Shape Selectivity Research Articles

Size separation of sodium dodecyl sulfate-proteins by capillary electrophoresis in dilute and ultra-dilute dextran solutions.

SDS capillary gel electrophoresis is a widely used in the biopharma and the biomedical fields for rapid size separation of proteins. However, very limited information is available on the use of dilute and ultra-dilute sieving matrices for SDS-protein analysis. Here, background electrolytes (BGEs) containing 1%-0% dextran were used in borate-based BGE to separate a protein sizing ladder (PSL)≤225kDa and the intact and subunit forms of a therapeutic monoclonal antibody (mAb). The separation performance for the PSL and mAb components differed significantly with decreasing dextran concentration. Ferguson and reptation plots were used to elucidate the separation mechanism. Highly diluted dextran solutions resulted in linear Ferguson plots for both solute types (cf. Ogston theory) in spite of this model assumes a rigid pore structure, thus cannot describe the separation mechanism in ultra-dilute polymer solutions with no reticulations. The saddle differences between the resolution of the PSL and the intact/subunit mAb forms in ultra-dilute dextran-borate matrices suggested the importance of shape selectivity, manifested by the adequate separation of the SDS covered intact as well as light and heavy chain subunits of the therapeutic mAb even at zero dextran concentration.

Products and coke shape-selectivity during anisole disproportionation over HZSM-5

Anisole conversion on an MFI zeolite (SiAl = 43) at atmospheric pressure and 673 K is a model for the upgrading of bio-oils produced by catalytic fast pyrolysis (CFP) of biomass. Catalyst activity and selectivity are measured experimentally, while deactivation is studied experimentally and by DFT calculations. Anisole dismutation is a Friedel-Crafts-type alkylation between two anisole molecules. Consecutive reactions produce phenol and methyl anisoles, cresols and xylenols, as primary, secondary, and tertiary products respectively. The resonance effect of the hydroxyl and methoxy groups, combined with the "product shape selectivity" of the zeolite, results in a high selectivity for p-methyl-anisole, o-cresol and 2,4 xylenol. Coke deposition consists exclusively of methyl phenols with 0–4 methyl groups. Their retention is due to steric hindrance for the larger ones and strong adsorption on stronger acid sites for the smaller ones, as confirmed by DFT calculations. Catalyst deactivation occurs by progressive fouling of the micropores rather than by poisoning.

Shape Selectivity in the Gold‐Catalyzed Hydration of Alkynes Using a Cavity‐Shaped Phosphine

AbstractA cavity‐shaped gold(I) complex derived from a bulky tri‐(ortho‐biaryl)‐phosphine ligand shows preferred selectivity towards terminal functionalities in the gold(I)‐catalysed hydration of alkynes under mild heating due to a well‐defined pocket as catalytic active site. The confinement‐induced size‐exclusion selectivity investigated for eight alkynes contrasts with other gold(I) complexes bearing bulky phosphine ligands that show reduced selectivity or even similar behaviour towards both internal and terminal alkynes. We also interrogate the potential of gold(III) derivatives for the same catalytic process.

Effect of Fe substitution over ZSM-48 on performance of n-dodecane hydroisomerization and distribution of isomers

A series of Fe substituted ZSM-48 nanocrystal aggregates were synthesized by the dynamic hydrothermal method. Through incorporation of Fe into ZSM-48 framework, the acidity of ZSM-48 zeolites has been modified. The hydroisomerization performances of n-dodecane were investigated on Pt supported ZSM-48 and Fe incorporated ZSM-48 zeolites. Fe incorporated ZSM-48 catalysts show higher n-C12 hydroisomerization selectivity compared with parent sample due to weaker acid strength of Si–OH–Fe than Si–OH–Al. High temperature favor the generation of the multi-branched isomers (mainly dimethyl isomers) following the “Key-Lock” mode. Meanwhile, 5-methylundecane is the main product in the monomethyl isomers (MOB) and 2,6-dimethyldecane is the main product in the multi-branched isomers (MUB), which suggests the excellent shape selectivity of ZSM-48 and the “Key-Lock” mode is preferred on ZSM-48 catalysts regardless Fe substituted or not. The relationship between the mono-methyl isomers and the butene-TPD results has been correlated and investigated. Four desorption peaks corresponding to butene weakly adsorbed over Si–OH, Fe–OH or Lewis acid sites, the “Key-Lock” mode adsorbed butene, the “Pore-Mouth” mode adsorbed butene and the strongly adsorbed butene respectively, can be distinguished. ZLC results indicate that Fe modified ZSM-48 can effectively increase n-hexane diffusion rate and reduce the activation energy of diffusion of n-hexane, which is benefit for the hydroisomerization performance. SMOB/SMUB can be altered by Fe modification, however, the distribution of the mono-methyl isomers can hardly be affected.

