Silica Pillars Research Articles

Reversed Mg-Based Smectites: A New Approach for CO2 Adsorption

Addressing climate change requires transitioning to cleaner energy sources and adopting advanced CO2 capture techniques. Clay minerals are effective in CO2 adsorption due to their regenerative properties. Recent advancements in nanotechnology further improve their efficiency and potential for use in carbon capture and storage. This study examines the CO2 adsorption properties of montmorillonite and saponite, which are subjected to a novel microwave-assisted acid treatment to enhance their adsorption capacity. While montmorillonite shows minimal changes, saponite undergoes significant alterations. Furthermore, the addition of silica pillars to smectites results in a new nanomaterial with a higher surface area (653 m2 g−1), denoted as reversed smectite, with enhanced CO2 adsorption capabilities, potentially useful for electrochemical devices for converting captured CO2 into value-added products.

Catalytic activity enhancement in pillared zeolites produced from exfoliated MWW monolayers in solution

Layered zeolites, especially MWW, have been used to synthesize pillared micro/mesoporous hybrids with the goal to enhance catalytic activity towards larger molecules. The critical preparation step is expansion of the interlayer space by swelling with cationic surfactants at high pH. Recently reported solutions of MWW monolayers, obtained by exfoliation of the zeolite MCM-56, can be used to prepare analogous materials by flocculation (precipitation) with surfactant solutions. This previously unavailable approach is studied in this work with both high pH (0.2 M, pH > 13.3) and lower pH (0.01 M, pH~12) solutions, and resulted in finding conditions for preparation of catalysts that can be more active than pure microporous layers (zeolite) despite large content of unreactive silica pillars. Both the high and low pH conditions afforded similar expanded surfactant-MWW composites, but their behavior differed during pillaring with TEOS. Well-defined expanded interlayer distances indicated by a low angle reflection in XRD above 3 nm d-spacing was observed only with the high alkalinity preparations. An additional benefit was observed with TEOS treatment in the presence of isopropyl alcohol leading to a product showing high catalytic activity in model alkylation reaction (mesitylene with benzyl alcohol) proceeding faster than with the pure zeolite. The products obtained under different conditions were characterized by textural methods, nitrogen adsorption and QE-TPDA, and FT-IR to investigate acid site parameters. Notably, the MCM-56 used in this work was prepared with addition of aniline as the structure-promoting agent and is the second formulation affording exfoliable MWW materials.

Pillared Mo2TiC2 MXene for high-power and long-life lithium and sodium-ion batteries.

In this work, we apply an amine-assisted silica pillaring method to create the first example of a porous Mo2TiC2 MXene with nanoengineered interlayer distances. The pillared Mo2TiC2 has a surface area of 202 m2 g−1, which is among the highest reported for any MXene, and has a variable gallery height between 0.7 and 3 nm. The expanded interlayer distance leads to significantly enhanced cycling performance for Li-ion storage, with superior capacity, rate capably and cycling stability in comparison to the non-pillared analogue. The pillared Mo2TiC2 achieved a capacity over 1.7 times greater than multilayered MXene at 20 mA g−1 (≈320 mA h g−1) and 2.5 times higher at 1 A g−1 (≈150 mA h g−1). The fast-charging properties of pillared Mo2TiC2 are further demonstrated by outstanding stability even at 1 A g−1 (under 8 min charge time), retaining 80% of the initial capacity after 500 cycles. Furthermore, we use a combination of spectroscopic techniques (i.e. XPS, NMR and Raman) to show unambiguously that the charge storage mechanism of this MXene occurs by a conversion reaction through the formation of Li2O. This reaction increases by 2-fold the capacity beyond intercalation, and therefore, its understanding is crucial for further development of this family of materials. In addition, we also investigate for the first time the sodium storage properties of the pillared and non-pillared Mo2TiC2.

