Silicon Skeleton Research Articles

Structural Features of Porous Silicon Formed on Heavily Doped Plates of Single-Crystal Silicon with Electron Conductivity

Using scanning electron microscopy, the structures of the surface and internal regions of porous silicon obtained by anodizing heavily doped plates of single-crystal silicon with electron conductivity in a hydrofluoric acid solution at different current densities were studied. It is found that the porous silicon surface has dark gray and light gray pores, which differ in size and surface distribution density. Dark gray pores possess larger sizes, and their density is about 5–10 times less than that of light gray pores. Based on the cross-section imagery, it is shown that light gray pores correspond to underdeveloped channels of small depth, while dark gray pores are the entrance points of deep bottle-shaped channels passing from the surface into the depth of the silicon wafer. The equivalent diameters of light gray pores on the surface of porous silicon are 12–15 nm and are practically independent of the anodic current density. At the same time, the equivalent diameters of dark gray pores and average distances between their centers increase linearly from 15 to 35 nm on the surface and from 35 to 120 nm in the volume of porous silicon when the current density is increased from 30 to 90 mA/cm2. The average thickness of silicon skeleton elements is about 3 nm on the surface and increases to 5–6 nm in the volume. By setting the density of the anode current, it is possible to obtain layers of porous silicon with different structural parameters. The obtained research results have practical significance for the formation of composite materials based on porous silicon, which can be used as a porous matrix for the deposition of metals and semiconductors.

Synthesis of coal slime-based catalytic material for peroxymonosulfate activation through molten salt thermal treatment

Coal slime-based peroxymonosulfate (PMS) catalytic materials were synthesized utilizing coal slime (CS) as raw material under the N2 atmosphere in this study. Raw and calcined coal slime were systematically characterized concerning morphologies, pore structures, chemical bonds, phase constituents, and defect density. Results show that after being calcinated with FeCl2∙4H2O at 750 °C, the specific surface area increased by nearly 21.0 times higher than that of CS. The addition of FeCl2∙4H2O has certain effect on the chemical bonds of calcined coal slime, i.e. the Fe elements can be fixed by the silicon aluminum skeleton with sekaninaite (Fe2Al4Si5O18) and aluminum iron silicon (Al0.5Fe3Si0.5, above 650 °C) generation, and the ID/IG values increase to 2.96 for CS-FeCl2-750. In addition, the degradation percentage of phenol by using CS-FeCl2 can achieve 100% in 25 min under optimized conditions. Furthermore, 1O2 plays a major role in phenol degradation, followed by •OH and SO4•− in the CS-FeCl2/PMS system from the quenching experiments and electron paramagnetic resonance results. Overall, the novel CS-FeCl2 material displays good application prospects in the field of effluent treatment, with efficient utilization of carbonaceous and silicon aluminum skeleton in coal slime.

Facile preparation of Bacterial cellulose‐SiO2 composite aerogel with good mechanical strength and thermal stability

AbstractBacterial cellulose is a natural biopolymer with an ultrafine nanofibrous reticulated matrix and crosslinking active sites. In this work, composite silica aerogels were prepared using a two‐step acid‐base catalysis and ambient pressure drying, with methyltriethoxysilane as the silica precursor and a trace amount of bacterial cellulose as the silicon skeleton reinforcement. The preparation was carried out in water without the addition of other organic solvents and without solvent replacement. The preparation conditions were optimized, and the prepared composite aerogels were characterized using SEM, BET, FTIR, a contact angle meter, and GA‐DTA. The results showed that the introduction of a trace amount of bacterial cellulose could obviously change the morphology and hydrophilicity of the aerogels, and increase its compressive strength up to 5 MPa without significant changes in its specific surface area and density. The addition of bacterial cellulose helped the aerogels maintain its original shape after being heated at high temperatures.

Interactions between alkali-activated and cement-based mortars and biofilm and their influence on microbial induced mortar deterioration

Alkali-activated fly ash (AAF) and alkali-activated slag (AAS) mortars were exposed to the sewage environment with acidophilic sulfur oxidizing bacteria and high concentration H2S to study their interaction. It was found that when they suffered the biogenic sulfuric acid attack, both the AAF and calcium aluminate cement (CAC) mortars had a strong inhibition effect on the growth and activity of microorganisms. There are more aluminum ions dissolved from AAF mortar into sewage, and its structure was characterized by the increase in 100-1000 nm pores. Still, a hard high silicon skeleton was formed to keep the apparent intact. However, CAC mortar had a higher killing rate for the bacteria in the attached biofilm. For AAS mortar, the formation of excessive gypsum increased the macropores content and was more conducive to the growth of biofilm, thereby accelerating the penetration of corrosive medium and the deterioration of mortar.

