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Polysaccharide-Stabilized Pd-Ag Nanocatalysts for Hydrogenation of 2-Hexyn-1-Ol

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29 August 2023

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30 August 2023

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Abstract
A new one-pot technique for preparation of polysaccharide-based Pd- and Pd-Ag nanocatalysts by sequential supporting natural polymer (2-hydroxyethyl cellulose (HEC), chitosan (Chit), pectin (Pec)) and metals on zinc oxide was developed. The nanocatalysts based on polysaccharide were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), infrared spectroscopy (IRS), X-ray powder diffraction (XRD), X-ray photoelectron spectra (XPS) and elemental analysis methods. The catalyst characterization results indicated complete adsorption of polysaccharide and metal ions on zinc oxide, forming polymer-stabilized Pd nanoparticles of ~2 nm in size, evenly distributed on the support surface. The catalysts were studied in the hydrogenation of 2-hexyn-1-ol under mild conditions (0.1 MPa, 40 °C). The catalysts demonstrated nearly the same conversion of 2-hexyn-1-ol. The selectivity to cis-hexen-1-ol of the catalysts decreases in the following order: 0.5%Pd-Ag(3:1)HEC/ZnO>0.5%Pd-Ag(3:1)Pec/ZnO>0.5%Pd-Ag(3:1)Chit/ZnO. The optimum reaction temperature and catalyst loading for the Pd-Ag catalysts modified with HEC and Chit have been determined (40 °C, 0.05 g).
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Subject: Chemistry and Materials Science  -   Nanotechnology

1. Introduction

According to requirements of green chemistry, the study of nontoxic natural polymers to create new environmentally friendly nanomaterials and stabilized metal nanoparticles for wide purposes is in the focus of researcher’s interest [1,2,3,4,5,6,7,8,9]. Polysaccharides derived from natural sources are suitable alternatives to synthetic polymers produced from petroleum products [5,6,7,8,9]. Polysaccharides have different functional groups in their structure and therefore are able to form composites with mineral sorbents [10,11,12,13,14,15] and transition metal ions [15,16,17,18].
For example, the hydroxyl and amino groups in chitosan structure, interacting with transition metal ions, form metal nanoparticles, which are very promising as catalysts [19,20]. The multifunctionality of pectin (Pec) is due to the nature of its molecule consisting of linear chains of 1,4-linked residues of α-d-galacturonic acid [3,21,22,23] with a large number of -OH-, -COOH- groups allows its use in the design of various polymer-inorganic materials such as nanocatalysts [3], sorbents for wastewater treatment [4], thickeners, emulsifiers, gelling agents for food industry and biomedicine [21,22,23].
The cellulose is the most common natural polysaccharide. Its soluble derivative 2-hydroxyethyl cellulose (HEC) is formed by treating cellulose with alkali and reacting with ethylene oxide [24]. The advantages of this derivative are good water solubility, biodegradability, biocompatibility and film formation [25,26,27,28]. It is widely used in pharmaceuticals [29], textile industry [30], paper making [31], cosmetology [32], etc.
The use of polysaccharides as new auxiliary materials for design of heterogeneous catalysts is on the rise. Chitosan (Chit) is started to use [8], mainly due to its high affinity for metal ions [20]. A number of works are devoted to catalysts for processes such as hydrogenation [1,33,34], oxidation [3,4], coupling reaction and other possible catalytic syntheses [8,19,20,35]. There are few data on pectin-containing catalysts, their design and use as stabilizer for metal nanoparticles [2,3,4,36,37,38,39].
Different types of cellulose were used as supports for catalysts for hydrogenation, oxidation, dye reduction, coupling reactions [3,4,35,40,41,42,43]. Bearing abundant reactive -OH groups on its chains, HEC can acts as both reducing agent for transition metal ions and stabilizing agent for metal nanoparticles formed. Despite the fact that HEC is an attractive biopolymer [44,45,46], its full potential for use in the design of heterogeneous catalysts has not yet been adequately developed [47,48,49].
To take into account the growing need of environmental friendly nanocatalyst syntheses, this work aims to prepare polysaccharide containing PdAg/ZnO catalysts. The HEC, pectin and chitosan were used as green stabilizers and water was the only medium for the syntheses of the catalysts in ambient conditions without high temperature processes of calcination and reduction. To that end, we used a new green one-pot technique for catalyst synthesis by sequential supporting polysaccharide and metals on zinc oxide. In this study, we have evaluated and compared the efficiency of the developed HEC, Chit, Pec stabilized bimetallic PdAg catalysts supported on zinc oxide with that of the monometallic Pd-HEC/ZnO nanocatalyst in the hydrogenation of 2-hexyn-1-ol.