A versatile hybrid catalyst platform of Na/ZnFe2O4 and zeolite for selective hydrocarbon production from CO2 hydrogenation

Catalytic CO2 hydrogenation faces great challenges in both reaction rates and selectivity to desired high-value products. Herein, we present a one-pot reaction platform that converts CO2 into various long-chain hydrocarbons selectively by combining a Na/ZnFe2O4-based catalyst for CO2 activation and carbon–carbon coupling, and a zeolite for fine-tuning the selectivity of desired products by exploiting its shape selectivity. Thus, the Na/ZnFe2O4 catalyst without zeolite produces highly olefinic diesel range hydrocarbons, and a hybrid catalyst with ZSM-5 produces highly aromatic gasoline range hydrocarbons, that with ZSM-11 produces branched kerosene range hydrocarbons, and that with SSZ-13 produces hydrocarbons rich in C2-C4 olefins. In all cases, high CO2 conversions of over 35% and low CO selectivity of near 10% are maintained. Therefore, the hybrid catalyst platform proposed here demonstrates that an elaborate catalyst design enables fine-tuning of the reaction pathway of CO2 hydrogenation to produce selectively versatile value-added hydrocarbons.

Highly Stable and Selective Catalysts for m-Cresol Hydrogenolysis to Aromatics

Deoxygenation is an essential link for upgrading bio-based low-value phenolics to aromatics, which is achieved through the catalytic hydrogenation way. Herein, we designed and synthesized kinds of Ni-confined catalysts via tailoring the preparation procedures. The highest hydroxyl hydrogenolysis performance (r[garomatics·gNi–1·h–1] = 103.5 at 290 °C) in vapor m-cresol conversion so far was obtained over Ni@Silicalite-1 prepared via the in situ encapsulation method due to the average 2.5 nm Ni nanoparticles uniformly encapsulated within silicalite-1 crystals for improving the shape selectivity with the vertical adsorption mode of phenolics. The appropriate porosities are proven to play a crucial role in shape-selective catalysis for hydroxyl hydrogenolysis of m-cresol. In addition, Ni@Silicalite-1 showed outstanding stability in 200 h long run with 95.5% conversion and 74.2% aromatics yield without obvious deactivation. This work provided a novel insight into tuning hydroxyl hydrogenolysis of phenolics by designing metal@zeolite catalysts with different microenvironments and ultrasmall metal nanoparticles.

Chemical recycling of plastic waste to monomers: Effect of catalyst contact time, acidity and pore size on olefin recovery in ex-situ catalytic pyrolysis of polyolefin waste

The catalytic upgrading of post-consumer mixed polyolefinic waste pyrolysis vapors has been studied over five different solid acid catalysts to understand the effect of vapor-catalyst contact time and catalyst properties on the recovery of light olefins (C2-C4). HZSM-5 with Si/Al ratio of 11 and 27 produced the highest light olefins yields (83.4 wt% and 72.1 wt%, respectively) at a very short contact time of ∼5 ms while extending the contact time (≥23 ms) decreased the olefin yields and resulted in higher yields of aromatics (∼30.0 wt% and ∼40.0 wt%, respectively). Short contact time (5 ms) was not sufficient for HZSM-22 to crack pyrolysis vapors to C2-C4 olefins (8.0 wt%) but increasing contact time improved C2-C4 yields to 53.5 wt% without increased production of aromatics, which may be attributed to its small pore size (shape selectivity). SAPO-34, SAPO-11, and Al-MCM-41 were not selective to light olefins at any studied contact time.

