Zerovalent Silicon Research Articles

Rapid mechanochemical dechlorination of hexachlorobenzene with zero-valent silicon as a novel additive: The attempt of zero-valent nonmetallic

The used of different zero-valent metals as reducing agents for dechlorination of polychlorinated biphenyl (PCB) based on mechanochemical method have been studied intensively, while few studies have been conducted on zero-valent nonmetallic reducing agents. Therefore, in this study, zero-valent silicon (ZVS), easily obtainable from photovoltaic panel waste in near future, has been used to serve the purposes as additive to dechlorinate persistent organic pollutants i.e., hexachlorobenzene (HCB). High-energy ball milling rapidly removes the passivated layer from the ZVS surface, exposing its fresh surface to release free electrons for the dechlorination of HCB at shortened time with eventually condensed amorphous carbon and hydrated silica as final products, safe to the soil. The degradation efficiency could achieve at 96.12% and dechlorination rate at 83.91% within only 60 min milling and 10% additive amount even if the concentration of HCB in simulated contaminated samples was as high as 5%. Various characterizations have been performed to fully analyze the intermediate phases and to understand the decomposition mechanism. Cyclic voltammograms (CV) curve and Tafel polarization scanning test as electrochemical evaluation method were performed to explore the involvement of electron release in the HCB remediation, for explaining the superior performance of ZVS compared to other additives based on radical dechlorination pathway. Overall, this work is expected to introduce a brand-new alternative toward practical application of mechanochemical remediation for the polluted soils, not only from environmental remediation perspective, but also the waste disposal.

Selective removal of copper from heavy-metals-containing acidic solution by a mechanochemical reduction with zero-valent silicon

It is always a challenge to deal with industrial wastewater containing heavy metals. Selective removal and recovery of valuable metals such as copper particularly from acidic solutions are very important from both stances of resource supplying and environmental purification. Among all the divalent heavy metals, copper has the potential of being reduced to monovalent state. A new approach was proposed to ball mill zero-valent silicon (ZVS) in CuSO4 solution and the solutions containing multi-metal ions to serve the purpose. In this study, both quantitative and qualitative data were reported on the induced mechanochemical reduction reaction to convert Cu2+ into Cu2O by ball milling with ZVS. Experimental parameters such as milling time and Si to Cu ratio were found to play important role in stimulating the reaction, and as a result, 97.32% Cu2+ was removed from the solution under Si/Cu molar ratio of 100. Even at a high concentration of sulfuric acid up to 4.6 mol/L, the copper removal percentage maintained a high value of 82.57%. Furthermore, in the sulfate solutions containing several other heavy metals of Zn, Mn, Ni, Cd, Co, Fe etc., the reduction reaction by zero-valent silicon occurred only on Cu2+ with other heavy metal ions remaining in the solutions. The feasibility of this technology has been verified by treating a leaching solution of NiS mineral, from which copper has been removed selectively from nickel, iron etc. This research may serve another purpose for applying waste silicon from the solar energy devices after lifetime in the future.

Mechanically activated zero-valent silicon by coating silica to decolorize Acid Red 73 dye

For the treatment of various contaminants involving the reduction action, utilization of zero-valent silicon (ZVS) may be expected due to its higher reactivity than zero-valent iron (ZVI) which has been predominantly investigated. However, the higher reactivity also means the easier passivation of ZVS, to which new approaches are required to prepare an active ZVS sample. In this study, a novel composite material consisting of microscale ZVS and silica was prepared by one-step mechanical ball milling. Such an easy operation of co-grinding allowed the generation of active ZVS sample through coating silica layer on the newly-generated surface of ZVS particles, which could prevent the easy passivation to lose reactivity when exposed to air. Typical experimental conditions for preparing active ZVS were: the mass ratio of ZVS to silica of 50 wt%, milling parameters with speed at 600 rpm and time for 60 min. A nearly complete decolorization of AR73 at a concentration of 100 mg/L was achieved by using the sample dosage at 2 g/L and stirring for 120 min. Results from this study suggest that tremendous amounts of Si sample from the solar cell after the termination of service life could serve as raw sample for preparing active reductive agents to deal with various contaminants, offering alternative to ZVI with high potentials for environmental remediation.

Entering new chemical space with isolable complexes of single, zero-valent silicon and germanium atoms

Herein, we present the chemistry of isolable monatomic silicon(0) and germanium(0) complexes, highlighting their synthesis, structure, and reactivity, with a particular focus on the cyclic bis-carbene- and bis-silylene-supported silylones and germylones.