2. Results

2.1. Characterization of catalysts

Mono- and bimetallic palladium and palladium-silver catalysts modified with polysaccharides such as chitosan (Chit), 2-hydroxyethyl cellulose (HEC) and pectin (Pec) have been prepared by adsorption method in aqueous medium under ambient conditions and constant stirring. The solution of polymer and then metal salts were sequentially added into a zinc oxide suspension. The resulting composites were washed with water and dried in air. As result the following catalysts were obtained: Pd-HEC/ZnO, PdAg-HEC/ZnO, PdAg-Chit/ZnO, PdAg-Pec/ZnO.
The palladium and silver content in the catalysts was evaluated using spectrophotometry and potentiometry methods, respectively. Analysis of supernatant solution before and after sorption process showed that 91-99% of the introduced Pd and 99-100% of Ag were adsorbed on the polymer-modified ZnO. The calculated total metal content (Pd and Ag) in the obtained polymer-modified mono- and bimetallic catalysts was 0.46-0.49 wt % that close to expected value of 0.5 wt% (Table 1).
Table 2 shows the results of elemental analysis of the catalysts. The palladium content in Pd-HEC/ZnO, PdAg-HEC/ZnO, PdAg-Chit/ZnO and PdAg-Pec/ZnO catalysts was found to be 0.49%, 0.36%, 0.44% and 0.48%, respectively. The silver content in the bimetallic PdAg-HEC/ZnO, PdAg-Chit/ZnO and PdAg-Pec/ZnO catalysts was found to be 0.18%, 0.15%, and 0.16 %, respectively. Thus, the total metal content in all catalysts was not less than 0.5 wt%, suggesting the almost complete adsorption of metals (Pd and Ag) on the polymer modified support materials (Table 2). This is consistent with data obtained using spectrophotometry and potentiometry methods.
The results of X-ray powder diffraction analysis (XRD) of the ZnO, HEC/ZnO and PdAg-HEC/ZnO is shown in Figure 1. All XRD patterns showed the characteristic peaks at 37.0⁰, 40.2⁰, 42.3⁰,55.8⁰, 66.7⁰, 74.5⁰, 78.9⁰, 80.9⁰, 82.3⁰, 86.8⁰, 92.6⁰, which correspond to the [100], [002], [101], [102], [110], [103], [200],[112], [201], [004], [202] planes of ZnO wurtzite structure (JCPDS card no. 79-0206) [50]. A broad peak at 23º observed in polymer modified materials can be attributed to the amorphous phase of the HEC [51]. No peaks related to palladium (Pd or PdO) and silver (Ag, AgCl) species were observed on the XRD pattern of PdAg-HEC/ZnO catalyst. This can be explained by low metal (Pd, Ag) content in the catalyst and small particles sizes [52].
The modification of ZnO with HEC is also confirmed by standardized Brunauer–Emmett–Teller (BET) method (Table 3). HEC/ZnO and PdAg-HEC/ZnO composites were characterized by decreased surface area in comparison with initial zinc oxide. It should be noted that PdAg-HEC/ZnO catalyst demonstrated the higher surface area to compare with that of HEC/ZnO composite. This is consistent with data obtained for similar systems modified with polyvinylpirollidone (PVP) [53]. In [53] decreasing the surface area from ZnO to PVP/ZnO explained by blockage of micropores in inorganic material after modification with the polymer, while increasing the surface area from PVP/ZnO to Pd-PVP/ZnO explained by a decreasing the surface coverage of the ZnO with PVP shell through changing in an orientation of the polymer functional groups from ZnO to Pd. Another explanation for such changing the surface area from unmodified zinc oxide to polymer-modified catalyst can be in changing the degree of agglomeration of ZnO particles after modification with a polymer and following adsorption of metal ions on the polymer/ZnO composite. That is, adding of HEC solution to zinc oxide suspension can lead to agglomeration of ZnO particle probably due to ZnO-HEC-ZnO bonding. Adsorption of metal ions on HEC/ZnO composite, on the contrary, can lead to decrease the agglomeration of ZnO particles due to formation ZnO-HEC-Pd(Ag) bonds. It should be noted that according to these assumptions, HEC can interact with both zinc oxide and metal (Pd and Ag) ions.
