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Article

Cr-Zn/Ni-Containing Nanocomposites as Effective Magnetically Recoverable Catalysts for CO2 Hydrogenation to Methanol: The Role of Metal Doping and Polymer Co-Support

by
Svetlana A. Sorokina
1,
Nina V. Kuchkina
1,
Maxim E. Grigoriev
2,
Alexey V. Bykov
2,
Andrey K. Ratnikov
1,
Valentin Yu. Doluda
2,
Mikhail G. Sulman
2 and
Zinaida B. Shifrina
1,*
1
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow 119991, Russia
2
Department of Biotechnology and Chemistry, Tver State Technical University, 22 A. Nikitina St., Tver 170026, Russia
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(1), 1; https://doi.org/10.3390/catal13010001
Received: 17 November 2022 / Revised: 9 December 2022 / Accepted: 15 December 2022 / Published: 20 December 2022
(This article belongs to the Special Issue Transition-Metal-Containing Bifunctional Catalysts: Design and Catalytic Applications)
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Abstract

:
CO2 hydrogenation to methanol is an important process that could solve the problem of emitted CO2 that contributes to environmental concern. Here we developed Cr-, Cr-Zn-, and Cr-Ni-containing nanocomposites based on a solid support (SiO2 or Al2O3) with embedded magnetic nanoparticles (NPs) and covered by a cross-linked pyridylphenylene polymer layer. The decomposition of Cr, Zn, and Ni precursors in the presence of supports containing magnetic oxide led to formation of amorphous metal oxides evenly distributed over the support-polymer space, together with the partial diffusion of metal species into magnetic NPs. We demonstrated the catalytic activity of Cr2O3 in the hydrogenation reaction of CO2 to methanol, which was further increased by 50% and 204% by incorporation of Ni and Zn species, respectively. The fine intermixing of metal species ensures an enhanced methanol productivity. Careful adjustment of constituent elements, e.g., catalytic metal, type of support, presence of magnetic NPs, and deposition of hydrophobic polymer layer contributes to the synergetic promotional effect required for activation of CO2 molecules as well. The results of catalytic recycle experiments revealed excellent stability of the catalysts due to protective role of hydrophobic polymer.