Redox Noninnocent Monoatomic Silicon(0) Complex ("Silylone"): Its One-Electron-Reduction Induces an Intramolecular One-Electron-Oxidation of Silicon(0) to Silicon(I).

A monatomic zerovalent silicon(0) complex ("silylone") stabilized by the chelating bis(silylenyl)-ortho-carborane ligand, 1,2-(LSi)2-1,2-C2B10H10 [L = PhC(NtBu)2], has been synthesized from the redox reaction of the dipotassium bis(silylenyl)-nido-carboranate salt, 1,2-(LSi)2-1,2-C2B10H10K2, and NHC-SiCl2 (NHC = {[HCN(2,6-iPr2C6H3)]2C:}). Markedly different from previous examples, this silylone undergoes reduction due to the closo-C2B10 cluster backbone, which is prone to accept up to two electrons to form the cage-opened dianionic nido-C2B10 cluster core. Surprisingly, the closo-C2B10 core of the silylone consumes only one molar equiv of potassium naphthalenide, in addition, one electron is intramolecularly transferred from the Si0 atom to the C2B10 core to form an elusive bis(silylene)-stabilized SiI radical cation which undergoes homocoupling to the corresponding isolable dicationic SiI-SiI complex.

Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules.

The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound, [SiII(Xant)SiII]Si0 (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[SiII(Xant)SiII]SiCl}+Cl- (2). The electronic structure of silylone 4, whose molecular structure is confirmed spectroscopically and crystallographically, is investigated by DFT calculations and Natural Bond Orbital analysis, showing two perpendicular lone-pairs of electrons on the central Si0 atom, i.e., an sp0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si0 center, silylone 4 exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation of silylone 4 by N2O can be controlled to generate distinct products by regulating the amount of added N2O. Exposing 4 to an excess or two molar equivalents of N2O yields the unexpected oxidation product 5, bearing a central six-membered Si4O2 ring. When 4 is mixed with one molar equivalent of N2O, the unique compound 6 is obtained, resulting from a rare 1,4-addition of two central silicon atoms to a phenyl ring of an amidinate ligand coordinated to the SiII atom. In addition, cleavage of the strong N-H bond in ammonia is also readily accomplished by silylone 4, representing the first example of NH3 activation in silylone chemistry. In the presence of the Lewis acid BPh3, silylone 4 achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which represent a new type of frustrated Lewis pair based on an electron-rich Si0 donor and a borane acceptor.

A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues.

Monoatomic zerovalent main-group element complexes emerged very recently and attracted increasing attention of both theoretical and experimental chemists. In particular, zerovalent silicon complexes and their congeners (metallylones) stabilized by neutral Lewis donors are of significant importance not only because of their intriguing electronic structure but also because they can serve as useful building blocks for novel chemical species. Featuring four valence electrons as two lone pairs at the central atoms, such complexes may form donor-acceptor adducts with Lewis acids. More interestingly, with the central atoms in the oxidation state of zero, they could pave a way to new classes of compounds and functional groups that are otherwise difficult to realize. In this Account, we mainly describe our contributions in the chemistry of monatomic zerovalent silicon (silylone) and germanium (germylone) supported by a chelate bis-N-heterocyclic carbene (bis-NHC) ligand in the context of related species developed by other groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene [:SiCl]+ and chlorogermyliumylidene [:GeCl]+ as suitable starting materials, we successfully isolated silylone (bis-NHC)Si and germylone (bis-NHC)Ge, respectively. The electronic structures of the latter complexes established by theoretical calculations and spectroscopic data revealed that they are genuine metallylone species with electron-rich silicon(0) and germanium(0) centers. Accordingly, they can react with 1 molar equiv of GaCl3 to form Lewis adducts (bis-NHC)E(GaCl3) (E = Si, Ge) and with 2 molar equiv of ZnCl2 to furnish (bis-NHC)Si(ZnCl2)2. Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon(II) and germanium(II) monochalcogenides (bis-NHC)EX(GaCl3) (X = Se, Te), representing molecular heavier congeners of CO. Moreover, their reaction with elemental chalcogens can also yield monomeric silicon(IV) and germanium(IV) dichalcogenides (bis-NHC)EX2 (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO2. Moreover, (bis-NHC)Si could even activate CO2 to afford the monomolecular silicon dicarbonate complex (bis-NHC)Si(CO3)2 via the formation of SiO and SiO2 complexes as intermediates. Furthermore, starting with a chelate bis-N-heterocyclic silylene supported [:GeCl]+, we developed two bis-N-heterocyclic silylene stabilized germylone→Fe(CO)4 complexes. Our achievements in the chemistry of metallylones demonstrate that the characteristic of monatomic zerovalent silicon and its analogues can provide novel reaction patterns for access to unprecedented species and even extends the series of functional groups of these elements. With this, we can envision that more interesting zerovalent complexes of the main-group elements with unprecedented reactivity will follow in the near future.