The interaction of the polysaccharide with other components of PdAg-HEC/ZnO catalyst and formation of polymer-metal complex on ZnO surface was confirmed by infrared spectroscopy (IRS). Figure 2 shows the IR spectra of HEC, HEC/ZnO and PdAg-HEC/ZnO. HEC shows characteristic bands at 1061 and 1122 cm-1, corresponding to the C-O-C stretching vibrations in the glucopyranose structure and C-O anti-symmetric vibrations, respectively.
Other characteristic bands at 1460 and 1385 cm–1, have been attributed to O-H “plane deformation” and for C-H “symmetric bending vibrations” respectively in -CH2O- [54,55]. The shifting of absorption bands of O-H, C-O-C, C-O and -CH2O- groups in the IR spectrum of the catalyst, compared to the same bands in HEC and HEC/ZnO (Figure 2, spectra 1 to 3), confirms the interaction of HEC with both ZnO and Pd (or Ag) ions.
In our prior studies [53,56] have been shown that interaction of polymers with metal ions on a support surface led to formation smaller active phase nanoparticles (~2 nm) to compare with those in similar unmodified supported catalysts. The transmission electron microscopy (TEM) image of Pd-HEC/ZnO catalyst showed the formation of finely dispersed palladium nanoparticles of ~2 nm evenly distributed on the surface of zinc oxide modified with HEC (Figure 3). This confirms the stabilization role of the polysaccharide immobilized on ZnO.
The study of the Pd-HEC/ZnO catalyst using X-ray photoelectron spectroscopy (XPS) also confirms the presence of both polysaccharide and palladium on the surface of zinc oxide (Figure 4).
Analysis of XPS spectrum of the Pd 3d(5/2) region (Figure 4a) showed that palladium in the catalyst occurred in both oxidized (Pd2+) and reduced (Pd0) states with the binding energies of ~337.3 and ~336.0 eV, respectively [57]. The presence of the small amount of zero-valent palladium is probably caused by the photocatalytic reduction of Pd2+ in the presence of zinc oxide, which is known as an efficient photocatalyst [58]. This suggests that a small part of the introduced palladium interacts with zinc oxide. This is confirmed by the fact that binding energy of Pd0 has a positive shift (ca. 0.6 eV) due to strong metal-support interaction and formation of PdZn species [59]. In the case of palladium in oxidized state such shifting was not observed, confirming stabilization of PdO particles with the polymer. The peak of the Zn 2p(3/2) at 1020.4 eV corresponds to Zn2+ ions in a zinc oxide crystal lattice (Figure 4b) [60]. The XPS spectrum of the O 1s region (Figure 4c) was de-convoluted into two peaks with binding energies at 530.6 (peak 1) and 532.5 eV (peak 2). According to the literature data [61], peak 1 is related to O2− ions in the Zn-O bonding of the wurtzite structure, and peak 2 is attributed to oxygen of OH groups on the zinc oxide surface. Peak 2 can also be assigned to the C-O oxygen in the HEC repeated unit [62]. The de-convolution of the C 1s line in the XPS spectrum of the catalyst indicated that carbon is represented by two components (Figure 4d). The main component centered at 285.0 eV is attributed to C-C bond in HEC macromolecule and no shifting of binding energy for this peak is observed. In case of the minor peak at 288.0 eV related to C-O bond in HEC some positive shift of binding energy (ca. 1.3 eV) was observed probably due to the interaction of C-O-C, C-O and -CH2O- groups of HEC with both Pd2+ and Zn2+ of zinc oxide [62].
Thus, the characterization of polysaccharide modified catalysts using physical chemical methods such as spectrophotometry, potentiometry, elemental analysis, XRD, IRS, BET, TEM and XPS indicated that polysaccharides and metal (Pd, Ag) ions quantitatively adsorbed on zinc oxide and the polymers interacted with both ZnO and active phase particles formed. The role of polymers is both to fix Pd and Ag species on zinc oxide surface and their stabilization. This is consistent with data obtained in our prior study for similar Pd-PVP/ZnO catalyst [53]. However, in case of Pd-HEC/ZnO a small amount of Pd was also interacted with ZnO forming PdZn species. This can be explained by lesser affinity of HEC functional groups to metal ions in comparison with those of PVP.

