Elsevier

Journal of Catalysis

Volume 411, July 2022, Pages 15-30
Journal of Catalysis

Aqueous phase hydrodechlorination of trichloroethylene using Pd supported on swellable organically modified silica (SOMS): Effect of support derivatization

https://doi.org/10.1016/j.jcat.2022.04.031Get rights and content

Highlights

  • Effect of support derivatization on activity of Pd catalyzed HDC of TCE is shown.

  • Lack of derivatization causes loss of support hydrophobicity and swellability.

  • Support swellability is essential to deposit Pd particles deep inside the pores.

  • Hydrophobicity protects the intrapore Pd by repelling hydrophilic HCl molecules.

  • TCE absorption in pores restricts HCl entry by creating localized non-polarity.

Abstract

Herein, the role of swellability and hydrophobicity of swellable organically modified silica (SOMS) in shielding Pd against deactivation due to HCl produced during hydrodechlorination (HDC) of trichloroethylene (TCE) is investigated. The extent of surface derivatization during sol gel synthesis of SOMS was found to directly impact the extent of hydrophobicity, swellability and surface area, as confirmed by infrared spectroscopy and N2 physisorption, respectively. Furthermore, after Pd impregnation, the resultant particle size, location, and atomic environment of Pd were also dictated by the extent of support derivatization such that the least derivatized material provided lowest protection to Pd from HCl. Post HCl-treatment, the batch activity rate constants decreased by 66% for the least derivatized sample and 17% for the most derivatized one, suggesting that hydrophobicity and swellability are essential for obtaining high resistance to HCl which could potentially impact the economic viability of HDC of TCE process.

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 Sisingle bondOsingle bondSi 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 single bondOH groups) [7]. This removal of single bondOH 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 single bondOH 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 Sisingle bondOH 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 Sisingle bondOH 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)

  • G. Celik et al.

    Formation of carbonaceous deposits on Pd-based hydrodechlorination catalysts: Vibrational spectroscopy investigations over Pd/Al2O3 and Pd/SOMS

    Catal. Today

    (2019)
  • S. Ailawar et al.

    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

    (2021)
  • S. Ailawar et al.

    Elucidating the role of ethanol in aqueous phase hydrodechlorination of trichloroethylene over Pd catalysts supported on swellable organically modified silica (SOMS)

    Appl. Catal. B

    (2021)
  • F.-D. Kopinke et al.

    Catalytic hydrodechlorination of groundwater contaminants in water and in the gas phase using Pd/γ-Al2O3

    Appl. Catal. B

    (2003)
  • A. Azzellino et al.

    Groundwater diffuse pollution in functional urban areas: The need to define anthropogenic diffuse pollution background levels

    Sci. Total Environ.

    (2019)
  • I. Rusyn et al.

    Trichloroethylene: Mechanistic, epidemiologic and other supporting evidence of carcinogenic hazard

    Pharmacol. Ther.

    (2014)
  • C.G. Schreier et al.

    Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water

    Chemosphere

    (1995)
  • E. López et al.

    Deactivation of a Pd/Al2O3 catalyst used in hydrodechlorination reactions: Influence of the nature of organochlorinated compound and hydrogen chloride

    Appl. Catal. B

    (2006)
  • R. Navon et al.

    Protection of palladium catalysts for hydrodechlorination of chlorinated organic compounds in wastewaters

    Appl. Catal. B

    (2012)
  • D. Comandella et al.

    Efforts for long-term protection of palladium hydrodechlorination catalysts

    Appl. Catal. B

    (2016)
  • M.O. Nutt et al.

    Improved Pd-on-Au bimetallic nanoparticle catalysts for aqueous-phase trichloroethene hydrodechlorination

    Appl. Catal. B

    (2006)
  • G. Yuan et al.

    Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects

    Appl. Catal. B

    (2004)
  • G. Yuan et al.

    Role of base addition in the liquid-phase hydrodechlorination of 2, 4-dichlorophenol over Pd/Al2O3 and Pd/C

    J. Catal.

    (2004)
  • A.L. Smith

    Infrared spectra-structure correlations for organosilicon compounds

    Spectrochim. Acta

    (1960)
  • G. Camino et al.

    Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

    Polymer

    (2002)
  • I.P. Lisovskii et al.

    IR spectroscopic investigation of SiO2 film structure

    Thin Solid Films

    (1992)
  • R.D. Gonzalez et al.

    Sol—Gel preparation of supported metal catalysts

    Catal. Today

    (1997)
  • J. Miller et al.

    The effect of gold particle size on AuAu bond length and reactivity toward oxygen in supported catalysts

    J. Catal.

    (2006)
  • J. Andersin et al.

    Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism

    J. Catal.

    (2012)
  • M.H. Seo et al.

    The graphene-supported palladium and palladium–yttrium nanoparticles for the oxygen reduction and ethanol oxidation reactions: Experimental measurement and computational validation

    Appl. Catal. B

    (2013)
  • K.J. Shea et al.

    Bridged polysilsesquioxanes. Molecular-engineered hybrid organic− inorganic materials

    Chem. Mater.

    (2001)
  • K. Shea et al.

    Arylsilsesquioxane gels and related materials. New hybrids of organic and inorganic networks

    J. Am. Chem. Soc.

    (1992)
  • D.A. Loy et al.

    Bridged polysilsesquioxanes. Highly porous hybrid organic-inorganic materials

    Chem. Rev.

    (1995)
  • G. Cerveau et al.

    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.

    (2002)
  • C.M. Burkett et al.

    Organic-inorganic hybrid materials that rapidly swell in non-polar liquids: nanoscale morphology and swelling mechanism

    Chem. Mater.

    (2008)
  • J.H. Song et al.

    Chemical modification of crystalline porous silicon surfaces

    Comments Inorg. Chem.

    (1999)
  • M. Ojeda et al.

    Silylation of a Co/SiO2 catalyst. Characterization and exploitation of the CO hydrogenation reaction

    Langmuir

    (2006)
  • N.A. Fellenz et al.

    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.

    (2017)
  • D. Basu et al.

    Effect of high temperature on swellable organically modified silica (SOMS) and its application for preferential CO oxidation in H2 rich environment

    ChemCatChem

    (2020)
  • D.J. Kim et al.

    Enhancement in the reducibility of cobalt oxides on a mesoporous silica supported cobalt catalyst

    Chem. Commun.

    (2005)
  • S. Yuanyuan et al.

    Effect of silylation of SBA-15 on its supported cobalt catalysts for Fischer-Tropsch synthesis

    Chin. J. Catal.

    (2009)
  • P.L. Edmiston et al.

    Adsorption of gas phase organic compounds by swellable organically modified silica

    Ind. Eng. Chem. Res.

    (2016)
  • E.K. Stebel et al.

    Absorption of short-chain to long-chain perfluoroalkyl substances using swellable organically modified silica

    Environ. Sci. Water Res. Technol.

    (2019)
  • G. Celik et al.

    Aqueous-phase hydrodechlorination of trichloroethylene over pd-based swellable organically modified silica: catalyst deactivation due to sulfur species

    Ind. Eng. Chem. Res.

    (2019)
  • Cited by (5)

    • Animated organic-inorganic hybrid materials and their use as catalyst scaffolds

      2023, Catalysis Today
      Citation 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.

    View full text