Computer-aided design of molecularly imprinted polymer reinforced by double hybrid monomers for selective purification of hydroxycamptothecin.

A kind of hydroxycamptothecin (HCPT) hybrid molecularly imprinted polymer (AT/MA-HMIPs) with high selectivity and hard silicon skeleton was successfully prepared based on double hybrid monomers. The relationship between templates and functional monomers was studied through computer molecular simulation and experiments. Three single-monomer molecularly imprinted polymers were prepared as controls. The Langmuir isotherm and pseudo-second-order kinetic models were found to fit well with the adsorption results. The maximum adsorption capacity was 18.79 mg/g, and equilibrium was reached within 20 min. Moreover, it shows excellent selectivity (imprinting factor is 10.73) and good recoverability (after 10 adsorption-desorption cycles, the adsorption capacity only decreases by 7.75%) for HCPT. The purity of HCPT can reach 80.86% after being put into a solid phase extraction column and used in an actual sample, and the yield was 61.43%. This study lays the fundament for the development of excellent HCPT molecularly imprinted composites.

Dual Role of Silicon-based Matrices in Electron Exchange Matrices for Waste Treatment.

Para chloro aniline (PCA) is a common toxic pollutant found in pharmaceutical wastewater. Our study suggests a novel PCA treatment method based on a heterogeneous advanced oxidation process (AOP) that proceeds in an electron exchange matrix (EEM) prepared by the incorporation of redox-active specie in silica matrices using the sol-gel synthesis route. The results, which are supported by DFT calculations, show that the silicon skeleton of the EEM has two important roles, both as a porous matrix that hosts the redox species and as an oxidant species involved in the AOP. The calculations indicate that the formation of a radical on the nitrogen is favored. The suggested mechanism could shed light on the AOP, which proceeds in a heterogenous system, and on its application inside the understudied EEMs that, until now, have been a virtual black box. A better understanding of the mechanism could lead to improved control over the heterogeneous processes that can play a critical role in industries with the need to treat small amounts of toxic compounds at low concentrations, such as in the pharmaceutical industry.

High-Branched Organosilicon Epoxy Resin with Low Viscosity, Excellent Toughness, Hydrophobicity, and Dielectric Property

Rapidly developing technology places higher demands on materials, thus the simultaneous improvement of materials' multiple properties is a hot research topic. In this work, a high-branched silicone epoxy resin (QSiE) was synthesized and applied to the curing system of bisphenol A epoxy resin (DGEBA) for modification investigations. When 6 wt% QSiE was added to the system, the viscosity dropped by 51.8%. The mechanical property testing results indicated that QSiE could significantly enhance the material's toughness while preserving good rigidity. The impact strength was enhanced by 1.31 times when 6wt% of QSiE was introduced. Additionally, the silicon skeleton in QSiE has low surface energy and low polarizability, which could endow the material with good hydrophobic and dielectric properties. This work provided a new idea for the preparation of high-performance epoxy resin additives, and provided a broad prospect for cutting-edge applications of epoxy resins.

The regulation of electron distribution on Fe Lewis acidic sites within silicon skeleton and its contribution to Ketoprofen ozonation

Lewis acid site was vital center for activating O3 during catalytic ozonation process. Herein, we proposed a strategy to enhance the Lewis acid strength of Fe-MCM-48 (FeM) by fluorination. Fluorinated Fe-MCM-48 (FFeM) exhibited the better catalytic ozonation efficiency towards Ketoprofen (KET), a kind of nonsteroidal anti-inflammatory drug. 87.4 % TOC removal was obtained by F2.5Fe0.8M/O3 process, but only 27.5 % and 52.7 % were found by sole ozonation and Fe0.8M/O3 processes, respectively. Comparative characterizations, experiments and DFT calculations showed that fluorinated FFeM was regular sphere with three dimensional duct channel and large SBET. Doping of F reduced particle size, implanted electron withdrawing Si-F group and formed the electron poor center around Fe atom, which enhanced Lewis acid strength of FFeM. Protonated hydroxyl groups on Lewis acid sites were the main active sites for O3 activation. Greater amount of O3 were absorbed on the surface of FFeM and the greater O3 utilization rate, •OH and •O2– generation rates were obtained by FFeM/O3 process. Common anions such as Cl−, SO42− and NO3– and Humic acids (HA) had little effect on KET removal but inhibited TOC removal in FFeM/O3 process. •OH, •O2– and Fe(IV) = O oxidation were accounted for KET degradation. 16 kinds of intermediates generated from KET degradation and 4 pathways were deduced.