2.2. Catalytic test

The obtained palladium and palladium-silver catalysts modified with natural polysaccharides (chitosan (Chit), 2-hydroxyethyl cellulose (HEC) or pectin (Pec)) were tested in hydrogenation process under mild conditions (0.1 MPa, 40 °C). The hydrogenation of 2-hexyn-1-ol was chosen as the model reaction. The possible pathways of the reaction are illustrated in Scheme 1. The first step triple bond of the alkynol is reduced to C=C double one forming cis/trans-isomers which then hydrogenated to alkanol.
The test results of the developed catalysts in the hydrogenation process of 2-hexyn-1-ol are given in Table 4. Polysaccharide-containing palladium-silver catalysts were found to be effective in this process. In descending order of influence on the hydrogenation rate, the stabilizing polymers form the following series: HEC > Pec > Chit (Figure 5). However, the maximum reaction rate is achieved on monometallic Pd catalysts modified with HEC (Table 4), probably due to the higher palladium content. The introduction of silver into the HEC-containing catalyst improved selectivity to 2-hexen-1-ol to compare with the selectivity on monometallic Pd catalyst (Figure 6). The cis-alkenol selectivity of process is maximal on PdAg catalyst containing HEC and achieves 97.2%. Thus, the maximum yield of the alkenol was observed on the bimetallic PdAg catalyst stabilized with HEC and reached 89.4% at 93.0% conversion (Table 4).
*Reaction conditions presented in Figure 5.
The optimum temperature and catalyst loading for the PdAg catalysts modified with HEC and Chit have been determined (Table 5). The initial reaction rate increased with increasing temperature up to 40°C.
Further temperature increase to 50°C lead to a significant decrease in the reaction rate, which could be explained by the shrinking the surface polymer layer of the catalyst and blocking the active centers [53].
The increase in the catalyst loading from 0.01 g to 0.1 g leads to the increase in the reaction rate. The highest rate was achieved on the 0.1 g load of the both catalysts. At the same time, the maximum yield and selectivity to cis-hexene-1-ol decreased, probably due to accelerated hydrogenation of double bond of the cis-alkene into an alkane.
According to the chromatographic analysis, in the presence of HEC-modified catalyst, the conversion of 2-hexyn-1-ol to cis-2-hexen-1-ol bigan in the first half of the process (Figure 7a). After almost complete disappearance of acetylenic alcohol in the reaction medium, a process of isomerisation of cis-alkenol to trans-form is initiated and parallel reduction of double bond of alkenols to saturated alcohol is performed. A similar change in the composition of the reaction mixture is observed when the process is carried out with the catalyst containing chitosan. A little difference is that the isomerization of cis- to trans-2-hexen-1-ol on this catalyst starts slightly before the complete conversion of the initial alkynol (Figure 7b).
Reusability of PdAg-Chit/ZnO and PdAg-HEC/ZnO catalysts was studied by hydrogenation of successive portions of 2-hexyn-1-ol on one the same load of catalyst (Figure 8). High stability demonstrated by reaction rate on the both catalysts remains nearly the same at least in 10 runs demonstrating high stability. This can be explained by swelling ability of polymer-metal shell of the catalysts in ethanol and negligible leaching active phase by preventing effect of the polymers [53].
Thus, a comparison of the performance of palladium and palladium-silver catalysts stabilised with derivatives of natutal polysaccharies (Table 5) as well as pectin confirms the prospectives of their use in synthesis of metal nanoparticles and further application in catalysis.

