Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic, Green, and Sustainable Sources

Sustainable Vat Photopolymerization-Based 3D-Printing through Dynamic Covalent Network Photopolymers

Vat photopolymerization (VPP) based three-dimensional (3D) printing, including stereolithography (SLA) and digital light projection (DLP), is known for producing intricate, high-precision prototypes with superior mechanical properties. However, the challenge lies in the non-recyclability of covalently crosslinked thermosets used in these printing processes, limiting the sustainable utilization of printed prototypes. This review paper examines the recently explored avenue of VPP 3D-printed dynamic covalent network (DCN) polymers, which enable reversible crosslinks and allow for the reprocessing of printed prototypes, promoting sustainability. These reversible crosslinks facilitate the rearrangement of crosslinked polymers, providing printed polymers with chemical/physical recyclability, self-healing capabilities, and degradability. While various mechanisms for DCN polymer systems are explored, this paper focuses solely on photocurable polymers to highlight their potential to revolutionize the sustainability of VPP 3D printing.

Closed-Loop Recyclable Silica-Based Nanocomposites with Multifunctional Properties and Versatile Processability

Most plastics originate from limited petroleum reserves and cannot be effectively recycled at the end of their life cycle, making them a significant threat to the environment and human health. Closed-loop chemical recycling, by depolymerizing plastics into monomers that can be repolymerized, offers a promising solution for recycling otherwise wasted plastics. However, most current chemically recyclable polymers may only be prepared at the gram scale, and their depolymerization typically requires harsh conditions and high energy consumption. Herein, it reports less petroleum-dependent closed-loop recyclable silica-based nanocomposites that can be prepared on a large scale and have a fully reversible polymerization/depolymerization capability at room temperature, based on catalysis of free aminopropyl groups with the assistance of diethylamine or ethylenediamine. The nanocomposites show glass-like hardness yet plastic-like light weight and toughness, exhibiting the highest specific mechanical strength superior even to common materials such as poly(methyl methacrylate), glass, and ZrO2 ceramic, as well as demonstrating multifunctionality such as anti-fouling, low thermal conductivity, and flame retardancy. Meanwhile, these nanocomposites can be easily processed by various plastic-like scalable manufacturing methods, such as compression molding and 3D printing. These nanocomposites are expected to provide an alternative to petroleum-based plastics and contribute to a closed-loop materials economy.

Biobased Silicon and Biobased Silica: Two Production Routes Whose Time has Come

This study offers an updated bioeconomy perspective on biobased routes to high-purity silicon and silica in the context of the societal, economic and environmental trends reshaping chemical processes. We summarize the main aspects of the green chemistry technologies capable of transforming current production methods. Coincidentally, we discuss selected industrial and economic aspects. Finally, we offer perspectives of how said technologies could/will reshape current chemical and energy production.

Influence of Different Treatments on the Structure and Conversion of Silicon Species in Rice Straw to Tetraethyl Orthosilicate (TEOS)

The production of tetraethyl orthosilicate (TEOS) from biomass provides a new way for TEOS production and biomass valorization. In this study, rice straw was treated using different fractionation methods, and the content, state, and reactivity of Si in the treated samples were investigated. It was found that acid treatment and ethanol extraction kept most Si in the biomass, while alkali treatment caused significant Si loss. Si was mainly present in the SiOx , Si-O-C, and Si-O-Si states in the surface of raw rice straw, cellulose and Klason lignin. The results showed that the Si-O-Si state in rice straw was beneficial for the formation of TEOS. The removal of lipids from rice straw facilitated the production of TEOS, giving the highest TEOS yield of 76.2 %. In contrast, the production of TEOS from other samples became difficult; the simultaneous conversion of the three organic components of rice straw also facilitated the production of TEOS.