An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone.

The first isolable molecular silicon dicarbonate complex (bis-NHC)Si(CO3 )2 2 (bis-NHC=H2 C[{NC(H)=C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) was synthesized by facile reaction of the bis-N-heterocyclic carbene stabilized silylone (bis-NHC)Si 1, bearing a zero-valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis-NHC)SiO2 3 supported by the bis-NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero-valent silicon atom in 1 by two molar equivalents of CO2 under liberation of CO; its subsequent Lewis acid-base reaction with CO2 leads to 2 which has been fully characterized including an single-crystal X-ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum-chemically.

From Silylone to an Isolable Monomeric Silicon Disulfide Complex.

The synthesis and characterization of the first bis-N-heterocyclic carbene stabilized monomeric silicon disulfide (bis-NHC)SiS2 2 (bis-NHC=H2 C[{NC(H)C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) is reported. Compound 2 is prepared in 89 % yield from the reaction of the zero-valent silicon complex ('silylone') 1 [(bis-NHC)Si] with elemental sulfur. Compound 2 can react with GaCl3 in acetonitrile to give the corresponding (bis-NHC)Si(S)S→GaCl3 Lewis acid-base adduct 3 in 91 % yield. Compound 3 is also accessible through the reaction of the unprecedented silylone-GaCl3 adduct [(bis-NHC)Si→GaCl3 ] 4 with elemental sulfur. Compounds 2, 3, and 4 could be isolated and characterized by elemental analyses, HR-MS, IR, (13) C- and (29) Si-NMR spectroscopy. The structures of 3 and 4 could be determined by single-crystal X-ray diffraction analyses. DFT-derived bonding analyses of 2 and 3 exhibited highly polar Si-S bonds with moderate pπ -pπ bonding character.

Enhanced dechlorination of tetrachloroethylene by polyethylene glycol-coated zerovalent silicon in the presence of nickel ions

In this study, the combined effect of polyethylene glycol (PEG) and nickel ions on the dechlorination of tetrachloroethylene (PCE) by zerovalent silicon (Si(0)) under anoxic conditions was investigated. PEG was first coated onto the surface of Si(0) to form PEG-coated Si(0) (SiPEG) and the concentration effect of Ni(II) ions at 0.001–2mM on the dechlorination of PCE by SiPEG was evaluated and optimized. Results of X-ray photoelectron spectroscopy and electron probe microanalysis showed that addition of PEG can not only inhibit the formation of SiO2 but also change the distribution pattern of SiO2 from homogeneous distribution to discrete and localized dispersion, resulting in the enhancement of dechlorination efficiency and rat of PCE by Si(0). The dechlorination of PCE by SiPEG followed pseudo-first-order kinetics and the pseudo-first-order rate constant (kobs) for PCE dechlorination was 0.36±0.02h−1. Addition of Ni(II) exhibited the synergistic effect on the enhanced dechlorination of PCE by SiPEG, and the maximum kobs for PCE dechlorination was 0.79±0.02h−1, which corresponded to 220–335 times higher than that by Si(0) alone. Hydrogenolysis and hydrodechlorination occurred simultaneously for PCE dechlorination by bimetallic Ni/SiPEG system in the presence of Ni(II) ions, and the product distribution was highly dependent on the mass loading of Ni ions. The surface coverage of Ni onto SiPEG was used to well describe the synergistic effect of Ni(II) and SiPEG on the enhanced dechlorination of PCE by Si(0) under anoxic conditions.

Binding Waste Anthracite Fines with Si-Containing Materials as an Alternative Fuel for Foundry Cupola Furnaces

An alternative fuel to replace foundry coke in cupolas was developed from waste anthracite fines. Waste anthracite fines were briquetted with Si-containing materials and treated in carbothermal (combination of heat and carbon) conditions that simulated the cupola preheat zone to form silicon carbide nanowires (SCNWs). SCNWs can provide hot crushing strengths, which are important in cupola operations. Lab-scale experiments confirmed that the redox level of the Si-source significantly affected the formation of SiC. With zerovalent silicon, SCNWs were formed within the anthracite pellets. Although amorphous Si (+4) plus anthracite formed SiC, these conditions did not transform the SiC into nanowires. Moreover, under the test conditions, SiC was not formed between crystallized Si (+4) and anthracite. In a full-scale demonstration, bricks made from anthracite fines and zerovalent silicon successfully replaced a part of the foundry coke in a full-scale cupola. In addition to saving in fuel cost, replacing coke by waste anthracite fines can reduce energy consumption and CO2 and other pollution associated with conventional coking.