3. Materials and Methods

3.1. Materials

The 2-hexyn-1-ol, palladium chloride (PdCl2, 59-60% Pd), silver nitrate (AgNO3, 99.0%), potassium chloride (KCl, reagent grade), 2-hydroxyethyl cellulose (HEC, Mw 90,000), pectin (Pec, Mw 15,000), chitosan (Chit, Mw 250.000), zinc oxide (chemically pure) were acquired from Sigma-Aldrich, St. Louis, USA. Ethanol (reagent) was purchased from Talgar Alcohol LLP (Kazakhstan) and purified by distillation.

3.2. Preparation of K2PdCl4 precursor solution

A K2PdCl4 precursor solution was prepared by crushing 168.4 mg of palladium (II) chloride and 155.7 mg of KCl in an agate mortar according to procedure described in [53]. The obtained potassium (II) tetrachloropalladate was dissolved in 50 mL of distilled water. The process was carried out at 70°C and constant magnetic stirring for 2 hours. The concentration of palladium ions in the resulting solution was 0.019 M.

3.3. Synthesis of Pd-HEC/ZnO catalyst

The nanocatalyst was prepared by adsorption method, according to procedure described in [53,56]. А 5 mL water solution of 0.9×10-2 M 2-hydroxyethylcellulose (HEC, 0.0192 g in 5 mL of water) was added dropwise to the aqueous suspension of inorganic sorbent (1 g ZnO in 15 mL of water) and stirred for 2 hours. Then 5 mL of a 0.9×10-2 M water solution of potassium (II) tetrachloropalladate was added. The process was carried out at room temperature and constant stirring for 3 hours. The concentration of 2-hydroxyethylcellulose and potassium (II) tetrachloropalladate solutions corresponding to a palladium content of 0.5% and a molar ratio of [Pd:HEC] = 1:1. The synthesized catalyst was kept in the mother liquor for 12-15 hours. Then washing with distilled water and air drying were carried out. The amount of immobilized palladium was monitored by photoelectrocolorimetry.

3.4. Synthesis of PdAg-polysaccharide/ZnO catalysts

The method of polysaccharide adsorption on an inorganic sorbent followed by metal ions deposition was used to prepare 0.5% bimetallic PdAg polysaccharide-inorganic nanocatalysts [53,56]. 5 mL of 0.9×10-2 M polysaccharide solution (HEC, Chit or Pec) was added to the aqueous suspension of the support (1 g ZnO in 15 mL of water). The preparation process was carried out at room temperature and constant stirring for 2 h. After that, aqueous solutions of palladium and silver salts (K2PdCl4 and AgNO3) were added under constant stirring. The duration of the process was 3 h. The concentration of potassium (II) tetrachloropalladate and silver (I) nitrate water solutions corresponded to 0.5% metal content (Pd:Ag ratio = 3:1). The amount of polymer for catalyst preparation was calculated at the rate of one transition metal atom per monomer unit. After keeping the synthesized nanocatalyst in the mother liquor for 12-15 h, it was washed with distilled water and dried in air. The completeness of palladium and silver fixation was monitored by photoelectrocolorimetry and direct potentiometry, respectively.