Carbon-Free Conversion of SiO2 to Si via Ultra-Rapid Alloy Formation: Toward the Sustainable Fabrication of Nanoporous Si for Lithium-Ion Batteries

Silicon has the potential to improve lithium-ion battery (LIB) performance substantially by replacing graphite as an anode. The sustainability of such a transformation, however, depends on the source of silicon and the nature of the manufacturing process. Today's silicon industry still overwhelmingly depends on the energy-intensive, high-temperature carbothermal reduction of silica─a process that adversely impacts the environment. Rather than use conventional thermoreduction alone to break Si-O bonds, we report the efficient conversion of SiO₂ directly to Mg₂Si by a microwave-induced Mg plasma within 2.5 min at merely 200 W under vacuum. The underlying mechanism is proposed, wherein electrons with enhanced kinetics function readily as the reductant while the "bombardment" from Mg cations and electrons promotes the fast nucleation of Mg₂Si. The 3D nanoporous (NP) Si is then fabricated by a facile thermal dealloying step. The resulting hierarchical NP Si anodes deliver stable, extended cycling with excellent rate capability in Li-ion half-cells, with capacities several times greater than graphite. The microwave-induced metal plasma (MIMP) concept can be applied just as efficiently to the synthesis of Mg₂Si from Si, and the chemistry should be extendable to the reduction of multiple metal(loid) oxides via their respective Mg alloys.

Fully recyclable multifunctional adhesive with high durability, transparency, flame retardancy, and harsh-environment resistance

Recyclable/reversible adhesives have attracted growing attention for sustainability and intelligence but suffer from low adhesion strength and poor durability in complex conditions. Here, we demonstrate an aromatic siloxane adhesive that exploits stimuli-responsive reversible assembly driven by π-π stacking, allowing for elimination and activation of interfacial interactions via infiltration-volatilization of ethanol. The robust cohesive energy from water-insensitive siloxane assembly enables durable strong adhesion (3.5 MPa shear strength on glasses) on diverse surfaces. Long-term adhesion performances are realized in underwater, salt, and acid/alkali solutions (pH 1-14) and at low/high temperatures (-10-90°C). With reversible assembly/disassembly, the adhesive is closed-loop recycled (~100%) and reused over 100 times without adhesion loss. Furthermore, the adhesive has unique combinations of high transparency (~98% in the visible light region of 400-800 nm) and flame retardancy. The experiments and theoretical calculations reveal the corresponding mechanism at the molecular level. This π-π stacking-driven siloxane assembly strategy opens up an avenue for high-performance adhesives with circular life and multifunctional integration.

Hybrid Tri-Cure Organo-Silicon Coatings for Monument Preservation

A coating system integrating three distinct chemistries was developed to protect materials used in monuments and construction. Initial curing is achieved using a UV-initiated thiol-ene reaction to form a non-impressionable/non-sticky surface. Second, amine/epoxy reactions form a firm surface adhesion and give mechanical strength through consolidation. Third, alkoxysilane sol-gel curing integrates the siloxane network while adding thermal stability, hydrophobicity, and a hardened surface. The final design utilizes a photoacid generator to increase the reaction speed of the second and third curing steps. The coating can be applied by spray, dip, or wipe on methods and exhibits a rapid non-impressionable surface (as fast as 10 min) that resists graffiti and environmental conditions, and is used and stored as a single-component system with a pot life exceeding six months. A series of experiments were used to determine the coating properties and durability, including field testing and accelerated weathering.

Inorganic Salts and Dehydrating Agents Cooperatively Promoted Ru-Catalyzed Ethylene Methoxycarbonylation Using CO2 as a CO Surrogate

The use of CO2 as a CO surrogate for the carbonylation of olefin has attracted considerable attention due to its abundance, readily availability, nontoxicity, and recyclability. In this work, we describe the synthesis of methyl propionate (MPA), a key intermediate for methyl methacrylate in the commercial Lucite Alpha process, by the ruthenium-catalyzed methoxycarbonylation of ethylene with CO2 as a carbonyl source. An efficient approach to producing MPA has been developed by adding metal halide promoters and dehydrating agents. Control experiments suggest that the NHC-Ru-hydride may be the real active species formed in situ by the reaction of Ru3(CO)12 with ionic liquid (IL). NMR data demonstrate that inorganic salts favor the formation of active species, which is an important issue for their promotion effect. In terms of the strategy to overcome chemical equilibrium by the addition of dehydrating agents and IL participation in the formation of NHC-Ru-hydride active species, a tasked IL containing a siloxyl group was employed to Ru-catalyze the methoxycarbonylation of ethylene, which showed higher catalytic efficiency in comparison to IL without a siloxyl group.