Catalytic conversion of biomass-derived compounds to high added value products using an acid treated natural mordenite

Herein, the mordenite catalyst TECHΝOSA-H2 that originates from volcanic soils in Greek islands is utilized for the synthesis of ethers and acetals from renewable raw materials. Di-furfuryl ether and furfuryl ethyl ether are used in perfumery and fuels, respectively, and are usually produced by employing either homogeneous catalysts or heterogeneous catalysts which however may lead to undesirable polymer products. Using TECHNOSA H2 we achieved yields in di-furfuryl ether and furfuryl ethyl ether 30% and 60%, respectively, in 3 h and 150 °C. We also report the synthesis of citral propylene glycol acetal and citral diethyl acetal, two important compounds in perfumery industry whose industrial synthesis often requires homogeneous catalysts while with TECHNOSA-H2 50% yields are achieved in 20 min and mild temperatures. The nature of the active sites of TECHNOSA-H2 was inspected by its application to relevant test reactions revealing that Brønsted acidity dominates in this catalyst. To that end, the one-step isomerization of a-pinene oxide to campholenic aldehyde and the one-step cyclization of citronellal to isopulegol in 10min were achieved. Finally, the shape selectivity of the catalyst was examined, proving TECHNOSA-H2 to be selective between the (E)- and (Z)- isomers of Citral. Based on our findings, TECHNOSA-H2 applications vary and include acetalization, isomerization and etherification reactions, as well as reactions where shape selectivity between reactants is desired.

Synergistic shape selectivity of H-Beta and H-ZSM-5 for Xylene-rich BTX production by hydrocracking of heavy-aromatic compounds

Catalytic hydroconversion of C10+ heavy aromatics (C10+ Aro) from an aromatic complex was studied to produce benzene, toluene, and xylenes (BTX), rich in xylenes. Two catalytic schemes, direct hydrocracking (HDC) and hydrotreating (HDT) followed by HDC, were explored. The Mo2C/γ-Al2O3 catalyst exhibits excellent selectivity for mono-aromatics and catalytic stability in the HDT of C10+ Aro. HDC catalysts containing BZ (H-Beta/H-ZSM-5 = 9/1 weight ratio) exhibited a significantly enhanced yield of BTX but significantly reduced the formation of alkylbenzenes (other than BTX) and heavy aromatic residues compared to the HDC catalyst without H-ZSM-5. H-ZSM-5 with smaller pore size and higher Brönsted acidity than H-Beta promoted dealkylation of alkylbenzenes into BTX while limiting heavy residue formation. HDC of HDT-C10+ Aro over NiMo/BZ catalyst resulted in BTX and BTX + C9 Aro yields as high as 47.7 and 61.1 wt%, respectively, which were significantly higher than those obtained from direct HDC of C10+ Aro (44.4 and 53.8 wt%). Unlike model tetralin HDC, the BTX distribution was in the order X > T > B from the HDC of HDTC10+ Aro. Transalkylation reactions between benzene/toluene and tetramethylbenzenes (in C10+ Aro) have been suggested as possible mechanisms for the formation of xylene-rich BTX.

Construction Archimedean tiling patterns based on soft materials from block copolymers and covalent organic frameworks

Soft materials based on block copolymers or covalent organic frameworks (COFs) that exhibit Archimedean tiling patterns have attracted significant interest due to their potential applications in nanopatterning, nanocomposites, and shape selectivity. Additionally, researchers have widely investigated the corresponding microporous or mesoporous materials with Archimedean tiling from COF or block copolymers as templates with high surface area and pore volume, or tunable porosity with different length scales, in separation, energy storage, drug delivery, photo-catalysis, photovoltaic solar cells, and chemical sensing. This review article highlights recent progress in constructing Archimedean tiling patterns based on the creation of ordered structures from block copolymers by self-assembly and the direct synthesis of COF materials with various topologies.

Preparation, characterization, and separation mechanism of a dehydroabietic-acid-based shape-selective chromatographic stationary phase1

Chromatographic stationary phases with molecular-shape selectivity are advantageous for the separation and analysis of geometric isomers. Herein, dehydroabietic acid is bonded on the surface of silica microspheres via 3-glycidoxypropyltrimethoxysilane to form a monolayer dehydroabietic-acid stationary phase (Si-DOMM) with a racket-shaped structure. Various characterization techniques indicate that Si-DOMM is successfully prepared, and the separation performance of a Si-DOMM column is evaluated. The stationary phase has a low silanol activity and metal contamination and a high hydrophobicity and shape selectivity. The resolutions of lycopene, lutein, and capsaicin on the Si-DOMM column confirm that the stationary phase exhibits high shape selectivity. The elution order of n-alkyl benzene on the Si-DOMM column indicates its high hydrophobic selectivity and suggests that the separation is an enthalpy-driven process. Repeatability experiments reveal highly stable preparation processes of the stationary phase and column and indicate that the relative standard deviations of retention time, peak height, and peak area are less than 0.26%, 3.54%, and 3.48%, respectively. Density functional theory calculations using n-alkylbenzenes, polycyclic aromatic hydrocarbons, amines, and phenols as model solutes provide an intuitive and quantitative description of the multiple retention mechanisms. The Si-DOMM stationary phase exhibits superior retention and high selectivity for these compounds via multiple interactions. The bonding phase of the monolayer dehydroabietic acid stationary phase with a racket-shaped structure has a unique affinity for benzene, strong shape selectivity, and good separation performance for geometrical isomers with different molecular shapes.