3.5. Characterization of catalysts [53,56]

The concentration of metals (Pd, Ag) in nanocatalysts was monitored by the change in the concentration of palladium and silver ions in aqueous solution before and after immobilization of Ag+ and/or PdCl42- on an inorganic support (ZnO) modified with polysaccharide. The quantitative content of Pd in aqueous solutions was detected by photoelectrocolorimetry (PEC). The measurement was carried out on spectrophotometer SF-2000 UV/Vis (OKB "Spektr", Russia) according to the calibration curves (wavelength λ = 425 nm). Ag concentration in aqueous solutions was monitored by potentiometric method (direct potentiometry method). The measurement was carried out on an ANION 4100 ionometer ("Infraspak-Analit", Russia), using an ion-selective electrode ELIS-131Ag.
A powder X-ray diffractometer DRON-4-0.7 (Burevestnik, Russia) with monochromatized radiation of cobalt Co-Kα (λ = 0.179 nm) was used to obtain powder X-ray diffractograms.
The specific surface area and pore size distribution of the obtained nanocatalysts were investigated using the low-temperature N2 adsorption-desorption method. The study was carried out on an Accusorb instrument (Micromeritics, USA).
The catalyst samples were studied by FTIR spectroscopic method. A Nicolet iS5 instrument (Thermo Scientific, USA) was used to study the samples by FTIR spectroscopy, in the 4000-400 cm-1 region. A mixture of 1 mg of sample with 100 mg of dry potassium bromide was ground to obtain pellets for IR analysis. The mixture was then pressed in a mold.
Transmission electron microscopy (TEM) micrographs were obtained on a JEM-2100 transmission electron microscope (Jeol, Japan) with an accelerating voltage of 100 kV. The elemental analysis of the nanocatalysts was performed on a JSM-6610LV scanning electron microscope with EDX detector (Jeol, Japan).
Nanocatalysts were investigated by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra DLD photoelectron spectrometer (Kratos Analytical LTD, UK).

3.6. Hydrogenation of 2-hexyn-1-ol

The hydrogenation process was carried out in a thermostated glass reactor, according to procedure described in [53]. The reaction was carried out in ethanol medium (25 mL) at atmospheric hydrogen pressure, temperature of 20-50°C and intensive stirring (600-700 oscillations per minute). Before hydrogenation, the nanocatalyst (0.05 g) was reduced with hydrogen in the reactor for 30 min under intensive stirring. After hydrogen treatment, 2.23 mmol (0.09 mol/L) alkynol was added to the reactor. The amount of alkynol corresponded to an uptake of 100 mL of hydrogen. The hydrogenation rate was calculated from the hydrogen uptake per unit time. For this purpose, the volume of hydrogen uptake was measured after a certain time interval using a burette connected to the reactor.
To determine the selectivity for the main products of the hydrogenation reaction, a syringe sample of the reaction mixture were taken at proper time intervals.
The hydrogenation products were analyzed by gas-liquid chromatography on a Chromos GC1000 chromatograph (Russia) with a flame ionization detector in isothermal mode. A BP21 capillary column (FFAP) with polar phase (PEG modified with nitroterephthalate) of 50 m length and 0.32 mm inner diameter was used. The column temperature was 90°C, the injector temperature was 200°C, and helium served as the carrier gas. The amount of the investigated sample was 0.2 mL. The selectivity for alkenol was calculated as the fraction of the target product in the reaction products at a given degree of substrate conversion.
To determine the stability of the catalysts, hydrogenation of successive portions of alkynol (2.23-4.46 mmol) was carried out for the same sample of nanocatalyst (0.05 g).

4. Conclusions

The PdAg/ZnO nanocatalysts modified with pectin (Pec), 2-hydroxyethyl cellulose (HEC) and chitosan (Chit) have been prepared by green one-pot method by consequent introduction of water solutions of polysaccharide and metal salts into water suspension of ZnO under ambient conditions. Study of the resulting catalysts using spectrophotometry, potentiometry, elemental analysis, XRD, IRS, BET, TEM and XPS methods indicated that polysaccharides and metal (Pd, Ag) ions quantitatively adsorbed on zinc oxide and the polymers interacted with both ZnO and active phase particles formed. In case of Pd-HEC/ZnO the interaction of a small amount of Pd with ZnO and formation of PdZn species was also observed. This suggests that by varying the polymer nature in such type catalysts is it possible to regulate the composition of active phase particles.
The catalysts have shown excellent activity in the hydrogenation of model 2-hexyn-1-ol substrate at 400C and 1 atm of H2. However, in this comparative study, Pd-HEC/ZnO proved to be slightly more effective than others.
The hydrogenation reaction takes place in a swollen bulk metal-polymer surface layer that increasing lifetime of the Pd-HEC/ZnO. Thus, this catalyst combines the advantages of both homogeneous and heterogeneous catalysts.
Excellent performance at low catalyst loadings and mild reaction conditions makes polysaccharide containing metal nanocatalysts highly attractive for further improvement and testing in both hydrogenation of different types of unsaturated organic compounds and other important catalytic processes.

