Siloxane Adsorption Research Articles

Quantitative Simulations of Siloxane Adsorption in Metal-Organic Frameworks.

We present a transferable force field (FF) for simulating the bulk properties of linear and cyclic siloxanes and the adsorption of these species in metal-organic frameworks (MOFs). Unlike previous FFs for siloxanes, our FF accurately reproduces the vapor-liquid equilibria of each species in the bulk phase. The quality of our FF combined with the Universal Force Field using standard Lorentz-Berthelot combining rules for MOF atoms was assessed in a wide range of MOFs without open metal sites, showing good agreement with dispersion-corrected density functional theory calculations. Predictions with this FF show good agreement with the limited experimental data for siloxane adsorption in MOFs that is available. As an example of using the FF to predict adsorption properties in MOFs, we present simulations examining entropy effects in binary linear and cyclic siloxane mixture coadsorption in the large-pore MOF with structure code FOTNIN.

Removal of linear siloxanes and dimethyl sulfone from water using hierarchical zeolite porous carbon adsorbents

Low molecular weight linear siloxanes are problematic compounds known to occur in water bodies, especially in water reclaimed from closed volumes. Remediation was attempted using a hierarchical porous composite adsorbent containing a Faujasite zeolite imprinted in activated carbon, namely CFAU. Although CFAU was prepared using procedures reported elsewhere, it was modified also to include extra-framework Ag+ cations. The composites long-range, textural, and compositional characteristics were verified via XRD, nitrogen porosimetry, water contact angle, zeta potential, TGA, and ICP elemental analyses. Furthermore, the composites were tested for the adsorption of siloxanes (i.e., monomethylsilanetriol, dimethylsilanediol, and trimethylsilanol (TMS)) as well as dimethyl sulfone (DMSO2) from water (mg L−1 level) in single and multicomponent fashion at ambient temperature and neutral pH. DMSO2 is highly ubiquitous in reclaimed waters where these siloxanes occur. Ag+-CFAU was the superior adsorbent, with siloxane and DMSO2 loadings of up to 20 and 87 mg cm−3 (or 17 and 72 mg g−1), respectively. These are at least an order of magnitude larger than commercial alternatives like carbon and resins. FTIR data suggest that the adsorption interaction with siloxanes is of Van der Waals type, whereas interactions between Ag+-CFAU and DMSO2 plausibly involve complexation. Moreover, multicomponent adsorption tests showed that Ag+-CFAU excels probably because of the complexation with DMSO2 and a TMS-driven siloxane co-adsorption.

Enhanced hydrophobicity of modified ZIF-71 metal-organic framework for biofuel purification

Hydrophobic materials are desirable for biofuel purification applications. In this work, the atomic substitution of Cl atom with Br atom in the imidazole organic linker was carried out to produce a more hydrophobic ZIF-71, named ZIF-71(ClBr) and ZIF-71(ClBr)-SE for the material with an additional solvent exchange procedure. The physicochemical characterization revealed that ZIF-71(ClBr) and ZIF-71(ClBr)-SE are isostructural to ZIF-71, have a high surface area (769 and 969 m2/g), and are hydrophobic (126 and 130° water contact angle). As a proof of concept, D4 siloxane adsorption capacity onto ZIF-71(ClBr) and ZIF-71(ClBr)-SE was estimated (1.76 and 2.66 mg/g, respectively). Furthermore, a superior adsorption capacity was obtained when ZIF-71(ClBr)-SE was used due to its higher hydrophobic nature, and similar results were obtained with ZIF-8, used as a frame of reference. Therefore, the high surface area and the hydrophobic nature of ZIF-71(ClBr)-SE make it compelling material for biofuel purification applications.