Directly probing surfactant adsorption on nanoscopic trenches and pillars

HypothesisConfinement causes a change in the amount of surfactant adsorbed and in the adsorption morphology. ExperimentsTwo cationic surfactants, tetradecyltrimethylammonium bromide (TTAB) and cetylpyridinium chloride (CPC) were adsorbed at the silica-water interface. Atomic force microscopy (AFM) force curves were measured on 50 nm and 80 nm wide trenches. Force curves were also measured on silica pillars, and the results were quantified based on distance from the edge. FindingsTrenches: Adsorbed surfactants films in 50 nm and 80 nm trenches showed the same break-through values. However, compared to unconfined values, TTAB in trenches had decreased break-through and adhesion forces while CPC in trenches had increased break-through and adhesion forces, indicating that surfactant identity varies the confinement effect.Pillars: Near the edge, few surfactants adsorb, and those that do extend in the direction normal to the surface. While the experimental data agree qualitatively with previous coarse-grained molecular dynamic simulations, the length scales at which the phenomena are detected differ by ~ half-order of magnitude. Specifically, experimental data show measurable effects on adsorbed surfactant morphology at a distance from the edge 10–20 times the length of a surfactant molecule after accounting for the ~8 nm size of the probe.

In-situ mesoporous silica pillared clay synthesis and effect of titanium and iron incorporation to structural properties

Abstract Standard Wyoming SWy-2 mineral as a clay source was modified by use of cetyltrimethylammonium bromide as surfactant before Silica Pillared Clays (SPCs) synthesis, while octylamine (OA) was chosen as an auxiliary surfactant. Using tetraethyl orthosilicate as a silica source by ranging silica/modified clay (Si/C+) molar ratio within 20–150, SPC supports were synthesized keeping OA/C+ molar ratio as 20. Hydrothermal syntheses of titanium and iron containing SPC catalysts were carried out with M/Si molar ratios in between 0.03 and 0.15. Structural characterization results explained well the proposed mechanism for formation of MCM-41 like silica pillars between clay layers. With an increase of (001) peak reflection yielding basal spacing values up to 6.9 nm, polycrystalline structure observed in XRD patterns was confirmed by TEM images in SPCs. TEM also showed silica phase formations between clay bundles. Structural properties at Si/C+ molar ratio as 50 were the best with a surface area of 1874 m2/g. Titanium loading resulted in some decreases in surface area values while structural properties of iron incorporated catalyst at low iron content were found to be closer to that of the support. The highest Lewis and Bronsted acidities were observed by low titanium and iron incorporation to high silica containing support, respectively. TGA results showed that calcination temperature of 550 °C was sufficient for stable structures formation. ICP analysis and TEM images indicated that titanium species settled locally nearly 100% loading success and presence of titanium increased iron loading success. XPS results showed presences of TiO2 and mostly Fe2O3 forms.

Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70

SSZ-70 is one of several layered forms of zeolite MMW obtained by direct synthesis with consecutive layers shifted by approximately ±1/3 of a unit cell in the horizontal direction (⟨110⟩), according to the recent structure solution report. Zeolite MWW is a valuable industrial alkylation catalyst and SSZ-70 itself was called by the inventors 'a successful catalytic material'. SSZ-70 shows characteristic powder X-ray diffraction pattern (XRD), containing a unique broad band with a roughly triangular shape and maximum at 8.5–9.0° 2θ (Cu Kα radiation), which can be used to distinguish SSZ-70 from the other MWW materials with different layer arrangements. This work concerns post-synthesis treatments of SSZ-70 to investigate its ability to produce more expanded structures by silylation, swelling and pillaring. Both boron- and aluminum-containing SSZ-70 were investigated. They showed the ability for interlayer expansion by swelling with cationic surfactants and were pillared with silica to produce micro/mesoporous hybrids with increased BET area. Not too surprisingly, the original unique layer stacking of SSZ-70 seems to have become more random and not distinguishable from the swollen forms of the other swellable MWW materials MCM-22 P and the monolayer MCM-56. Calcined SSZ-70 zeolites showed an unusual peak, assigned to (002) reflection, with d-spacing shorter than c-unit cell of the 3D MWW framework, which suggests collapse with formation of the so-called sub-zeolite form. Silylation resulted in different outcomes for B and Al SSZ-70 samples. The former produced interlayer expanded (IEZ) forms of B-SSZ-70 but did not show significant increase in pore volume and BET surface area. Silylation of Al-SSZ-70 did not yield the stabilized (IEZ) form but had a positive effect as it prevented the collapse to sub-zeolite upon calcination. Catalytic testing was carried out for the Al-containing SSZ-70 obtained by direct synthesis and for B-SSZ-70 activated by Al insertion. The former showed much higher concentration of Brønsted acid sites and 5 times faster conversion in a model alkylation reaction between benzyl alcohol and mesitylene. Pillared Al-SSZ-70 showed highest activity despite lower Al concentration due to silica pillars. Both silylation and pillaring had a positive effect on the activity in alkylation reaction with bulky molecules.