The Effects of Heat Treatment on Microstructure and Mechanical Properties of Selective Laser Melting 6061 Aluminum Alloy.

Selective laser melting technology can be used for forming curved panels of 6061 aluminum alloy thermal shield devices for the International Thermonuclear Experimental Reactor (ITER), in order to make the formed parts with better performance. This study proposes different heat treatment processes, including annealed treatment at 300 °C for 2 h, solution treatment at 535 °C and then aging at 175 °C over 2 h, to control the mechanical behavior of the 6061 aluminum alloy samples prepared by selective laser melting (SLM). The mechanical properties such as ductility, tensile strength, and hardness of SLM 6061 aluminum alloy were investigated, and the microstructure of the samples was analyzed. The eutectic silicon skeleton shape disappeared after annealing treatment at 300 °C for 2 h. The tensile strength decreased by 22.86% (from 315 MPa to 243 MPa of the deposited state samples), and the elongation increased from 2.01% to 6.89%. Moreover, the hardness reduced from 120.07 HV0.2 to 89.6 HV0.2. After solution aging, the unique microstructure of SLM disappeared. Furthermore, the precipitation of massive Si particles on the α-Al matrix increased, and a trace amount of the Mg2Si(β) phase was generated. Compared with the deposited samples, the tensile strength decreased by 12.06%, while the hardness of specimens was 118.8 HV0.2. However, the elongation showed a remarkable increase of 297% (from 2.01% to 7.97%). Therefore, solution aging can critically improve the plasticity without losing significant tensile stress in the SLM 6061 aluminum alloy. This study proposes the use of SLM 6061 aluminum alloy for the thermal shields on the ITER and provides a reference for choosing a reasonable heat-treatment method for the optimal performance of the SLM 6061 aluminum alloy.

Acoustic properties study of porous materials with orthotropic symmetry and specific pore shapes - I. Analytical calculations

Artificially prepared porous materials are usually highly anisotropic, which could affect the performance of porous materials-based ultrasonic devices. In a previous work, the elastic moduli of the porous silicon skeleton have been evaluated by FEM simulations based on five typical pore shapes with orthotropic symmetry. To study on their acoustic properties, in this article, the compliance tensor Sps for orthotropic porous materials (OPMs) is derived instead of getting directly the stiffness tensor Cps,through the "Gedanken experiments" proposed by Biot, for its easier relation to the measurable elastic moduli of skeleton. Cps for orthotropic porous skeleton is consequently acquired by the inversion ofSps. Besides, the 3-D wave equations for OPMs are derived using Biot's theory and the Christoffel's equations. Four propagating wave modes are therefore obtained from their analytic solutions. Numerical and experimental results for porous silicon are used to validate the proposed methodology. A wave modes distinction method is proposed by using the derivative of the wave velocity with respect to the porosity. The results are compared with literature and good agreements have been observed.

Hierarchical Carbon Shell Compositing Microscale Silicon Skeleton as High-Performance Anodes for Lithium-Ion Batteries

Si is a promising high-capacity anode material; however, its practical implementation is hindered by its huge volume expansion, low initial Coulombic efficiency (ICE), poor cycling life, and high c...

Porous Silicon Skeleton as Catalysts for Hydrocarbon Decomposition at Low Temperature Synthesis of Graphene Nanocomposites

The energy of low-temperature synthesis of graphene on the surface of nanoporous silicon (nc-PS) is considered. It was shown that the energy of low-temperature synthesis of graphene on nc-PS is due to the excess energies of the nanocrystal surface, the interfaces of the nanocrystal nc-Si/c-Si monocrystal matrix, and the free dangling bonds of silicon atoms of nanoscale silicon grains in the skeleton of porous silicon. This opens up a new perspective for the development of methods for the low-temperature synthesis of graphene without metal catalysts for the decomposition of carbon precursors, including the use of the ALD (atomic layer deposition) method.