Environmental assessment of metal-organic framework DUT-4 synthesis and its application for siloxane removal

In this study, the effect of different solvents in metal-organic framework (MOF) synthesis was evaluated in a technical study and an environmental study. The technical study considered the octamethylcyclotetrasiloxane (D4) adsorption on MOF DUT-4 for biogas purification, while the environmental study was implemented using the Life Cycle Assessment (LCA) methodology. In addition, the obtained DUT-4 adsorbents were characterized by X-ray diffraction, N2 adsorption-desorption isotherms, solid-state 27Al magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. The results indicated that technically similar materials are obtained through solvothermal synthesis with similar D4 siloxane adsorption capacities (9.8 mg/g). Conversely, hydrothermal synthesis led to a low D4 siloxane adsorption capacity (1.8 mg/g). The environmental study demonstrated that the solvothermal synthesis had the highest contribution to the environmental damage due to the use of toxic solvents in the synthesis, cleaning, and solvent exchange stages (environmental score factor up to 244 mPt). In addition, a techno-environmental impact factor was introduced to identify the most suitable synthesis route of DUT-4. The overall results pointed out that solvothermal synthesis is the best scenario for DUT-4 production due to its low environmental impact/adsorption capacity ratio (11.44). This study is a step toward a more holistic approach integrating technical and environmental criteria in MOF synthesis for adsorption applications.

Siloxane adsorption by porous silica synthesized from residual sand of wastewater treatment

Wastewater from municipal sewage may be an interesting source of biogas, in which case siloxanes are commonly present above the spec limit to be used in power engines. On the other hand, residual sand from Wastewater Treatment (WWT) Plants is also rich in inorganic matter, which can be a suitable feedstock for the synthesis of porous inorganic adsorbents. This work describes the synthesis of an alternative adsorbent for siloxane adsorption using residual sand from a WWT plant. The ultimate goal was to find a substitute for commercial activated carbons or silicas currently employed in siloxane removal from biogas. The residual sand was subject to thermal and acid treatment, which successfully increased its silicon content while developing adequate internal porosity. The textural characteristics and the chemical composition of the new adsorbent were similar to those found in commercial silica gels. Liquid phase adsorption isotherms and uptake rate curves were measured for siloxanes Octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The siloxane uptakes and adsorption kinetics were comparable and even superior to those found in commercial silicas, especially at low adsorbate concentrations. These results demonstrate the potential of residual sand from WWT as a suitable raw material to produce a good adsorbent for the removal of cyclic siloxanes from biogas.

Investigating the degradation mechanism of the solid oxide fuel cell nickel-yttria stabilized zirconia anode under siloxane contamination

In order to investigate the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode degradation mechanism due to poisoning by Octamethylcyclotetrasiloxane (D4), four different experiments utilizing H2+H2O, H2+H2O+D4, H2+D4 and H2+CO+D4 as fuels at 750 °C are conducted in this study. The electrochemical characterization and morphology results are analyzed and compared for anode degradation phenomenon. The results contradict the previously proposed degradation mechanism as the experimental results show that water can inhibit the silicon deposition and anode degradation. Based on the experimental results in this study and previous studies about siloxane deposition on metal oxides, a new anode degradation mechanism is proposed. The proposed degradation mechanism for siloxane is due to siloxane adsorption resulting in silicon deposition and carbon deposition after siloxane decomposition.

Desiloxanation process of biogas using an amorphous iron hydroxide-based adsorbent: A comparison between laboratory and field-scale experiments

Amorphous iron hydroxide-based material was selected as a suitable adsorbent for the siloxanes from biogas and landfill gas. The adsorptive characteristics of the desiloxanation (DeSiloxs) process were studied through a lab-scale experiment using a simulated gas containing a mixture of siloxanes (D4, and D5) and a field experiment on an actual environment scale. The iron hydroxide-based adsorbents exhibited high activity in adsorption of siloxanes during lab-scale experiments, where the obtained adsorption capacity was about 26.3 % with N2 as a carrier gas. However, from an applicability point of view, the effectiveness of the siloxane removal process was significantly affected by the space velocity during the in-situ test in anaerobic digestion gas plant using wastewater sludge. In the experimental case of various space velocities (50, 100, 300 h−1), a very high removal efficiency (100 %) was obtained.