Selective Oxidation of Hydrogen Sulfide to Sulfur Using Vanadium Oxide Supported on Porous Clay Heterostructures (PCHs) Formed by Pillars Silica, Silica-Zirconia or Silica-Titania.

Vanadium oxide (V2O5) species has been supported on different porous clay heterostructures (with silica pillars, silica-zirconia with a molar ratio Si/Zr = 5 and silica-titania with a molar ratio Si/Ti = 5) by wetness incipient method. All catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption at −196 °C, NH3 thermoprogrammed desorption (NH3-TPD), Raman spectroscopy, diffuse reflectance UV-Vis and X-ray photoelectron spectroscopy (XPS). After that, the catalytic activity of the vanadium-based catalysts was evaluated in the selective oxidation of H2S to elemental sulfur. The catalytic data show that both the activity and the catalytic stability increase with the vanadium content, obtaining the highest conversion values and sulfur yield for the catalysts with vanadium content of 16 wt.%. The comparison among all supports reveals that the incorporation of TiO2 species in the pillars of the PCH improves the resistance to the deactivation, attaining as best results a H2S conversion of 89% for SiTi-PCH-16V catalyst and elemental sulfur is the only compound detected by gas chromatography.

Porous clay heterostructures intercalated with multicomponent pillars as catalysts for dehydration of alcohols

Abstract Montmorillonites intercalated with silica, silica-alumina, silica-titania and silica-zirconia pillars by surfactant directed method were studied as catalysts for dehydration of methanol and ethanol to dimethyl ether as well as diethyl ether and ethylene, respectively. Moreover, the samples of montmorillonite intercalated with silica pillars were doped with aluminum using template ion-exchange method. In the case of all the samples deposited metal (Al, Ti, Zr) species were present in highly dispersed forms and resulted in generation of acid sites catalytically active in dehydration of methanol and ethanol. The best catalytic results in both studied processes were obtained for the samples doped with aluminum, especially introduced by using template ion-exchange method (first time applied for deposition of metal cations into PCHs), which were more catalytically active in the process of methanol to dimethyl ether conversion than γ-Al2O3 (one of the most important commercial catalysts of this process). High catalytic activity of the aluminum doped samples was related to the nature and strength of acid sites active in the processes of alcohols dehydration.

Baeyer–Villiger Oxidation of Cyclic Ketones by Using Tin–Silica Pillared Catalysts

AbstractThe Baeyer–Villiger oxidation is an important transformation of ketones into esters and particularly for cyclic ketones to lactones. We report here preparation and catalytic activity of layered Sn–silicate catalysts and mesoporous ordered silica catalysts in this reaction. Sn was introduced by using so‐called tin–silica pillaring or by impregnation with a mixture of tetraethylorthosilicate and tin(IV) alkoxide. The prepared catalysts were characterized by XRD, N2 physisorption, SEM, UV/Vis, and inductively coupled plasma optical emission spectroscopy (ICP‐OES) techniques and the catalysts were studied in the Baeyer–Villiger oxidation of cyclopentanone, norcamphor, and 2‐adamantanone with aqueous hydrogen peroxide. Norcamphor and 2‐adamantanone were oxidized easily with selectivity up to 99 %. Sn‐MS and IPC‐1‐SnPI materials exhibited the highest conversions (e.g., norcamphor: Sn‐MS 37 %, IPC‐1‐SnPI 36 % after 8 h vs. Sn‐MCM‐41 22 %). On the other hand, oxidation of cyclopentanone suffered from product hydrolysis to the corresponding 4‐hydroxybutanoic acid.

