within 28 h at a TEAOH/SiO2 of 0.1 with the aid of 1% seeds.

Micron-Sized Single-Crystal-like CoAPO-5/Carbon Composites Leading to Hierarchical CoAPO-5 with Both Inter- and Intracrystalline Mesoporosity

Commercial mesopores-containing carbon (carbon black BP2000) has been used as crystallization host as well as hard template of intracrystalline mesopores in CoAPO-5 materials prepared with N-methyldicyclohexylamine as specific structure-directing agent. As a result, micrometer-sized AFI-structured terminated-tip (pencil-like) hexagonal prims have been obtained. Despite the appearance of single crystals, the crystals are actually composites formed by a large number of elongated-shape CoAPO-5 and carbon nanodomains. Indeed, the removal of carbon by calcination leaves an inorganic Co-doped AlPO4-based material having a complex hierarchical pore system, formed by the expected microporosity for an AFI-structured material and both intra- and intercrystalline mesoporosity. In good agreement with the nanocomposite nature of these micrometer-sized crystals, the size of their fibers generated by burning the carbon matrix can be relatively controlled by the conditions of the calcination processes. Under optimized synthesis and calcination conditions, AFI-structured materials with high mesoporous surface area and pore volume can be afforded.

Towards the control of intercrystalline mesoporosity in inorganic microporous materials: The case of CoAPO-5

The generation of mesoporosity in microporous materials has been postulated as a valid strategy to limit the diffusional problems of organic substrates within the pores of these materials, without affecting their intrinsic catalytic behavior. This article shows the effect of systematic variations of different synthesis parameters on the nature of the intercrystalline mesoporosity in CoAPO-5 materials. The hard impositions of this approach, including a structure-directing agent (SDA) possessing enough structural specificity towards AFI materials under a very wide range of synthesis conditions, and at the same time having the ability of generating intercrystalline mesoporosity, severely reduces the possible choices. Fortunately, N-methyldicyclohexylamine (MCHA) simultaneously obeys both conditions. We have examined some parameters conventionally affecting the crystallization process such as time, temperature and dilution, and some other parameters with particular effect on the results of the crystallization of microporous materials such as heteroatom content, heteroatom source, pH of the starting gel and heating source (conventional or assisted by microwave radiation). Except Co source, the rest of the parameters did have a significant influence on the nature and/or magnitude of intracrystalline mesoporosity. The effect of pH and dilution was particularly relevant, so that ordered intracrystalline mesoporosity is favored when CoAPO-5 is crystallized from relatively diluted and acidic gels. In addition, high Co content produces a higher and better-defined mesoporosity, although it implies a modification of the catalytic activity as the number of active centers is inherently varied. Finally, a relevant improvement in the mesoporosity is obtained when heating is assisted by microwave radiation, since the resultant CoAPO-5 is composed by smaller crystals efficiently and orderly agglomerated in pseudospherical particles with a well-defined mesoporosity of narrower pores.

Non-templated intercrystalline mesoporosity in heteroatom-doped AlPO 4-5 using N-methyldicyclohexylamine as structure-directing agent

Non-doped and 11 (Si, Co, Mg, Zn, Mn, Sn(II), Sn(IV), Cr, Fe, V and Zr) heteroatom-doped AlPO 4-5 samples have been crystallized in a high purity using N-methyldicyclohexylamine as structure-directing agent by simply changing the heteroatom source. Needle- or rod-like crystals of nano size, arranged in cylindrical or spherical agglomerates containing intercrystalline mesopores, compose all samples, excepting VAPO-5, whose nanocrystals and agglomerates are shapeless, and ZnAPO-5 composed by larger crystals not defining any mesorpore in their arrangement. Crystal sizes and dimensions were evaluated by direct application of Scherrer equation to the most intense XRD peaks and by Williamson–Hall plots of h 0 0 and 0 0 l reflections, respectively. Scherrer equation systematically underestimated crystal sizes. Williamson–Hall method gives different crystal dimensions, which were much closer to those observed by SEM and TEM studies. Moreover, Williamson–Hall plots consider separately crystal dimensions and strain/stress contributions to XRD peaks broadening, which have been used as key information to rationally understand the role of different heteroatom ions in the sample morphology and their mesoporosity. Intercrystalline mesoporosity could be studied by N 2 adsorption/desorption isotherms in as-prepared samples, as well as in calcined ones, confirming the stability of mesopores under calcinations conditions. Both maxima of pore size distribution and external (extra-micropore) area well correlate with crystal size of MeAPO-5 with a given agglomerates morphology, suggesting different compact packing for each morphology.

Cyclic diquaternary ammoniums for nanocrystalline BEA, MTW and MFI zeolites with intercrystalline mesoporosity

3,10-Diazoniabicyclo[10.2.2]hexadeca-12,14,15-triene-3,3,10,10-tetramethyl-dichloride is an effective structure-directing agent for BEA zeolite under a high concentration of Na+. The pseudomorphic crystallization process led to nanocrystalline BEA particles possessing a large volume of intercrystalline mesopores. The synthesis of nanocrystalline zeolite has been extended to other cyclic diammonium compounds that can generate BEA, MTW and MFI zeolite structures. Phase diagrams for these nanocrystalline zeolites have been constructed using sodium silicate as a silica source. The nanocrystalline morphologies and intercrystalline mesoporosity have been characterized by electron microscopy and N2 adsorption. The result indicates that the pseudomorphic crystallization process is limited to BEA, and not MTW or MFI despite their nanocrystalline morphologies.

Sorption kinetics of linear paraffins in zeolite BEA nanocrystals

The objective of this study was to evaluate the sorption kinetics of linear paraffins in zeolite beta crystals using the ZLC technique to better understand its catalytic properties. The experiments were made under the influence of desorption flow rate and temperature. The diffusivity values, estimated using a transient Fickian diffusion model, were very low compared to others reported in the literature for zeolites with similar pores aperture. This indicates the probable influence of extracrystalline diffusion due to the very small size of zeolite beta crystals and also very pronounced heat effects in the experiments. The n-paraffins desorption rates in zeolite beta were much faster when compared to those under similar conditions obtained for zeolite HUSY. The very short diffusional length and probable intercrystalline mesoporosity in zeolite beta may facilitate the molecules kinetics through the pores and thus the catalytic performance in the isomerization reaction of n-alkanes.