Study of Siloxane Adsorption on Hard Disk Media Surface.

Siloxane adsorption on disk surfaces is investigated as a function of lubricant film thickness and carbon film composition through several analytical technologies including FTIR, XPS, TOF-SIMS, GC-MS, and Candela OSA. Disk parameters that govern the propensity for siloxane adsorption are identified. Lubricant film coverage, taken as the fractional monolayer thickness, is found to be the most significant determinant. Siloxane adsorption decreases with increasing lubricant film coverage. The nitrogen content of the underlying carbon film is also found to be a determinant for submonolayer lubricant films: On the one side, siloxane adsorption on carbon films increases with increasing atom % nitrogen in carbon films. On the other side, the lubricant monolayer film thickness increases with increasing atom % N in carbon films, i.e., lubricant film coverage decreases with increasing atom % N. Thus, increasing atom % N in carbon films leads to increasing siloxane adsorption.

Purification of wastewater digester biogas from siloxanes via adsorption-desorption with NaOH-reformed SiO2 adsorbent

This study was performed in order to investigate the adsorption of siloxane for biogas treatment using a novel SiO2-based adsorbent and its regeneration. The siloxane adsorption capacity of the NaOH-modified SiO2 was significantly increased. At low temperature (25–50 °C), the siloxane (D5) adsorption capacity of the novel SiO2 adsorbent was 200 mg per g of adsorbent, which is approximately four times than that observed for silica gel or SiO2 prior to treatment with NaOH.The siloxane retention mechanism of silica particles is the influence of the hydrogen bonds formed between the Si–O–Si groups on the siloxane surface and the OH groups on the silica surface. Therefore, increasing the concentration of NaOH used in the reforming step will increase the density of OH group density increased proportionally and siloxane adsorption capacity also increased. The number of OH groups, determined based on the TGA analysis, was increased to 4.7 OH/nm2 using 3 M NaOH from 1.3 OH/nm2 of pure SiO2. This also supports the hypothesis that the concentration of OH groups on the SiO2 is proportional to the NaOH concentration.

Removal of siloxane (L2) from biogas using methyl-functionalised silica gel as adsorbent

Volatile methylsiloxanes (VOSiC) are impurities present in trace amounts in biogas produced by the anaerobic digestion of organic wastes in landfill and sewage plants. Efficient removal of these impurities is required due to the restrictions posed by them on the application of biogas as an alternative energy resource. In this study, methyl-functionalised silica gels (MFS) have been prepared via a hydrolysis and condensation approach using double precursor (TEOS and MTS) in methanol–water medium for achieving effective removal of a model VOSiC - hexamethyldisiloxane (L2). The developed gels have been characterized by N2-BET, FTIR, SEM-EDS and XRD. The results reveal that the as-synthesized MFS materials have well-developed micropore structure and superhydrophobicity. Among the developed materials, MFS2 sample obtained with a MTS/TEOS ratio of 1/1 (v/v) exhibits the highest specific surface area (1261.3 m2 g−1), micropore volume (0.52 cm3 g−1) and total pore volume (1.03 cm3 g−1), along with superior water contact angle (123.5°). The non-crystalline nature and sponge-like structure of MFS are confirmed by XRD and SEM analyses. The adsorptive performance of MFS materials for L2 siloxane is measured to be 14.9–18.3 times higher than that of the unmodified silica in terms of the breakthrough adsorption capacity (QB). Also, the adsorption capacities of MFS materials are noted to be linearly related to the specific surface area and micropore volume, along with positively affected by the total pore volume and water contact angle. From a series of the experimental results of adsorption of L2 by MFS2, an empirical equation is developed for reflecting the correlation of the breakthrough time and inlet concentration. Use of lower temperature is concluded to result in better overall siloxane adsorption levels. The QB reaches an optimal value of 346.3 mgL2 g−1 at 0 °C using 83.82 mg L−1 of L2 with N2 as the carrier gas. The developed MFS adsorbent can be readily regenerated for reuse by heating at 80 °C, and an insignificant fluctuation in the dynamic adsorption behavior is observed even after consecutive 5 adsorption/desorption cycles.