Sustainability Evaluation of AquaSun Antifouling Coating Production

Relying on a cost-effective and green process that is technically and economically feasible on large scale is not enough to call a technology “sustainable”. We thus evaluate the sustainability of the production of the new “AquaSun” antifouling sol–gel coating with reference to each of the three main dimensions (economic, social, and environmental) of sustainable development. This study will hopefully assist in overcoming the “not invented here” syndrome that still affects many sol–gel technologies, including antifouling coatings derived via the sol–gel process.

Superior Hard but Quickly Reversible Si-O-Si Network Enables Scalable Fabrication of Transparent, Self-Healing, Robust, and Programmable Multifunctional Nanocomposite Coatings

A coating with programmable multifunctionality based on application requirements is desirable. However, it is still a challenge to prepare a hard and flexible coating with a quick self-healing ability. Here, a hard but reversible Si-O-Si network enabled by aminopropyl-functionalized poly(silsesquioxane) and triethylamine (TEA) was developed. On the basis of this Si-O-Si network, basic coatings with excellent transparency, hardness, flexibility, and quick self-healing properties can be prepared by filling soft polymeric micelles into hard poly(silsesquioxane) networks. The highly cross-linked continuous network endows the coating with a hardness (H = 0.83 GPa) higher than those of most polymers (H < 0.3 GPa), while the uniformly dispersed micelles decrease the Young's modulus (E = 5.89 GPa) to a value as low as that of common plastics, resulting in excellent hardness and flexibility, with an H/E of 14.1% and an elastic recovery rate (W_e) of 86.3%. Scratches (∼50 μm) on the coating can be healed within 4 min. The hybrid composition of poly(silsesquioxane) networks also shows great advantages in integration with other functional components to realize programmable multifunctionality without diminishing the basic properties. This nanocomposite design provides a route toward the preparation of materials with excellent comprehensive functions without trade-offs between these properties.

From SiO2 to Alkoxysilanes for the Synthesis of Useful Chemicals

The transformation of silica (SiO₂) to useful chemicals is difficult to explore because of the strength of the Si-O bond and thermodynamic stability of the SiO₂ structure. The direct formation of alkoxysilanes from SiO₂ has been explored as an alternative to the carbothermal reduction (1900 °C) of SiO₂ to metallic silicon (Si_met) followed by treatment with alcohols. The base-catalyzed depolymerization of SiO₂ with diols and monoalcohols afforded cyclic silicon alkoxides and tetraalkoxysilanes, respectively. SiO₂ can also be converted to alkoxysilanes in the presence of organic carbonates, such as dimethyl carbonate. Alkoxysilanes can be further converted to useful chemicals, such as carbamates, organic carbonates, and chlorosilanes. An interesting and highly efficient pathway to the direct conversion of SiO₂ to alkoxysilanes has been discussed in detail along with the corresponding economic and environmental implications. The thermodynamic and kinetic aspects of SiO₂ transformations in the presence of alcohols are also discussed.

Sustainability in Heritage Wood Conservation: Challenges and Directions for Future Research

Conserving the world’s cultural and natural heritage is considered a key contributor to achieving the targets set out in the United Nation’s Sustainable Development Goals, yet how much attention do we pay to the methods we use to conserve and protect this heritage? With a specific focus on wooden objects of cultural heritage, this review discusses the current state-of-the-art in heritage conservation in terms of sustainability, sustainable alternatives to currently used consolidants, and new research directions that could lead to more sustainable consolidants in the future. Within each stage a thorough discussion of the synthesis mechanisms and/or extraction protocols, particularly for bio-based resources is provided, evaluating resource usage and environmental impact. This is intended to give the reader a better understanding of the overall sustainability of each different approach and better evaluate consolidant choices for a more sustainable approach. The challenges facing the development of sustainable consolidants and recent research that is likely to lead to highly sustainable new consolidant strategies in the future are also discussed. This review aims to contribute to the ongoing discussion of sustainable conservation and highlight the role that consolidants play in truly sustainable heritage conservation.