Shape-selective processing in deep networks: integrating the evidence on perceptual integration

Understanding how deep neural networks resemble or differ from human vision becomes increasingly important with their widespread use in Computer Vision and as models in Neuroscience. A key aspect of human vision is shape: we decompose the visual world into distinct objects, use cues to infer their 3D geometries, and can group several object parts into a coherent whole. Do deep networks use the shape of objects similarly when they classify images? Research on this question has yielded conflicting results, with some studies showing evidence for shape selectivity in deep networks, while others demonstrated clear deficiencies. We argue that these conflicts arise from differences in experimental methods: whether studies use custom images in which only some features are available, images in which different features compete, image pairs that vary along different feature dimensions, or large sets of images to assess how representations vary overall. Each method offers a different, partial view of shape processing. After comparing their advantages and pitfalls, we propose two hypotheses that can reconcile previous results. Firstly, deep networks are sensitive to local, but not global shape. Secondly, the higher layers of deep networks discard some of the shape information that the lower layers are sensitive to. We test these hypotheses by comparing network representations for natural images and silhouettes in which local or global shape is degraded. The results support both hypotheses, but for different networks. Purely feed-forward convolutional networks are unable to integrate shape globally. In contrast, networks with residual or recurrent connections show a weak selectivity for global shape. This motivates further research into recurrent architectures for perceptual integration.

Catalytic cracking of heavy atmospheric gas oil to light olefins over ZSM-5 zeolite: Effect of crystal size in the absence/presence of steam

The catalytic cracking of heavy atmospheric gas oil (AGO) has been investigated to study the effect of ZSM-5 zeolite crystal size on the selectivity to light olefin in the absence and presence of steam. Dodecane (used as model compound) and heavy AGO feed streams were utilized to study the influence of changing pore shape selectivity when comparing cracking performance by nano and micro ZSM-5 zeolites. The zeolites were characterized using BET, XRD NMR, NH3-TPD and Py-FTIR. In the presence of steam, the short pores length nano ZSM-5 crystals showed enhanced catalytic cracking activity and selectivity to light olefins. The nano ZSM-5 catalyst promoted higher selectivity to light olefins through β-scission while reducing the extent of hydrogenation reactions towards the formation of liquefied petroleum gas components (LPG). In contrast, the long pores length of micro ZSM-5 tended to favor the production of more naphtha, kerosene and LPG through hydrogen transfer routes, resulting in lower selectivity to light olefins. The resulting propylene/ethylene (P/E) ratios were altered by the type of feed used, cracking reaction temperature, influence of steam and catalyst crystal size. Specifically, higher P/E ratio was achieved over nano ZSM-5 zeolite, especially with the cracking of heavy AGO feed in the presence of steam, due to an increase of monomolecular reactions through β-scission cracking mechanism.

Selective hydrogenation of carbon–carbon double bond catalyzed by FLP-MOFs

Selective hydrogenation of CC double bond in α, β-unsaturated carbonyl compounds is of great significance in the production of fine chemicals, and the design and preparation of hydrogenation catalysts with high selectivity is the key to realize the industrialization of this reaction. Controlling and adjusting the steric hindrance between substrate molecules and active center of catalysts is an effective strategy to achieve high selectivity. In this work, the spatially hindered “Frustrated Lewis pairs” (FLPs) was used as the active component and metal-organic frameworks (MOFs) materials with a certain window size were used as the catalyst support. We designed and synthesized a heterogeneous hydrogenation catalyst B(C6F5)3/DO-MIL-101 by immobilizing FLP onto metal-organic frameworks (MIL-101). Characterizations and catalytic performance tests revealed that the catalytically active species B(C6F5)3/DO was confined in MIL-101 nanocavities via Cr–O coordination bond. The resulting B(C6F5)3/DO-MIL-101 catalyst not only realized the recycling of FLP, but also showed excellent catalytic activity and high selectivity in the selective hydrogenation of CC double bond in α, β-unsaturated carbonyl ketones. It should be noted that the steric hindrance between FLP and substrate molecules, as well as the shape selectivity of MOFs nanocages, are the principal factors for the high selectivity of the catalyst.