Adsorption of linear and cyclic siloxanes on activated carbons for biogas purification: Sorbents regenerability

Two commercial activated carbons, namely STIX and AP4, were selected to investigate their ability for siloxane adsorption and further thermal regeneration in relation with their different physico-chemical properties. In the frame of biogas purification, the studied siloxanes were L2, L3, L4 (linear molecules) and D4, D5 (cyclic molecules). The maximum capacity of adsorption was estimated by gas chromatography and gravimetric methods. AP4 presents better adsorptive properties comparing with STIX irrespective of the type of siloxane. Thermal regeneration after siloxanes adsorption was followed up to 400 °C by both gas chromatography and in situ DRIFTS. Except L2, the siloxane polymerization proceeds on the AC containing alkali metals (K, Na) as revealed by the observed release of their decomposition products during the thermodesorption treatment. The same sites are responsible for the cleavage of SiO bonds in linear molecules and formation of L2 as primary product above 100 °C. The presence of such sites, possibly strong basic sites, is detrimental to the regenerability of the spent adsorbent and to the adsorption which could be limited by partial blockage of the AC porosity. Interestingly the regenerability of the alkali-free AC depends only on the volatility of the siloxane.

Novel One-Component Anisole- and Hydroxyl-Terminated Perfluoropolyethers for Boundary Lubrication

The tribological properties of some novel single component perfluoropolyether (PFPE) boundary lubricants with chemically integrated mixture end groups are investigated. Chemically integrated mixture end groups composed of hydroxyl- and anisole-terminated PFPE boundary lubricant films on the –(CF2CF2CF2O)– main chain are reported. These PFPE-based boundary lubricants explore a new method by which single component PFPE lubricants with mixture end groups might be used to tailor boundary film properties instead of using physical mixtures of two or more PFPEs with different end groups. Lubricant transfer to the low-flying read/write head, head wear, and siloxane adsorption as a function of PFPE film thickness and of type are compared. Normalization of the data to the monolayer fraction instead of film thickness allows direct comparison between anisole- and hydroxyl-terminated PFPEs. Lubricant transfer to the head and head wear are independent of the functional end groups. Siloxane adsorption decreases with increasing anisole substitution of the hydroxyl groups. One-component PFPEs with mixed end groups provide a methodology by which boundary film properties could be adjusted.

Siloxane adsorption on activated carbons: Role of the surface chemistry on sorption properties in humid atmosphere and regenerability issues

Commercial activated carbons (ACs) have been studied in dynamic adsorption of octamethylcyclotetrasiloxane (D4) in both dry and wet conditions. 1000 ppm (v/v) D4 with N2 as carrier gas and relative humidity (RH) up to 70% were used. Samples were characterized by physicochemical techniques such as N2 and H2O adsorption/desorption at −196 and 20 °C, respectively, XPS, Boehm titration, chemical and elemental analysis, thermogravimetric measurements. No correlation between adsorption capacity in dry conditions and textural properties could be established. In wet conditions, the presence of hydrophilic sites preferentially adsorbing H2O strongly decreases the ability of ACs to adsorb D4. The regenerability by thermal treatment after D4 adsorption in dry conditions was also addressed and followed by gas chromatography and in situ DRIFTS. The D4 polymerization into polysiloxanolate/PDMS was found to proceed on ACs containing alkali metals (K, Na) as revealed by the observed release of their decomposition products during the thermodesorption treatment. This property is detrimental to the regenerability of the AC by blockage of its porosity. The eventual formation of polysiloxanalolate type species during the adsorption could also be responsible for limiting the D4 uptake by partial blockage of the AC porosity.