Enhanced Dechlorination of Tetrachloroethylene by Zerovalent Silicon in the Presence of Polyethylene Glycol under Anoxic Conditions

The combination of zerovalent silicon (Si(0)) with polyethylene glycol (PEG) is a novel technique to enhance the dechlorination efficiency and rate of chlorinated hydrocarbons. In this study, the dechlorination of tetrachloroethylene (PCE) by Si(0) in the presence of various concentrations of PEG was investigated under anoxic conditions. Several surfactants including cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 were also selected for comparison. Addition of SDS and Tween 80 had little effect on the enhancement of PCE dechlorination, while CTAB and PEG significantly enhanced the dechlorination efficiency and rate of PCE by Si(0) under anoxic conditions. The Langmuir-Hinshelwood model was used to describe the dechlorination kinetics of PCE and could be simplified to pseudo-first-order kinetics at low PCE concentration. The rate constants (k(obs)) for PCE dechlorination were 0.21 and 0.36 h(-1) in the presence of CTAB and PEG, respectively. However, the reaction mechanisms for CTAB and PEG are different. CTAB could enhance the apparent water solubility of PCE in solution containing Si(0), leading to the enhancement of dechlorination efficiency and rate of PCE, while PEG prevented the formation of silicon dioxide, and significantly enhanced the dechlorination efficiency and rate of PCE at pH 8.3 ± 0.2. In addition, the dechlorination rate increased upon increasing PEG concentration and then leveled off to a plateau when the PEG concentration was higher than 0.2 μM. The k(obs) for PCE dechlorination by Si(0) in the presence of PEG was 106 times higher than that by Si(0) alone. Results obtained in this study would be helpful in facilitating the development of processes that could be useful for the enhanced degradation of cocontaminants by zerovalent silicon.

Concentration effect of copper loading on the reductive dechlorination of tetrachloroethylene by zerovalent silicon

The dechlorination of tetrachloroethylene (PCE) by zerovalent silicon (Si(0)) in the presence of low concentration of Cu(II) ion was investigated under anaerobic conditions. The mass loadings of Cu(II) in the Si(0)-H(2)O system were in the range 0.06-3 wt% (0.02-1 mM). In addition, the X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA) were used to characterize the change in chemical species and distribution patterns of metals, respectively. Results showed that the pre-incubation time of 3 d was needed to activate the reactive sites of Si(0) before the dechlorination of PCE. Addition of low concentration of Cu(II) at 0.06 wt% significantly enhanced the dechlorination of PCE, while high concentration of Cu(II) would occupy the reactive sites of Si(0), and subsequently decreased the dechlorination efficiency and rate of PCE. The pseudo first-order rate constant (k(obs)) for PCE dechlorination by 0.06 wt% Cu/Si was 0.028 h(-1), which was 2.8 times higher than that by Si(0) alone. However, the k(obs) for PCE dechlorination decreased to 0.0016 h(-1) when the loading of Cu(II) increased to 3 wt%. The EPMA results showed that the distribution of 0.06 wt% Cu on the Si(0) surface was homogeneous without any aggregation, which means that the maximum rate constant was observed before the total coverage of the active sites on the reductive metal by the catalytic metal layer. The surface coverage of Cu to Si(0) can theoretically calculate by estimation of the lowest energy fcc(111) crystallographic orientation. The calculated surface coverage of 0.06 wt% Cu onto Si(0) was approximately 43%, which is consistent with the experimental results obtained in this study.

Dechlorination of Tetrachloroethylene in Aqueous Solutions Using Metal-Modified Zerovalent Silicon

