Aqueous phase hydrodechlorination of trichloroethylene using Pd supported on swellable organically modified silica (SOMS): Effect of support derivatization
Graphical abstract
Introduction
Swellable organically modified silica (SOMS), a novel catalyst scaffold, belongs to the class of bridged polysilsesquioxanes which are hybrid organic–inorganic materials synthesized by sol–gel technique via co-polymerization of a monomer consisting of an organic group covalently bonded to alkoxysilyl groups. [1], [2], [3], [4]. In case of SOMS, bis(trimethoxysilylethyl)benzene (BTEB) is used as the monomer which consists of diethylbenzene (the bridging organic group) which is covalently bonded to two -Si(OCH3)3 groups (two trifunctional silyl groups) [5], [6]. The covalent bonds between the Si atoms and the terminal atoms of diethylbenzene are non-hydrolyzable which allows the polymerization to occur only at the methoxy groups attached to the trifunctional Si atoms [1], [2], [3]. Owing to the hydrolysis and condensation of the alkoxysilane groups occurring during polymerization, the resulting material often consists of surface silanol groups which are sterically prohibited to condense to SiOSi which makes it hydrophilic in nature.
Typically, a silica surface is comprised of isolated, geminal or vicinal silanol groups that are acidic in nature [7], [8]. Two major ways of removing or hydrophobizing these groups are thermal treatment and chemical functionalization [7], [9], [10]. Thermal treatments cause surface dehydration (removal of H-bonded water molecules) followed by dehydroxylation (removal of OH groups) [7]. This removal of OH groups from the surface leads to hydrophobization which is why a completely dehydroxylated siloxane surface is hydrophobic [9], [10]. Alternatively, chemical modification of the silica surface by replacing OH groups with long-chained alkyl groups has also been used for surface hydrophobization [7], [11]. However, the steric hindrance caused by these long-chained groups does not allow all SiOH groups to be covered during functionalization. Therefore, a silylating agent with a smaller organic group such as trimethylchlorosilane or similar aminosilane is used to ‘end-cap’ the residual SiOH groups and the process is referred to as derivatization [7], [11]. For example, silylation of the surface silanol groups was performed to impart hydrophobicity to Co supported catalysts used for Fischer-Tropsch synthesis and preferential oxidation (PROX) of CO from an H2-rich environment [12], [13], [14], [15]. The silylation performed on the silica supports for FT synthesis led to a more reducible Co species which resulted in better selectivity of the reaction. Furthermore, the hydrophobization prevented the poisoning of Co sites by water molecules [16], [17]. In case of SOMS, derivatization of its surface silanol groups (Si-OH) using hexamethyldisilazane (HMDS) was performed to obtain hydrophobicity [5], [6]. Besides being hydrophobic, SOMS is a mesoporous material which can swell to almost 3–4 times its original volume [5], [6]. Its application as an absorbent for many organic chemicals has been demonstrated, owing to its high porosity and surface area [18], [19], [20].
Our group has investigated the role of SOMS as a support for Pd catalyzed hydrodechlorination of trichloroethylene (HDC of TCE) [21], [22], [23], [24], [25], [26], [27], [28] which is an efficient, selective and environmentally friendly route for degradation of TCE [29], [30], [31], [32], [33], [34]. TCE, a common industrial solvent [35], [36], is a toxic volatile organic contaminant found in groundwater [37], [38], [39], [40] and is strictly regulated by US E.P.A. [41], [42]. Although Pd catalyzed HDC of TCE is an efficient technique, it suffers from deactivation via leaching (loss of expensive Pd particles) by HCl, an unavoidable by-product [23], [32], [43], [44]. To suppress inhibition by HCl, use of hydrophobic materials to support Pd [45], [46], addition of another metal (Fe, Au, Ni) to alter the electronic structure of Pd [47], [48], [49] and use of bases such as NaOH or KOH to scavenge the deactivating Cl- ions in the reaction medium have been explored in the literature [50], [51], [52]. Our research group has focused on the use of SOMS (a swellable and hydrophobic material) to support and protect Pd [21], [22], [23], [24], [25], [26]. In gas phase, Pd/SOMS was found to be more resistant to deactivation by H2O and H2S than the commercial Pd/Al2O3 catalyst [21], [22], [24]. Similarly, in liquid phase, Pd/SOMS showed higher deactivation resistance towards chlorine and sulfur species than Pd/Al2O3 [23], [24], [25], [26]. The main aim of this paper is to investigate the cause of high deactivation resistance of SOMS towards HCl (an unavoidable by-product), i.e., to investigate whether the deactivation resistance stems from hydrophobicity or swellability or a combination of these properties of SOMS.