Mechanism of Reactions of 1-Substituted Silatranes and Germatranes, 2,2-Disubstituted Silocanes and Germocanes, 1,1,1-Trisubstituted Hyposilatranes and Hypogermatranes with Alcohols (Methanol, Ethanol): DFT Study.

The mechanism of reactions of silatranes and germatranes, and their bicyclic and monocyclic analogues with one molecule of methanol or ethanol, was studied at the Density Functional Theory (DFT) B3PW91/6-311++G(df,p) level of theory. Reactions of 1-substituted sil(germ)atranes, 2,2-disubstituted sil(germ)ocanes, and 1,1,1-trisubstituted hyposil(germ)atranes with alcohol (methanol, ethanol) proceed in one step through four-center transition states followed by the opening of a silicon or germanium skeleton and the formation of products. According to quantum chemical calculations, the activation energies and Gibbs energies of activation of reactions with methanol and ethanol are close, their values decrease in the series of atranes–ocanes–hypoatranes for interactions with both methanol and ethanol. The reactions of germanium-containing derivatives are characterized by lower activation energies in comparison with the reactions of corresponding silicon-containing compounds. The annular configurations of the product molecules with electronegative substituents are stabilized by the transannular N→X (X = Si, Ge) bond and different intramolecular hydrogen contacts with the participation of heteroatoms of substituents at the silicon or germanium.

High Capacity Retention of Batteries Using Scalable Micro-Structured Silicon-Polymer Composite As Anode

The emerging demands in large energy storage for portable electronic devices and transportations are attracting more interest in developing next-generation lithium-ion batteries (LIBs) with a series of new electrode materials, thus achieving a 3-5 times higher capacity to meet a strategic perspective energy-density of 500 Wh/kg or higher. Silicon, with a theoretical capacity up to 4000 mAh/g, is considered as one of the promising anodes to achieve this goal. The industry R&D in Si-leaded anodes is widespread, and the release of commercial products for portable electronics seems imminent, although simultaneous achievement of high capacity and sufficient calendar life for automotive applications remains to be demonstrated due to a 360% volume expansion of silicon. The current ratio of silicon in graphite anode is less than 10%, which largely limits the overall energy density. The degraded battery performance and complex synthesis process of various nano-structured configurations pose challenges on mass production industry.Here we present the improved battery performance using micro-sized silicon skeleton caged by polymer as anode, which was synthesized via a facile wet-chemical strategy ( ACS Nano , 13 (2019), 12032-12041). The industry available, micro-sized AlSi alloy was used as precursor, which ensures a scalable production with low cost. The hollow skeleton configuration provides sufficient spaces to accommodate the drastic volume expansion/shrinkage upon charging/discharging while the minimum amount of conductive polymer serves as a protective layer and fast channel for Li+/e- transport. The battery with the micro-silicon cage as anode displays an excellent capacity retention upon long cycling at high charge/discharge rates and high material loadings. The areal capacity of as high as >6.0 mAh/cm2 was obtained, which suffices a high battery energy density in powering large devices such as electric vehicles (EVs).

FEM simulation on elastic parameters of porous silicon with different pore shapes

Porous silicon (PSi) seems to be a promising material for acoustic transducers due to its manufacturing ease and high abilities. Yet, the investigation on wave propagation in this porous material is required to enhance the sensitivity. According to Biot’s theory, wave propagation is governed by the porous matrix moduli (i.e. Young’s modulus, bulk modulus, and shear modulus). These parameters as well as Poisson ratios are difficult to measure and are often estimated using ultrasonic measurements. In this paper, the elastic parameters of porous silicon skeleton varying with porosity are investigated using FEM simulations. The silicon substrate from which the PSi is formed has a (100) crystallographic orientation and the pores are cylinder-like. Six pore shapes were designed in order to investigate the influence of pore morphology on elastic parameters. These shapes are chosen according to the pore geometries obtained by electrochemical etchingand designed as orthotropic structures. The results show that the morphology of the pores has a significant influence on the elastic moduli, therefore, should be taken into consideration in further research. The simulation results are in good agreement with the experimental data reported in literature for Young’s modulus. Moreover, the power-law fittings of Young’s moduli and the shear moduli as functions of porosity are proposed and the fitting errors are discussed.