Learning a Model of Shape Selectivity in V4 Cells Reveals Shape Encoding Mechanisms in the Brain.

The mechanisms involved in transforming early visual signals to curvature representations in V4 are unknown. We propose a hierarchical model that reveals V1/V2 encodings that are essential components for this transformation to the reported curvature representations in V4. Then, by relaxing the often-imposed prior of a single Gaussian, V4 shape selectivity is learned in the last layer of the hierarchy from Macaque V4 responses. We found that V4 cells integrate multiple shape parts from the full spatial extent of their receptive fields with similar excitatory and inhibitory contributions. Our results uncover new details in existing data about shape selectivity in V4 neurons that with additional experiments can enhance our understanding of processing in this area. Accordingly, we propose designs for a stimulus set that allow removing shape parts without disturbing the curvature signal to isolate part contributions to V4 responses.SIGNIFICANCE STATEMENT Selectivity to convex and concave shape parts in V4 neurons has been repeatedly reported. Nonetheless, the mechanisms that yield such selectivities in the ventral stream remain unknown. We propose a hierarchical computational model that incorporates findings of the various visual areas involved in shape processing and suggest mechanisms that transform the shape signal from low-level features to convex/concave part representations. Learning shape selectivity from Macaque V4 responses in the final processing stage in our model, we found that V4 neurons integrate shape parts from the full spatial extent of their receptive field with both facilitatory and inhibitory contributions. These results reveal hidden information in existing V4 data that with additional experiments can enhance our understanding of processing in V4.

Catalytic Upgrading of Lignite Pyrolysis Volatiles to Aromatics over Urea-Modified HZSM-5: Study on Desilication of Preserved Micropores

This work aimed to investigate the effect of the physical properties of urea-desilicated HZSM-5 on the catalytic performance of the lignite pyrolysis volatiles upgrading to aromatics. A mild urea treatment can create more secondary mesoporous and Brønsted acid sites while well-preserving micropores, of which the first two promoted the diffusion and active cleavage of macromolecules into more small fragments, while the latter ensured the shape selectivity to form aromatics. Consequently, the urea-desilicated catalysts exhibited better performance for the production of aromatics, especially benzene and toluene in monocyclic aromatic hydrocarbons (MAHs), compared to the parent HZSM-5. Among the catalysts, HZSM-5 treated with 5 wt % urea solution for 5 h gave the highest light aromatic (LA) yield of 25.8 mg/g, in which the increased MAH yield accounted for 77% of the total increased LAs. However, excessive desilication with a higher urea concentration or longer treatment time significantly reduced the aromatic yield as a result of a considerable loss of micropores and mesoporous and Brønsted acid sites. The strategy of preserving micropores and optimizing mesoporous and Brønsted acid sites provides a new way to enhance the formation of aromatics.

Silica microparticles modified with ionic liquid bonded chitosan as hydrophilic moieties for preparation of high-performance liquid chromatographic stationary phases.

Two novel stationary phases, 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan modified silica and 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan derivatized calix[4]arene modified silica stationary phase, were synthesized using 1-(4-bromobutyl)-3-methylimidazolium bromide bonding chitosan as a polarity regulator solvingthe limitation of the strong hydrophobicity of calixarene in the application of hydrophilic field. The resulting materials were characterized by solid-state nuclear magnetic resonance, Fourier-transform infrared spectra, scanning electron microscopy, elemental analysis, and thermogravimetric analysis. Based on the hydrophilicity endowed by 1-(4-bromobutyl)-3-methylimidazolium bromide bonded chitosan, the retention mode of ILC-Sil and ILCC4-Sil could be effectively switched from the hydrophilic mode to a hydrophilic/hydrophobic mixed mode and could simultaneously provide various interactions with solutes, including hydrophilic, π-π, ion-exchange, inclusion, hydrophobic, and electrostatic interactions. On the basis of these interactions, successful separation and higher shape selectivity were achieved among compounds that vary in polarity under both reverse-phase and hydrophilic interactive liquid chromatography conditions. Moreover, the ILCC4-Sil was successfully applied tothe determination of morphine in actual samples using solid-phase extraction and mass spectrometry. The LOD and LOQ were 15 pg/mL and 54 pg/mL, respectively. This work presents an exceptionally flexible adjustment strategy for the retention and selectivity of a silica stationary phase by tuning the modification group.