Amino-modified pillared adsorbent from water-treatment solid wastes applied to CO2/N2 separation

In this work, solid wastes from the flocculation step of a drinking water treatment plant were used as starting material to obtain CO2 adsorbents upon thermal treatment and a pillaring strategy. Thermally treated solid waste and pillared materials were characterized by XRD, FTIR, TGA, N2 adsorption–desorption isotherms at 77 K, XPS and elemental analysis. From the characterization results, it was found that the introduction of silica pillars was successful, leading to heterostructured materials with higher specific surface area and pore volume. In order to improve the CO2 adsorption capacity, pillared materials were functionalized by grafting with (3-aminopropyl)triethoxysilane (APTES)and impregnation with polyethylenimine (PEI). Pillared materials could be loaded with up to 50 wt% PEI. PEI-impregnated pillared adsorbents were tested for CO2 adsorption at 298 and 348 K and CO2/N2 adsorption at 348 K using a Magnetic Suspension Balance equipped with a mixture gas dosing (Rubotherm, Germany). Langmuir and Sips models were used to fit monocomponent and mixture experimental data, considering two different sites for CO2 adsorption. At 1 bar, 50PEI-SiFe (loaded with 50 wt% PEI) adsorbed 2.5 and 3.6 mmol CO2 g−1 at 298 and 348 K, respectively and exhibited an extremely high CO2/N2 selectivity (above 150 mol CO2/mol N2 at 348 K). Nevertheless, PEI loaded adsorbents still exhibit a lower amine efficiency as compared to APTES-grafted counterparts, due to diffusion restrictions caused by the high viscosity of the impregnated polymer.

Adsorption study of reactive dyes onto porous clay heterostructures

The present research evaluates the adsorption of reactive dyes, Remazol Violet 5R (RV5R) and Acid Blue 25 (AB25), using Porous Clay Heterostructures (PCHs) prepared from natural bentonite. Natural bentonite, PCH with silica pillars (Si-PCH) and PCH with silica-zirconia (SiZr-PCH) were characterized by elemental analysis, XRD, N2 adsorption-desorption at −196°C, FT-IR, TG and XPS. The adsorption experiments were carried out in a stirred tank in order to evaluate the effect of pH, contact time and initial concentration. The adsorption isotherms were well fitted by Langmuir (L), Langmuir-Freundlich (LF) and Redlich Peterson (RP) models. The equilibrium data were described using the Langmuir-Freundlich model for both dyes and both materials, obtaining a maximum adsorption capacity of 209.9mgg−1 and 265.9mgg−1 for AB25 using Si-PCH and SiZr-PCH, respectively. In the case of RV5R, the maximum adsorption capacity was 127.07mgg−1 and 185.7mgg−1 for Si-PCH and SiZr-PCH, respectively. The adsorption process takes place by electrostatic interactions between the silanol groups of the PCHs with functional groups of the dyes, such as amine or hydroxyl. From the obtained results, it can be concluded that PCHs showed to be a promising material for the adsorption of dye.

Perfectly plastic flow in silica glass

The plastic behavior of silicate glasses has emerged as a central concept for the understanding of glass strength. Here we address the issue of shear-hardening in amorphous silica. Using in situ SEM mechanical testing with a high stiffness device, we have been able to compress silica pillars to large strains while directly monitoring radial strain. The sizeable increase of pillar cross-section during compression directly demonstrates the significant role of homogeneous shear flow. From the direct evaluation of the cross section, we have also measured true stress-strain curves. The results demonstrate that silica predominantly experiences plastic shear flow but that there is no shear-induced hardening. The consequence of this finding for our understanding of glass strength is discussed.