Since Al2O3 is neither hydrophobic nor swellable, it cannot be used for comparison with SOMS which is both, hydrophobic and swellable. Therefore, for a better comparison, we synthesized different versions of SOMS using the same precursor (BTEB) such that each of them possessed different combinations of hydrophobicity and swellability. This was achieved by altering the derivatization method used for surface silylation during the synthesis of SOMS. Activity tests and characterization techniques performed with Pd supported on SOMS and its counterparts revealed that not only hydrophobicity but swellability is also essential for high deactivation resistance towards HCl. Such studies which involve the use of animated materials for catalytic applications are quite rarely found in the literature; moreover, those investigating the relationship between the structure of these animated materials and their catalytic performance are even more scarce.
Section snippets
Support synthesis
Synthesis of SOMS was previously reported by Edmiston and co-workers [5], [6]. Briefly, SOMS is synthesized using the sol–gel method with bis-(trimethoxysilylethyl)benzene (BTEB) as the precursor dissolved in a suitable water miscible organic solvent (tetrahydrofuran), followed by addition of water (with 3:1 mol ratio of water:BTEB) containing 0.155 M tetrabutylammonium fluoride as the catalyst. After gelation and ageing for 6 days at room temperature and ambient pressure, the resulting
Fourier transform infrared spectroscopy (FTIR)
FTIR spectroscopy was used to determine the structural changes occurring in SOMS caused by derivatization of the surface silanol groups. These FTIR spectra were collected in transmission mode and were divided in three regions: low-frequency (650–1250 cm−1), mid-frequency (1250–1700 cm−1) and high-frequency (1700–4000 cm−1), shown in Fig. 1a, b and c respectively.
Conclusions
Derivatization performed during synthesis of SOMS determines its extent of hydrophobicity and swellability. Derivatized samples were more hydrophobic than the underivatized (SOMD-UD) one; however, when derivatization was performed prior to drying (SOMS), the material obtained was more swellable than the one where derivatization was performed post-drying (SOMS-PDD).
When Pd was deposited on these supports, the resulting Pd particle size was found to follow the same trend as the swellability of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Science Foundation through the Grant CBET- 1436729. J.T.M. was supported as part of the National Science Foundation Energy Research Center for Innovative and Strategic Transformation of Alkane Resources (CISTAR) under the Cooperative Agreement No. EEC-1647722. EXAFS data was collected at the Materials Research Collaborative Access Team’s (MRCAT) sector 10- ID-B at Argonne National Laboratory. MRCAT operations are supported by the Department of
References (88)
- et al.
Highly swellable sol–gels prepared by chemical modification of silanol groups prior to drying
J. Non-Cryst. Solids
(2005) - et al.
Reducing residual silanol interactions in reversed-phase liquid chromatography: Thermal treatment of silica before derivatization
J. Chromatogr. A
(2003) - et al.
Mechanism and kinetics of hexamethyldisilazane reaction with a fumed silica surface
J. Colloid Interface Sci.
(2000) - et al.
Silylation of mesoporous silica MCM-41 with the mixture of Cl (CH2) 3SiCl3 and CH3SiCl3: combination of adjustable grafting density and improved hydrothermal stability
Microporous Mesoporous Mater.
(2004) Silica: backbone material of liquid chromatographic column packings
J. Chromatogr. A
(1991)- et al.
Silylated Co3O4-m-SiO2 catalysts for Fischer-Tropsch synthesis
Catal. Commun.
(2011) - et al.
Absorption of dissolved organic species from water using organically modified silica that swells
Sep. Purif. Technol.
(2009) - et al.
Effect of high-temperature on the swellable organically-modified silica (SOMS) and its application to gas-phase hydrodechlorination of trichloroethylene
Appl. Catal. B
(2017) - et al.
Hydrodechlorination of trichloroethylene over Pd supported on swellable organically-modified silica (SOMS)
Appl. Catal. B
(2017) - et al.
Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions
Appl. Catal. B
(2018)
Formation of carbonaceous deposits on Pd-based hydrodechlorination catalysts: Vibrational spectroscopy investigations over Pd/Al2O3 and Pd/SOMS
Catal. Today
On the dual role of the reactant during aqueous phase Hydrodechlorination of Trichloroethylene (HDC of TCE) using Pd supported on Swellable Organically Modified Silica (SOMS)
Appl. Catal. B
Elucidating the role of ethanol in aqueous phase hydrodechlorination of trichloroethylene over Pd catalysts supported on swellable organically modified silica (SOMS)
Appl. Catal. B
Catalytic hydrodechlorination of groundwater contaminants in water and in the gas phase using Pd/γ-Al2O3
Appl. Catal. B
Groundwater diffuse pollution in functional urban areas: The need to define anthropogenic diffuse pollution background levels
Sci. Total Environ.