Author Contributions

Conceptualization, A.S.A. and A.K.Z; methodology, E.T.T.; software, F.U.B. and A.I.J.; validation and formal analysis, A.K.Z., E.T.T., A.S.A.; investigation, A.I.J. and F.U.B.; resources, A.K.Z, E.T.T. and A.S.A.; data curation, F.U.B.; writing—original draft preparation, A.K.Z, E.T.T. and A.S.A.; writing—review and editing, A.K.Z. and A.S.A.; visualization, A.I.J. and F.U.B.; supervision, project administration and funding acquisition A.K.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP09259638).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

This work was carried out with the financial support of the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grants No. AP09259638).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD of ZnO, HEC/ZnO and PdAg-HEC/ZnO.
Figure 1. XRD of ZnO, HEC/ZnO and PdAg-HEC/ZnO.
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Figure 2. IR spectra of the HEC (1), HEC/ZnO (2) and PdAg-HEC/ZnO (3).
Figure 2. IR spectra of the HEC (1), HEC/ZnO (2) and PdAg-HEC/ZnO (3).
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Figure 3. TEM microphotograph of Pd-HEC /ZnO catalyst.
Figure 3. TEM microphotograph of Pd-HEC /ZnO catalyst.
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Figure 4. The lines of the Pd 3d(5/2) (a), Zn 2p(3/2) (b), O 1s (c) and C 1s (d) from the XPS spectrum of the Pd-HEC/ZnO catalyst.
Figure 4. The lines of the Pd 3d(5/2) (a), Zn 2p(3/2) (b), O 1s (c) and C 1s (d) from the XPS spectrum of the Pd-HEC/ZnO catalyst.
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Scheme 1. Hydrogenation of 2-hexyn-1-ol.
Scheme 1. Hydrogenation of 2-hexyn-1-ol.
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Figure 5. The rate of 2-hexyn-1-ol hydrogenation on the PdAg/ZnO catalysts modified with hydroxyethyl cellulose (HEC), chitosan (Chit) and pectin (Peс). Reaction conditions: T = 40°C, PH2 = 1 atm, mcat = 0.05 g, ethanol = 25 mL, and Calkynol = 0.09 mol/L.
Figure 5. The rate of 2-hexyn-1-ol hydrogenation on the PdAg/ZnO catalysts modified with hydroxyethyl cellulose (HEC), chitosan (Chit) and pectin (Peс). Reaction conditions: T = 40°C, PH2 = 1 atm, mcat = 0.05 g, ethanol = 25 mL, and Calkynol = 0.09 mol/L.
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Figure 6. Selectivity of 2-hexen-1-ol vs. conversion of 2-hexyn-1-ol on Pd-HEC and PdAg-HEC catalysts supported on ZnO. Reaction conditions presented in Figure 5.
Figure 6. Selectivity of 2-hexen-1-ol vs. conversion of 2-hexyn-1-ol on Pd-HEC and PdAg-HEC catalysts supported on ZnO. Reaction conditions presented in Figure 5.
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Figure 7. Changes in the composition of the reaction mixture at the hydrogenation of 2-hexyn-1-ol in the presence of PdAg-HEC/ZnO (a) and PdAg-Chit/ZnO (b). Reaction conditions presented in Figure 5.
Figure 7. Changes in the composition of the reaction mixture at the hydrogenation of 2-hexyn-1-ol in the presence of PdAg-HEC/ZnO (a) and PdAg-Chit/ZnO (b). Reaction conditions presented in Figure 5.