Chromium-based MIL-101 metal organic framework as a fully regenerable D4 adsorbent for biogas purification

In this work, the chromium-based MIL-101 metal organic framework has been synthesized and investigated as a fully regenerable, high capacity sorbent for siloxanes, which are common contaminants in raw biogas from anaerobic digestion. Adsorption isotherms of D4, the most common siloxane in waste treatment plants, have been collected at different temperatures and modelled by means of the Langmuir theoretical isotherm. The selected metal organic framework shows a remarkable affinity for siloxane adsorption and an outstanding saturation capacity weakly dependent on temperature. Moreover, repeated adsorption isotherms, collected after a reactivation treatment of the same adsorbent sample, clearly indicate full regeneration capabilities. Considering that chromium-based MIL-101 is also a noticeable H2S adsorbent, the results reported in this work pave the way to its potential application for the simultaneous removal of the two main poisoning compounds contained in biogas.

Competitive siloxane adsorption in multicomponent gas streams for biogas upgrading

Biogas, produced in anaerobic digesters, is used as a renewable fuel after undergoing several upgrading processes. Adsorption onto activated carbon is the most widely used technology to remove harmful volatile compounds, such as siloxanes. However, the competition for the adsorptive sites with other biogas impurities reduces the performance of the adsorbents increasing the operative costs.This work studies the competitive adsorption of volatile organic compounds and siloxanes into activated carbons of different sources and activation processes. Ten selected materials were exhaustively characterized in terms of textural and chemical properties. Dynamic competitive adsorption tests displayed different equilibrium uptakes for each target compound depending on the properties of each carbon type. Chemically activated carbons demonstrated a higher adsorption capacity and higher selectivity for bulky siloxanes, being the most suitable adsorbents for biogas upgrading. The performance of the adsorbents was also investigated in the presence of moisture, which enable siloxane hydrolysis reactions in the carbon surface, leading to the formation of α-ω-silanediols, especially on phosphoric acid-activated carbons.The technical implications of this work are discussed in terms of biogas volume treated per volume of adsorbent at the first siloxane breakthrough. Phosphoric acid activated carbons are capable of treating higher number of bed volumes, reducing the costs associated to the adsorbent replacement.

Low-temperature regeneration of novel polymeric adsorbent on decamethylcyclopentasiloxane (D5) removal for cost-effective purification of biogases from siloxane

Biogas, a fuel used to generate electricity, contains siloxanes that can damage combustion engines, leading to expensive repairs and service interruptions. Activated charcoal is used to reduce the silicon content. Since siloxanes are difficult to desorb from the material, the adsorbent beds have to be replaced regularly. Thermal swing adsorption (regeneration at 250 °C) using silica gel is an effective method to remove siloxane from biogas. Also, it would be advantageous and cost-effective if the adsorption material could be regenerated easily at a lower temperature. In the present study, a novel polyacrylic acid (PAA)-based polymer adsorbent was studied as a biogas siloxane adsorbent exposed to adsorption/regeneration cycles. At room temperature (25 °C), the D5 adsorption capacity of RPA was 21.55 mg/g adsorbent, which is ∼70% of the adsorption capacity of silica gel. The thermal desorption of siloxane (D5) from the exhausted polymer was more than 95%, which afforded a similar capacity for siloxane (D5) adsorption, at a low regeneration temperature of 80 °C, where no siloxane regeneration was observed using conventional silica gel.

Modified inverse micelle synthesis for mesoporous alumina with a high D4 siloxane adsorption capacity

In this work, mesoporous aluminas (MAs) with uniform and monomodal pores were fabricated via a modified inverse micelle synthesis method, using a non-polar solvent (to minimize the effect of water content) and short reaction time (for a fast evaporation process). The effects of reaction times (4–8 h), surfactant chain lengths (non-ionic surfactants), and calcination temperatures and hold times (450–600 °C; 1–4 h) on the textural properties of MA were studied. The targeted pore sizes of MA were obtained in the range of 3.1–5.4 nm by adjusting the surfactant and reaction time. The surface area and pore volume were controlled by the calcination temperature and hold time while maintaining the thermal stability of the materials. The tuned MA of the large mesopore volume achieved 168 mg/g octamethylcyclotetrasiloxane (D4 siloxane) adsorption capacity, a 32% improvement compared to commercially activated alumina. After three adsorption recycles, the synthesized MA still maintained approximate 85% of its original adsorption capacity, demonstrating a sustainable adsorption performance and high potential for related industrial applications.