Trichloroethylene: Mechanistic, epidemiologic and other supporting evidence of carcinogenic hazard
Pharmacol. Ther.
Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water
Chemosphere
Deactivation of a Pd/Al2O3 catalyst used in hydrodechlorination reactions: Influence of the nature of organochlorinated compound and hydrogen chloride
Appl. Catal. B
Protection of palladium catalysts for hydrodechlorination of chlorinated organic compounds in wastewaters
Appl. Catal. B
Efforts for long-term protection of palladium hydrodechlorination catalysts
Appl. Catal. B
Improved Pd-on-Au bimetallic nanoparticle catalysts for aqueous-phase trichloroethene hydrodechlorination
Appl. Catal. B
Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects
Appl. Catal. B
Role of base addition in the liquid-phase hydrodechlorination of 2, 4-dichlorophenol over Pd/Al2O3 and Pd/C
J. Catal.
Infrared spectra-structure correlations for organosilicon compounds
Spectrochim. Acta
Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms
Polymer
IR spectroscopic investigation of SiO2 film structure
Thin Solid Films
Sol—Gel preparation of supported metal catalysts
Catal. Today
The effect of gold particle size on AuAu bond length and reactivity toward oxygen in supported catalysts
J. Catal.
Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism
J. Catal.
The graphene-supported palladium and palladium–yttrium nanoparticles for the oxygen reduction and ethanol oxidation reactions: Experimental measurement and computational validation
Appl. Catal. B
Bridged polysilsesquioxanes. Molecular-engineered hybrid organic− inorganic materials
Chem. Mater.
Arylsilsesquioxane gels and related materials. New hybrids of organic and inorganic networks
J. Am. Chem. Soc.
Bridged polysilsesquioxanes. Highly porous hybrid organic-inorganic materials
Chem. Rev.
Hybrid materials and silica: drastic control of surfaces and porosity of xerogels via ageing temperature, and influence of drying step on polycondensation at silicon
J. Mater. Chem.
Organic-inorganic hybrid materials that rapidly swell in non-polar liquids: nanoscale morphology and swelling mechanism
Chem. Mater.
Chemical modification of crystalline porous silicon surfaces
Comments Inorg. Chem.
Silylation of a Co/SiO2 catalyst. Characterization and exploitation of the CO hydrogenation reaction
Langmuir
Changes in the surface hydrophobicity degree of a MCM-41 used as iron support: a pathway to improve the activity and the olefins production in the Fischer-Tropsch synthesis
J. Porous Mater.
Effect of high temperature on swellable organically modified silica (SOMS) and its application for preferential CO oxidation in H2 rich environment
ChemCatChem
Enhancement in the reducibility of cobalt oxides on a mesoporous silica supported cobalt catalyst
Chem. Commun.
Effect of silylation of SBA-15 on its supported cobalt catalysts for Fischer-Tropsch synthesis
Chin. J. Catal.
Adsorption of gas phase organic compounds by swellable organically modified silica
Ind. Eng. Chem. Res.
Absorption of short-chain to long-chain perfluoroalkyl substances using swellable organically modified silica
Environ. Sci. Water Res. Technol.
Aqueous-phase hydrodechlorination of trichloroethylene over pd-based swellable organically modified silica: catalyst deactivation due to sulfur species
Ind. Eng. Chem. Res.
Cited by (5)
Effect of Pt nanoparticle size on resistance to chloride-induced inhibition for hydrodechlorination of trichloroethylene in water
2024, Journal of Environmental Chemical EngineeringImprovement strategies for oil/water separation based on electrospun SiO<inf>2</inf> nanofibers
2024, Journal of Colloid and Interface ScienceAnimated organic-inorganic hybrid materials and their use as catalyst scaffolds
2023, Catalysis TodayCitation Excerpt :As discussed earlier, derivatization of the remaining silanol groups after crosslinking is an important step to obtain a swellable material. Ailawar et al. established that the changes in the derivatization step during SOMS synthesis had a noticeable effect on the location of the active metal sites within the support [107]. During the synthesis of SOMS, derivatization is carried out using hexamethyldisilazane (HMDS) followed by drying.