Lamellar and pillared ZSM-5 zeolites modified with MgO and ZnO for catalytic fast-pyrolysis of eucalyptus woodchips

Lamellar and pillared ZSM-5 materials modified with Mg and Zn oxides were synthesized and tested for in-situ catalytic upgrading of eucalyptus woodchips fast-pyrolysis vapors. The introduction of silica pillars into the lamellar ZSM-5 support led to a higher BET area, but also reduced the overall catalyst acidity. The incorporation of MgO and ZnO occurred with a high dispersion over the zeolitic supports, causing also a significant reduction in the value of their textural properties due to a partial blockage of the zeolite pores. Likewise, the acid features of the zeolitic supports underwent sharp changes by the addition of both MgO and ZnO with a strong decrease in the concentration of the Brønsted and Lewis acid sites present in the parent zeolite, as detected by pyridine adsorption followed FTIR spectroscopy. However, additional Lewis acid sites were created associated to the metal oxides deposited onto the zeolitic supports.Pyrolysis tests were accomplished using a lab-scale downdraft fixed-bed reactor working at atmospheric pressure and a temperature of 500°C. The use of zeolitic catalysts increased the gas yield, mostly due to the formation of CO and CO2, to the detriment of bio-oil production. However, the so obtained bio-oils presented higher quality in terms of H/C and O/C ratios, and larger heating values. The incorporation of MgO and ZnO allowed tailoring the zeolite activity to avoid an excessive cracking of the bio-oil, which in turn resulted in a higher yield of the organic compounds present in the bio-oil, and decreasing the formation of undesired polyaromatic hydrocarbons and coke.

Pillared HMCM-36 zeolite catalyst for biodiesel production by esterification of palmitic acid

Layered MWW zeolite has been studied for swelling/pillaring using CTAB and silica as swelling and pillaring reagents, respectively to synthesize pillared MCM-36 material. The swelling/pillaring efficiency was evaluated based on their X-ray diffraction, N2-physisorption, scanning electron microscopy and FTIR spectra after pyridine adsorption as indicator for acidity measurements and catalytic potential. There was an overall decrease in acid site concentration due to incorporation of inert silica pillars. However, after ion exchange, mesoporous HMCM-36 zeolite with the highest BET surface area showed increased Brönsted and Lewis acid sites compared to the MCM-22, suggesting enhanced accessibility of acid sites for bulky reacting molecules. MCM-22 and MCM-36, as well as the ion exchanged HMCM-22 and HMCM-36 samples were tested for esterification of palmitic acid with methanol. The HMCM-36 catalyst showed high activity in palmitic acid esterification with methanol. This catalyst can be readily separated from the reaction system for re-use for at least four cycles without losing any activity suggesting potential industrial applications in biodiesel synthesis.

Strain superlattices and macroscale suspension of graphene induced by corrugated substrates.

We investigate the organized formation of strain, ripples, and suspended features in macroscopic graphene sheets transferred onto corrugated substrates made of an ordered array of silica pillars with variable geometries. Depending on the pitch and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lie fully suspended between pillars in a fakir-like fashion over tens of micrometers. With increasing pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to organized domains of parallel ripples linking pillars, eventually leading to suspended tent-like features. Spatially resolved Raman spectroscopy, atomic force microscopy, and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the structural transition between fully suspended and collapsed graphene. For the arrays of high density pillars, graphene membranes stay suspended over macroscopic distances with minimal interaction with the pillars' apexes. It offers a platform to tailor stress in graphene layers and opens perspectives for electron transport and nanomechanical applications.

Montmorillonite intercalated with SiO2, SiO2-Al2O3 or SiO2-TiO2 pillars by surfactant-directed method as catalytic supports for DeNOx process

AbstractThe intercalation of natural montmorillonite with SiO2, SiO2-Al2O3 or SiO2-TiO2 pillars by the surfactant-directed method resulted in the formation of high surface area porous materials; these were tested as catalytic supports for the process of selective catalytic reduction of NO (DeNOx). The incorporation of titanium or aluminium into the structure of the silica pillars significantly increased the surface acidity of the clay samples. Iron and copper were deposited onto the surface of the pillared clays mainly in the form of monomeric isolated cations and oligomeric metal oxide species. The contribution of the latter species was higher in the clay intercalated with SiO2-TiO2 pillars than in the samples modified with SiO2 and SiO2-Al2O3 pillars. The pillared clay-based catalysts were active in the DeNOx process but, in this group, the best results were obtained for the clay intercalated with SiO2-TiO2 pillars and doped with iron and copper. The catalytic performance of the samples is discussed in respect of their surface acidity and active forms of transition metal species deposited.