The combination of zerovalent metal with a catalytic second metal ion (bimetallic materials) to enhance the dechlorination efficiency and rate of chlorinated compounds has received much attention. Bimetallic materials not only enhance the dechlorination process but also alter the reduction pathway and product distribution. In this study, the efficiency and rate of tetrachloroethylene (PCE) dechlorination by metal-modified zerovalent silicon was investigated as a potential reductant for chlorinated hydrocarbons under anoxic conditions. The X-ray photoelectron spectroscopic (XPS) results showed that metal ions including Ni(II), Cu(II), and Fe(II) could be reduced to their zerovalent forms on the Si surface. The dechlorination of PCE obeyed the pseudo-first-order kinetics, and the pseudo-first-order rate constants (k(obs)) for PCE dechlorination followed the order Ni/Si > Fe/Si > Cu/Si. Addition of Cu(II) lowered the dechlorination efficiency and rate of PCE by Si, while the k(obs) values for PCE dechlorination in the presence of 0.1 mM Fe(II) and Ni(II) were 1.5-3.8 times higher than that by Si alone. In addition, the efficiency and rate of PCE dechlorination increased upon increasing the mass loading of Ni(II) ranging between 0.05 and 0.5 mM and then decreased when the Ni(II) loading was further increased to 1 mM. The scanning electron microscopic (SEM) images and electron probe microanalytical (EPMA) maps showed that the Ni nanoparticles deposited on the Si surface and aggregated to a large particle at 1 mM Ni(II), which clearly depicts that the Ni(II) loading of 0.5 mM is the optimal value to enhance the efficiency and rate of PCE dechlorination by Si. Also, the reaction pathways for PCE dechlorination changed from hydrogenolysis in the absence of Ni(II) to hydrodechlorination when Ni(II) concentrations were higher than 0.05 mM. Results obtained in this study reveal that the metal-deposited zerovalent silicon can serve as an environmentally friendly reductant for the enhanced degradation of chlorinated hydrocarbons for long-term performance.

Coupled reduction of chlorinated hydrocarbons and heavy metals by zerovalent silicon

The feasibility of using zerovalent silicon (Si0) as a novel reductant to remove chlorinated compounds and heavy metals in contaminated sites was investigated. The kinetics and degradation mechanism of carbon tetrachloride (CT) by Si0 were also examined. Results showed that zerovalent silicon could effectively dechlorinate the chlorinated compounds. A nearly complete dechlorination of CT by Si0 was obtained within 14 h. The produced concentrations of chloroform (CF) accounted for 71-88% loss of CT, showing that reductive dechlorination is the major degradation pathway for the degradation of chlorinated hydrocarbons by Si0. The degradation followed pseudo first-order kinetics and the normalized surface reaction rate constant (k(sa)) for CT dechlorination ranged between 0.0342 and 0.0454 L m(-2) h(-1) when CT concentrations were in the range of 3-20 microM. A linear relationship between the k(sa) and pH value was also established. In addition, zerovalent silicon has a high capability in the removal of heavy metals. 83% of Cr(VI) was removed by 0.5g Si0 within 5 h, which is higher than that by Fe0. The removal efficiency of divalent metal ions by Si0 followed the order of Cu(II) > Pb(II) > Ni(II). This indicates that zerovalent silicon is an alternative reductant and can undergo coupled reduction of heavy metals and chlorinated hydrocarbons in contaminated groundwater.

Reductive dechlorination of carbon tetrachloride and tetrachloroethylene by zerovalent silicon-iron reductants.

Reductive dechlorination of carbon tetrachloride (CT) and tetrachloroethylene (PCE) by zerovalent silicon (ZVS, Si0) and the combination of Si0 with metal iron (Fe0) was investigated as potential reductants for chlorinated hydrocarbons. The X-ray photoelectron spectroscopy (XPS) was used to identify the surface characteristics of Si0. CT and PCE can be completely degraded via sequential reductive dechlorination to form lesser chlorinated homologues by Si0. Productions of chloroform (CF) and trichloroethylene (TCE) accounted for 80% of CT and 65% of PCE dechlorination, respectively. The degradation of CT and PCE by Si0 at pH 8.3 followed pseudo-first-order kinetics, and the normalized surface rate constants (k(sa)) were 0.288 and 0.003 L m(-2) h(-1), respectively, which react more efficiently than zerovalent iron in CT and PCE dechlorination. A linear relationship was also established between pH and the k(sa) value. The XPS results showed that the hydrogenated silicon surface and silicon oxides on the silicon surface were removed during the dechlorination processes, thus providing a relatively clean silicon surface for dechlorination reactions. The combination of zerovalent silicon with iron influences both the dechlorination rate and the distribution of products. Sequential reductive dechlorination was still the main reaction for CT dechlorination by Si0/Fe0, while reductive dechlorination and beta-elimination were the dominant reaction pathways for PCE dechlorination with ethane and ethene as the major end products. Also, the combination of silicon and iron constitutes a buffer system to maintain the pH at a stable value. A 0.3 unit of pH changed upon increasing the amount of Fe by a factor of 35 was observed, depicting that Si0 serves as a pH buffer in Si0/Fe0 system during dechlorination processes.