Largely Improved Battery Performance Using a Microsized Silicon Skeleton Caged by Polypyrrole as Anode.

Various architectures with nanostructured silicon have demonstrated promising battery performance while posing a challenge in industrial production. The current ratio of silicon in graphite as anode is less than 5 wt %, which greatly limits the battery energy density. In this article, we report a scalable synthesis of a large silicon cage composite (micrometers) that is composed of a silicon skeleton and an ultrathin (<5 nm) mesoporous polypyrrole (PPy) skin via a facile wet-chemical method. The industry available, microsized AlSi alloy was used as precursor. The hollow skeleton configuration provides sufficient space to accommodate the drastic volume expansion/shrinkage upon charging/discharging, while the conductive polymer serves as a protective layer and fast channel for Li+/e- transport. The battery with the microsilicon (μ-Si) cage as anode displays an excellent capacity retention upon long cycling at high charge/discharge rates and high material loadings. At 0.2 C, a specific capacity of ∼1660 mAh/g with a Coulombic efficiency (CE) of ∼99.8% and 99.4% was achieved after 500 cycles at 3 mg/cm2 loading and 400 cycles at 4.4 mg/cm2 loading, respectively. At 1.0 C, a capacity as high as 1149 mAh/g was retained after 500 cycles with such high silicon loading. The areal capacity of as high as 6.4 mAh/cm2 with 4.4 mg/cm2 loading was obtained, which ensures a high battery energy density in powering large devices such as electric vehicles.

Adaptation of SERS-Active Materials to Analysis of Certain Molecules By Electroless Corrosive Deposition of Coinage Metals on Silicon Nanostructures

Here we present results of our research work on an electroless corrosive deposition of coinage metals on nanostructured silicon. Principal steps of nanostructured silicon fabrication and further deposition of metals allowing growth of metallic films controlled by dopant type of a silicon substrate, morphology of silicon skeleton and electroless deposition regimes are presented. Imprinting of the nanostructured pattern to metallic films by means of the sculpted silicon template opens their new morphological, optical, mechanical and electrical properties, which can be successfully applied in micro- and nanosystems and biomedicine. In the context of the latter, we pay special attention to activity of the metal-coated silicon nanostructures in surface enhanced Raman scattering (SERS) spectroscopy and profits provided both by using silicon templating and electroless deposition of metals. The electroless corrosion is mostly associated with undesirable surface destruction of different materials. Indeed, corrosion can lead to the irreparable harm both in macro- and nanoscaled worlds. Oxidation/degradation of metallic components of massive constructions in aggressive environment are very dangerous as well as corrosion of conductive metallic wires in ICs and MEMS causing further failure of the whole device. Surprisingly, but it has opened an opportunity to turn corrosion effect into positive way in some applications. For instance, since the 90-th of the last century corrosion of monocrystalline silicon (c-Si) in solutions for chemical cleaning, containing trace amounts of Cu ions, has been actively studied [1]. Solutions for chemical cleaning usually contain deionized water and HF. However, the Cu ions can unpredictably appear in solutions from environment on different stages of ICs production. Corrosion of c-Si also takes a place in HF-based solutions containing ions of other coinage metals (Ag, Au, etc.). Understanding mechanism of corrosion has allowed to apply this process for fabrication of the Cu conductive interconnections, which possess improved adhesion to the Si wafers [2]. New wave of interest to this process has been raised due to an opportunity to fabricate metallic nanostructures of variable morphology by electroless corrosive deposition of metals on silicon nanostructures. During our work related to the electroless corrosive deposition of coinage metals on silicon nanostructures we found that this process occurs according to mechanism of island film growth. It was revealed that Si nanocrystallites corrode faster than c-Si because of a great number of the surface states. Metallic atoms were observed to prevalently deposit on an external surface of silicon skeleton based on n-Si because of excess of electrons from dopant atoms. On the other hand, the inner growth of metallic atoms has been found to dominate on the substrates based on p-Si due to absence of dopant atoms supplying electrons. In the last case the reduction of metallic ions to the atomic form is connected only with taking electronts from the silicon atoms. As a result of our systematic investigations we developed a technique of electroless corrosive deposition of Cu, Ag and Au on nanostructured Si, which provides precise control of metallic deposit morphology from quasi-continuous films consisted of metallic nanoparticles to dendrites or nanovoids. The obtained films were found to show activity in the SERS-spectroscopy that is known as an extremely sensitive method of qualitative and quantitative analysis in analytical chemistry, biomedicine, environment and food control, forensics, etc. It was shown that nanosculpted silicon acts not only as a source of electrons for reduction of metal ions and template defining shape, sizes and special location of metallic nanostructures, but also prevents their fast oxidation and coalescence, which usually lead to the inhibition of SERS-activity. SERS-activity of the substrates was observed even for the 2 years aged samples. Morphology of these substrates can be adjusted to detect certain analyte molecules in dependence on their molecular weight including to achieve attomolar detection limit. [1] H. Morinaga, M. Suyama, T. Ohmi, J. Electrochem. Soc. 1994, 141, 2834. [2] J.L. Gole, L.T. Seals, P.T. Lillehei, J. Electrochem. Soc. 2000, 147, 3785–3789.