Preparation, Characterization and Antimicrobial Properties of Nanosized Silver-Containing Carbon/Silica Composites from Rice Husk Waste

Rice husk, one of the main side products in the rice production, and its sustainable management represent a challenge in many countries. Herein, we describe the use of this abundant agricultural bio-waste as feedstock for the preparation of silver-containing carbon/silica nano composites with antimicrobial properties. The synthesis was performed using a fast and cheap methodology consisting of wet impregnation followed by pyrolysis, yielding C/SiO2 composite materials doped with varying amounts of silver from 28 to 0.001 wt %. The materials were fully characterized and their antimicrobial activity against ESKAPE pathogens, namely E. faecium, S. aureus, K. pneumoniae, A. baumannii, P. aeruginosa, and E. coli, and the pathogenic yeast C. albicans was investigated. Sensitivities of these strains against the prepared materials were demonstrated, even with exceptional low amounts of 0.015 m% silver. Hence, we report a straightforward method for the synthesis of antimicrobial agents from abundant sources which addresses urgent questions like bio-waste valorization and affordable alternatives to increasingly fewer effective antibiotics.

A Systematic Study on Bio-Based Hybrid Aerogels Made of Tannin and Silica

Tannin-silica hybrid materials are expected to feature excellent mechanic-chemical stability, large surface areas, high porosity and possess, after carbothermal reduction, high thermal stability as well as high thermal conductivity. Typically, a commercially available tetraethoxysilane is used, but in this study, a more sustainable route was developed by using a glycol-based silica precursor, tetrakis(2-hydroxyethyl)orthosilicate (EGMS), which is highly water-soluble. In order to produce highly porous, homogeneous hybrid tannin-silica aerogels in a one-pot approach, a suitable crosslinker has to be used. It was found that an aldehyde-functionalized silane (triethoxysilylbutyraldehyde) enables the covalent bonding of tannin and silica. Solely by altering the processing parameters, distinctly different tannin-silica hybrid material properties could be achieved. In particular, the amount of crosslinker is a significant factor with respect to altering the materials’ properties, e.g., the specific surface area. Notably, 5 wt% of crosslinker presents an optimal percentage to obtain a sustainable tannin-silica hybrid system with high specific surface areas of roughly 800–900 m² g⁻¹ as well as a high mesopore volume. The synthesized tannin-silica hybrid aerogels permit the usage as green precursor for silicon carbide materials.

Green routes to silicon-based materials and their environmental implications

The “greening” of silicon chemistry is fundamentally important for the future of the field. Traditional methods used to make silicon-based materials rely on carbon rich processes that are highly energy intensive, cause pollution, and are unsustainable. Researchers have taken up the challenge of developing new chemistries to circumvent the difficulties associated with traditional silicon material synthesis. Most of this work has been in the conversion of the “green” carbon neutral biogenic silica source rice hull ash (RHA, ~85 % silica) into useful silicon building blocks such as silica’s, silicon, and alkoxysilanes by using the inherently higher surface area and reactivity of RHA to sidestep the low reactivity of mined silica sources. This is a review of the work that has been done in the area of developing more environmentally benign methods for the synthesis and use of silicon containing materials to eliminate the negative impact on the environment.

Synthesis of hydrosilanes via Lewis-base-catalysed reduction of alkoxy silanes with NaBH4

Hydrosilanes were synthesized by reduction of alkoxy silanes with BH₃ in the presence of hexamethylphosphoric triamide (HMPA) as a Lewis-base catalyst. The reaction was also achieved using an inexpensive and easily handled hydride source NaBH₄, which reacted with EtBr as a sacrificial reagent to form BH₃in situ.

Direct synthesis of tetraalkoxysilanes from silica and alcohols

A simple and efficient process for synthesizing tetraalkoxysilanes from silica and alcohols was developed using molecular sieves as dehydrating agents.