Silica-pillared graphene sheets with iron–nitrogen units as an oxygen reduction catalyst

Graphene layers with silica pillars which widen the interlayer distance and functionalize the space as micropores have been prepared as one of the new porous carbon materials. In this study, the silica-pillared graphene with Fe–N units as a catalytic active site for cathodic oxygen reduction was formed as a new type of carbonaceous noble-metal-free oxygen reduction catalysts with the ordered pore structure. The formation of the pillared carbon was performed by introducing silylating reagents as the pillar source and also as the nitrogen source of the units in the graphite oxide interlayer spaces, introducing the Fe ions, and heat treatment in vacuum at 500°C. The X-ray diffraction (XRD) patterns showed the ordered layered structure. The X-ray absorption fine structure (XAFS) of the Fe-K edge showed that the unit was a square planar Fe–N4 moiety. The catalytic activity for the O2 reduction was demonstrated by measurements using rotating disk electrodes with the pillared carbon fixed on the surface and immersed in an oxygen-saturated acidic solution. The activity was enhanced by the combination of the nitrogen containing compound to the silylating reagent and using the nitrogen-rich silylating reagent.

Plastic deformation and residual stresses in amorphous silica pillars under uniaxial loading

We have carried out uniaxial compression of micron-scale amorphous silica pillars. We have measured load–displacement curves and observed the morphology of the pillars after unloading, providing strong evidence for large plastic deformations. Minor cracking is also observed, with a well-defined pattern. We find that the van Mises stress in compression is comparable to the intrinsic tensile strength of silica. Precise analysis of the deformation of the pillars has been carried out by finite element modeling (FEM) using the constitutive equation determined previously (G. Kermouche et al., Acta Materialia, 56 (2008) 3222), which quantitatively takes into account densification, shear flow and strain hardening. The residual stress distribution we predict by FEM matches the observed crack pattern well. Finally the calculated stress fields in pillar compression and cone indentation are compared. We propose an interpretation of the contrasts in terms of confinement.

Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2-Pillared HNb3O8

Carbon dioxide (CO2) photoreduction by gaseous water over silica-pillared lamellar niobic acid, viz. HNb3O8, was studied in this work. The physicochemical characteristics of samples were examined by techniques such as XRD, FT-IR, SEM, TEM, and UV–visible diffuse reflectance spectroscopy. Aspects that influence CO2 photoreduction, such as the layered structure, the protonic acidity, silica pillaring, and cocatalyst loading, were investigated in detail. Pt loading obvious promoted the activity for CO2 photoreduction to methane. The loading of Pt also promoted the formation of methane from catalyst associated carbon residues, although this contributes insignificantly to the overall amount of methane produced. The layered structure and the protonic acidity of the lamellar niobic acid have significant influences on CO2 photoreduction by water in gas phase. With layered structure, expanded interlayer distance, and stronger intercalation ability to water molecules, the silica pillared niobic acid showed much higher activity than the nonpillared niobic acid, Nb2O5, and TiO2. Because of the unique adsorption ability to water molecules through hydrogen bonding, the activity of silica pillared HNb3O8 increased more remarkably with elevated water content than the mostly investigated TiO2 photocatalyst.

Simultaneous measurement of photoluminescence and Raman scattering spectra from suspended single‐walled carbon nanotubes

Single‐walled carbon nanotubes (SWNTs) are interesting material, owing to their one‐dimensional structure. Photoluminescence (PL) spectroscopy and resonant Raman scattering spectroscopy are important measurement techniques to characterize SWNTs. The simultaneous measurements of PL and Raman scattering spectra have been performed for SWNT samples, which were wrapped with surfactant and dispersed on substrates. Here, we developed the simultaneous measurement technique of PL and Raman scattering from a perfectly isolated SWNT. The isolated SWNT was suspended between a pair of silica pillars, and it was free from any substances. The laser beam generated from a Ti : sapphire laser was focused with objective lens to spot size of 1.6 µm in diameter for the excitation of a suspended SWNT. Raman scattering and PL emission light from an SWNT were separated with a dichroic mirrors and introduced into each monochromator. By using the simultaneous measurement, the intensity profiles along a suspended SWNT were compared between PL and Raman scattering, which showed rapid quenching of PL signals at the pillars. Copyright © 2011 John Wiley & Sons, Ltd.