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Figure 8. Hydrogenation of 10 portions of 2-hexyn-1-ol on in PdAg-Chit/ZnO (a) and PdAg-HEC/ZnO (b). Reaction conditions presented in Figure 5.
Figure 8. Hydrogenation of 10 portions of 2-hexyn-1-ol on in PdAg-Chit/ZnO (a) and PdAg-HEC/ZnO (b). Reaction conditions presented in Figure 5.
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Table 1. Results of adsorption of Pd2+ and Ag+ ions on polymer modified ZnO.
Table 1. Results of adsorption of Pd2+ and Ag+ ions on polymer modified ZnO.
Catalysts Concentration of metal ions in mother liquor C(Me)×10-6, mol L-1 Degree of adsorption, % Pd and Ag content in catalyst, %
Before adsorption After adsorption
Pd2+ Ag+ Pd2+ Ag+ Pd2+ Ag+ Pd2+ Ag+ Total
Pd-HEC/ZnO 1889.14 - 164.36 - 91.3 - 0.4565 - 0.46
PdAg-HEC/ZnO 1416.86 465.85 98.58 0.0020 93.0 100.0 0.3489 0.1250 0.47
PdAg-Chit/ZnO 1416.86 465.85 21.16 0.0016 98.5 100.0 0.3694 0.1250 0.49
PdAg-Pec/ZnO 1416.86 465.85 45.07 1.25 96.8 99.7 0.3631 0.1247 0.49
Table 2. Results of elemental analysis of the catalysts obtained.
Table 2. Results of elemental analysis of the catalysts obtained.
Sample Elemental composition of the catalyst, wt%
Pdcalcd/detd Agcalcd/detd Zncalcd/detd
Pd-HEC/ZnO 0.50/0.49 - 79.0/77.8
PdAg-HEC/ZnO 0.37/0.36 0.13/0.18 79.0/81.5
PdAg-Chit/ZnO 0.37/0.44 0.13/0.15 79.0/81.6
PdAg-Pec/ZnO 0.37/0.48 0.13/0.16 79.0/82.0
Table 3. Surface area of ZnO, HEC/ZnO and PdAg-HEC/ZnO.
Table 3. Surface area of ZnO, HEC/ZnO and PdAg-HEC/ZnO.
Sample Surface area, m2 g-1
ZnO 8.7
HEC/ZnO 1.1
PdAg-HEC/ZnO 5.2
Table 4. The results of 2-hexyn-1-ol hydrogenation on PdAg/ZnO catalysts stabilized with hydroxyethyl cellulose (HEC), chitosan (Chit) and pectin (Peс).*.
Table 4. The results of 2-hexyn-1-ol hydrogenation on PdAg/ZnO catalysts stabilized with hydroxyethyl cellulose (HEC), chitosan (Chit) and pectin (Peс).*.
Catalysts Wmax·10-6 (mol s-1) Maximum yield of cis-hexen-1-ol, % Scis-hexen-1-ol,% Conversion, %
Pd-HEC/ZnO 4.3 82.8 90.6 91.4
PdAg-HEC/ZnO 4.0 89.4 97.2 93.0
PdAg-Chit/ZnO 2.6 85.9 92.3 93.1
PdAg-Pec/ZnO 2.8 86.4 93.5 92.4
Table 5. Effect of variation of temperature and catalyst loading on reaction conditions in hydrogenation of 2-hexyn-1-ol (0.09 mol/L) in ethanol (25 mL).
Table 5. Effect of variation of temperature and catalyst loading on reaction conditions in hydrogenation of 2-hexyn-1-ol (0.09 mol/L) in ethanol (25 mL).
Reactionparameters Catalysts
PdAg-Chit/ZnO PdAg-HEC/ZnO
Wmax·10−6 (mol s−1) Maximum yield of cis-hexen-1-ol, % Scis-hexen-1-ol, % Wmax·10−6 (mol s−1) Maximum yield of cis-hexen-1-ol, % Scis-hexen-1-ol, %
Temperature, °C
20 0.6 84.0 91.8 0.4 77.0 93.0
30 2.3 76.4 93.5 1.3 75.6 84.9
40 2.6 85.9 92.3 4.0 89.4 97.2
50 2.3 79.3 79.8 1.6 76.2 86.7
Catalyst loading, g
0.01 0.6 62.0 67.8 0.5 58.4 63.2
0.03 1.9 71.8 84.7 2.5 88.2 89.8
0.05 2.6 85.9 92.3 4.0 89.4 97.2
0.10 2.8 83.5 89.9 4.4 87.6 94.5
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