Siloxane D4 adsorption by mesoporous aluminosilicates

This study deals with the development and exploration of new zeolite-type materials with mesoporosity for siloxane adsorption. The mesopores in the adsorbents are preferred for adsorption of relatively large siloxanes, such as octamethylcyclotetrasiloxane (D4) (∼10Å). Mesoporous aluminosilicate (UCT-15) is a zeolite-type material in the family of mesoporous oxide materials developed at the University of Connecticut (UCT). In this study, UCT-15 materials are synthesized, using different aluminum dopant amounts and calcination heating rates to obtain tunable textural properties. The synthesized materials are applied in biogas cleanup, particularly the adsorption of siloxane D4. Under room temperature and 1.79mg/h D4 feed rate, the best adsorption performance (105mg/g adsorption capacity) is found for mesoporous aluminosilicate synthesized with a Si:Al=5 and a 10°C/min calcination heating rate, because of its large BET surface area, external surface area, mesopore volume, and total pore volume. In the best case scenario, more than twice the amount of D4 is adsorbed by the synthesized mesoporous aluminosilicate, compared with commercial ZSM-5. Moreover, the external surface area and mesopore volume are the most influential adsorbent textual properties that affect the adsorption capacity. Furthermore, polymerization of the adsorbed D4 is investigated and attributed to the adsorbent surface hydroxyl groups.

Surface modification of activated carbon for siloxane adsorption

The presence of siloxanes challenges the use of landfill gas (LFG) as a fuel for energy recovery, due to the formation of microcrystalline silica deposits during combustion. Activated carbon (AC) is often selected as an adsorbent for removing siloxanes from LFG. In order to find the key characteristics that affect the siloxanes adsorption capacity of AC, this paper studied the effects of AC textural structure and surface chemistry on siloxane adsorption. Anthracite AC was respectively treated by aqua ammonia, hydrochloric acid and heat to obtain modified AC with different surface properties. Adsorption capacities of the original and modified AC for octamethylcyclotetrasiloxane (D4) were measured. Results showed that most of the modified AC had a higher D4 adsorption capacity than the original AC. Several approaches were adopted to characterize the AC. The results obtained by nitrogen adsorption experiment revealed that all the employed modification methods changed the AC pore size distribution to some extent. The narrow mesopores on the AC surface are more desired for the siloxane adsorption. As for the AC surface functional groups, the results obtained by Boehm titration revealed that the alkaline and phenolic groups are favorable for siloxane adsorption, while the carboxylic groups are undesired for siloxane adsorption.

Adsorption and Electrothermal Desorption of Volatile Organic Compounds and Siloxanes onto an Activated Carbon Fiber Cloth for Biogas Purification

Although biogases mainly consist of a mixture of carbon dioxide and methane, traces of volatile organic compounds are present, and these undesirable compounds must be removed during the purification process. Adsorption onto an activated carbon fiber cloth (ACFC) was investigated and, in particular, the feasibility of electrothermal desorption. Five compounds were chosen, and their desorption was assessed by monitoring the electric resistance of the material as a function of the temperature. The results were confirmed by thermogravimetric analysis. Toluene, dichloromethane, and isopropanol were entirely desorbed at 420 K, whereas siloxane D4 and ethyl mercaptan (ethanethiol) were partially removed. These conclusions were confirmed by dynamic adsorption measurements. Cycles of adsorption followed by electrothermal desorption were first carried out for a single component (toluene). Although there was a loss of adsorption capacity between the first and second cycles, a steady performance was reached, shedding...