ZnO on rice husk: A sustainable photocatalyst for urban air purification

A cost-effective and sustainable De-NOx photocatalyst is prepared using zinc acetate and rice husk. ZnO@SiO2 samples are obtained from the calcination of a homogenised precursor mixture at 600 °C. ZnO nanoparticles (70–180 nm) grow aggregated in spheres and well dispersed (40–53 m2 g−1 surface area) covering the silicon skeleton. The corresponding band gap for ZnO@SiO2 photocatalysts was estimated at 3.1–3.2 eV. When the samples are irradiated by sunlight in a nitrogen oxide atmosphere the NO → HNO2 → NO2 → NO3− photochemical oxidation takes place. In comparison to unsupported ZnO and TiO2-P25, ZnO@SiO2 samples exhibit high NOX removal values (70%) and outstanding selectivity (>90%), the latter related to the sensitivity of zinc oxide towards NO2 gas. This new photocatalyst is easily recyclable and reusable.

The planarity of heteroatom analogues of benzene: Energy component analysis and the planarization of hexasilabenzene.

There are various nonplanar heteroatom analogues of benzene-cyclic 6π electron systems-and among them, hexasilabenzene (Si6 H6 ) is well known as a typical example. To determine the factors that control their planarity, quantum chemical calculations and an energy component analysis were performed. The results show that the energy components mainly controlling the planarity of benzene and hexasilabenzene are different. For hexasilabenzene, electron repulsion energy was found to be significantly important for the planarity. The application of the pseudo Jahn-Teller effect and the Carter-Goddard-Malrieu-Trinquier model for the interpretation of the planarity of the benzene analogues was also investigated. Furthermore, based on the quantitative results, it was revealed that the planarization of hexasilabenzene is realized by introducing substituents with π-accepting ability, such as the boryl group, that bring about a reduction of the π-electron repulsion on the silicon skeleton. © 2018 Wiley Periodicals, Inc.

Catalytic cracking of rubber seed oil using basic mesoporous molecular sieves K2O/MeO-SBA-15 (Me = Ca, Mg or Ba) as heterogeneous catalysts for the production of liquid hydrocarbon fuels

Liquid hydrocarbon fuels obtained from the catalytic cracking animal fats and plant oils have become one kinds of the attractive fuels because of their possible environment benefits and the current concern over the depletion of fossil fuel sources. In this work, using the combined methods of one-pot synthesis and wetness-impregnation, some basic mesoporous molecular sieves K2O/MeO-SBA-15 (Me = Ca, Mg or Ba) were prepared, characterized and used in the catalytic cracking of rubber seed oil (RSO). The results indicated that the catalysts K2O/MeO-SBA-15 had better catalytic performances than MeO-SBA-15, assigning to their stronger basicity. The catalyst K2O/MgO-SBA-15 obtained with 15 wt% KNO3 impregnation concentration showed the excellent catalytic performance with about 93.2% conversion and 78.3% yield of liquid hydrocarbon biofuel. The obtained liquid biofuel had similar chemical composition to diesel-based fuels and showed good cold flow property, high calorific and low acid value. Importantly, the catalyst K2O/MgO-SBA-15 was of excellent reusability, and it was reused with negligible loss in its catalytic performance for five times, attributing to the MgO layer between silicon skeleton and potassium species which prevents the reaction between silicon in the framework and potassium species.