Para-xylene production from toluene methylation: Novel catalyst synthesis, fabrication and ANFIS modelling

BackgroundToluene methylation to p-xylene is a key reaction in the petrochemical industry, with p-xylene being an important feedstock for the manufacturing of many chemical commodities. MethodsThe present contribution reports the synthesise of Al-HMS industrial catalysts with different Si/Al ratios and H-ZSM-11 industrial catalysts with surface modification (SM-H-ZSM-11). The properties of the catalysts were characterized by X-ray crystallography (XRD), scanning electron microscope (SEM), thermogravimetric Analysis (TGA), Fourier-transform infrared spectroscopy (FTIR) of pyridine adsorption, and NH3 TPD. catalytic performances were evaluated for the toluene methylation with methanol by assessing the effects of temperature, pore size, acidic strength, and residence time of reactants on the shape-selectivity of the product. Significant findingThe results show that whilst an increase in pore size has an unfavorable impact on the shape selectivity of p-xylene, an increase in the strength of the acid site enhances the shape selectivity of p-xylene, hence counterbalancing the effect of pore size. In particular, Al-HMS-5 has a larger pore size than that of H-ZSM-11 but the stronger acidity of Al-HMS-5 is more effective on the shape selectivity of the catalyst. Beside experimental studies, a computational process modelling was carried out to estimate percentage of toluene conversion as a function of the reaction temperature (K), WHSV (h−1), toluene/MeOH molar ratio, acidity of catalyst (mmol/g), and surface area (m2/g) using an adaptive neuro-fuzzy inference system optimized by a particle swarm optimization algorithm. Excellent agreement between measured and predicted toluene conversion confirmed the satisfactory performance of our model. The results show that by suitable tuning of catalyst acidity it is possible to retain high selectivity whilst increasing pore size, hence alleviating potential problems due to diffusion limitation in small pores. The relevancy factor calculated for reaction temperature, WHSV, acidity, and BET surface area of the catalysts were1.35, 0.096, 0.0071, and 10.42, respectively, which shows that highest sensitivity of toluene methylation relates to BET surface area of the catalysts.

Intensified shape selectivity and alkylation reaction for the two-step conversion of methanol aromatization to p-xylene

Two-step conversion of methanol to aromatics via light hydrocarbons can significantly improve the conversion stability compared with direct aromatization of methanol, but it remains a challenge to achieve a high p-xylene (PX) selectivity. Herein, silica coating was firstly used to passivate external acid sites of ZSM-5 catalyst for the aromatization of light hydrocarbons by the chemical liquid deposition method. With the increase of SiO2 deposition, the density of the external acid sites of the catalyst was decreased from 0.1 to 0.03 mmol·g−1, which inhibited the surface secondary reactions and increased the PX/X from 34.6% to 60.0%. In view of the fact that the aromatization process in the second step was partly inhibited as methanol was consumed in advance in the upper methanol-to-light hydrocarbons catalyst layer, part of methanol was directly introduced into the lower aromatization catalyst layer to promote the alkylation process during the aromatization, which decreased the toluene selectivity from 34.5% to 14.3% but increased the xylene selectivity from 40.0% to 55.3%. It was also found that an appropriate external acid density was needed for aromatization catalyst to strengthen the alkylation process and improve the selectivity of xylene under the conditions of methanol introduction.

Hierarchical Zeolite Synthesis by Alkaline Treatment: Advantages and Applications

Zeolites are of great interest to the scientific and industrial communities due to their interesting catalytic properties, such as high specific area, shape selectivity, and thermal and hydrothermal stability. For this reason, zeolites have been intensively studied and applied in several reactions of great industrial interest. However, the size of zeolite micropores may hinder the diffusion of bulky molecules in the pore system, limiting the use of zeolites in some reactions/applications that use bulky molecules. One way to address this limitation is to generate secondary porosity (in the range of supermicropores, mesopores and/or macropores) in such a way that it connects with the existing micropores, creating a hierarchical pore system. There are different hierarchical approaches; however, most are not economically viable and are complicated/time-consuming. Alkaline treatment has been highlighted in recent years due to its excellent results, simplicity, speed and low cost. In this review, we highlight the importance of alkaline treatment in the generation of secondary porosity and the parameters that influence alkaline treatment in different zeolitic structures. The properties and catalytic performance of hierarchical zeolites prepared by alkaline treatment are extensively discussed. It is expected that this approach will be useful for understanding how alkaline treatment acts on different hierarchical structures and will thus open doors to achieve other hierarchical zeolites by this method.