1. Introduction

Human impact on the environment has become a critical issue for the last 20 years. Emissions of greenhouse gases are now believed to be responsible for global warming and have gained worldwide concern. CO2 is one of the most emitted gases with level in the atmosphere raised from 315 ppm in 1958 when continuous observations began to 420 ppm in 2021 [1,2]. In this regard, CO2 utilization and transformation into valuable chemicals are of particular importance.
CO2 can be considered as a carbon feedstock alternative to hydrocarbons whose combustion depletes the carbon sources. The possible transformation routes for CO2 molecule via catalytic hydrogenation include the formation of formic acid, methane, methanol, and DME [3,4]. The production of methanol from CO2 is considered as one of the most effective ways for decreasing the CO2 concentration [5,6]. Methanol is an important platform molecule which can be converted into a variety of reagents, such as formaldehyde, acetic acids, and others [7,8,9]. It is a component of fuel blends, antifreezes, resins, and plastics [10]. Currently methanol is almost exclusively synthesized from syngas via Fischer-Tropsch process over the Cu-ZnO-Al2O3 catalyst [11]. However, recent reports have defined CO2 methanolization as one of the most exciting commercial applications for CO2 capture [4,10,12,13].
The heterogeneous catalysts employed for CO2 to methanol hydrogenation are usually ternary catalytic systems comprised of bimetallic species deposited on a solid support. While Cu-ZnO-based materials are the most investigated systems due to high activity and ability to operate under moderate reaction conditions, recent studies have been focused on expanding the scope of catalytic systems and chemical element combinations [13,14]. Thus, Cu-In [15], Pd-Zn [16,17,18], Rh-In [19], Pd-Cu [20], and Ni-Ga [21,22] catalytic systems have been explored. The key role of support is also highlighted in numerous papers [14,23]. A strong metal-support interaction may direct the reaction pathway, enhancing the selectivity and activity. For example, Ni-based catalysts without proper modification deliver only methane as a main product of CO2 hydrogenation, while Ni deposited on Ga2O3 provides methanol with high selectivity [22]. The effect of metal-support interaction is attributed to the electronic and structural modification due to formation of oxygen vacancies, electronic doping, formation of alloy interface, and charged metal moieties, etc. [14,24,25,26]. Al2O3, CeO2, ZrO2, SiO2, and TiO2 are the widely examined solid materials for this purpose.
However, the explored catalysts usually suffer from relatively poor activity and low selectivity due to the presence of reverse water-gas shift (RWGS) reaction that proceeds along with the target hydrogenation reaction. RWGS reaction leads to the formation of CO and H2O vapors lowering the methanol yield [14]. Moreover, water vapors also cause a notable decrease in catalyst stability and reusability [27,28]. Another major shortcoming of the nanosized catalysts used in methanol synthesis is the NP agglomeration that occurs due to the sintering process induced by high-reaction temperature and pressure [29,30]. Strong and reliable stabilization of NPs may suppress the sintering. This could be achieved by appropriate interactions with a support and/or incorporation of additional ligand molecules creating a barrier between metal species [23].
Therefore, novel catalysts developed for methanol synthesis from CO2 should withstand harsh reaction conditions, preserve stability under high water partial pressure, possess high thermostability, and have easily adoptable microstructures. The synthetic procedures employed in catalyst production should be simple and scalable due to potential industrial implementation of the process.
In our preceding work, we developed the catalysts based on metal NPs stabilized by aromatic polymers for different hydrogenation reactions, such as synthesis of methanol from syngas [31,32,33], synthesis of gamma-valerolactone from levulinic acid [34], and furfuryl alcohol from furfural [35]. These catalysts demonstrated exceptional activity and stability which have been preserved in several consecutive catalytic cycles. The synthesis of these nanocomposites has been carried out by thermal decomposition of metal precursors in the presence of a polymer acting as a stabilizing media. Despite the precious control over the NP size and morphology provided by this technique, its employment in CO2 methanolization is not fully reasonable because of low catalyst yield. Indeed, the approaches to nanocomposite formation providing several grams of catalyst during simple synthetic procedures are preferable. This could be achieved by addition of a solid support to the catalyst formulation. More recently, we developed an approach to the formation of a thin polymer layer on a mesoporous silica gel containing magnetic nanoparticles [36]. After incorporation of palladium species by coordination with pyridine groups of the polymer, the resulting nanocomposites were tested in Suzuki-Miyaura reaction. The results of recycle experiments revealed the important role of the branched polymer layer in NPs stabilization and superior catalytic activity under repeated use [36].
In this work, we applied this approach to fabricate the novel nanostructured catalytic systems for methanol synthesis from CO2 + H2. Deposition of a thin polymer layer which is hydrophobic in nature on a solid support (SiO2 or Al2O3) containing metal NPs could be one of the possible strategies for attenuating the negative influence of H2O vapors. Moreover, the polymer adsorption contributes to NP segregation and stabilization, thus preventing the possible sintering and leaching of NPs. Both processes are known to be responsible for significant drops in the catalyst activity [29,30].
Here we synthesized and explored Cr-, Cr-Zn-, and Cr-Ni-containing magnetically recoverable catalysts based on two different inorganic supports—SiO2 and Al2O3—covered by a polymer layer. Recently, Cr2O3 has been shown to possess a potential activity in CO2 methanolization [37] while having an established productivity in the methanol synthesis from syngas [33,38,39]. The oxygen vacancies presented on the surface of Cr2O3 make it suitable for activation of thermodynamically stable and inert CO2 molecule [37,40]. The activity of Cr2O3 can be adopted by doping with different metals like Ni, La, etc. [41]. In this work we expanded the studies of Cr2O3 catalytic activity in methanol synthesis via CO2 hydrogenation. To elucidate the main factors underlying the high-catalyst activity, we fabricated both monometallic and bimetallic catalytic systems. Magnetic NPs included in the catalyst structure do not possess the catalytic activity in this reaction; however, they may boost methanol productivity while ensuring the effective magnetic separation for repeated uses and more sustainable processes.

2. Results and Discussion

To meet requirements for effective CO2 methanolization the designed catalytic system should be (1) stable under harsh reaction conditions and high partial pressure of water vapors, (2) easily separable from reaction mixture to allow the repeated uses, and (3) synthesized via robust procedure. To develop such catalyst, we used a combination of synthetic methods.
At the first step, Fe3O4 NPs were formed in the pores of commercially available mesoporous solid support according to the procedure described previously [42]. Here we used two different supports: Al2O3 and SiO2 to further assess their influence on the catalytic activity. Thermal decomposition of iron nitrate in the presence of the support and mild reducing agent led to the formation of magnetic nanoparticles of 13.2 and 15.6 nm for SiO2- and Al2O3-based composites, respectively, oriented alongside the pores [42,43]. Magnetic NPs ensure the easy and simple separation of catalyst from the reaction mixture by external magnet. After formation of magnetic support, a thin layer of cross-linked pyridylphenylene polymer (PPP) has been deposited by Diels Alder polycondensation of two branched monomers in the presence of magnetic silica/aluminia. Structure of PPP together with the scheme of formation of a solid support -Fe3O4 –PPP composite are given in Figure S1 in SI. PPP is a thermally stable polymer whose thermal decomposition starts at 500 °C which is important for practical application [32,34]. It is worth noting that deposition of PPP on the surface of support does not decrease the porosity of material due to cross-linked hyperbranched structure, as was established earlier [36]. This means that a polymer layer does not hinder the active sites of the future catalyst and does not prevent the diffusion of substrate molecules. The presence of pyridine moieties in PPP structure may contribute to more effective chemisorption of CO2 molecules being the first step of catalysis because of electropositivity of CO2 carbon atom.
The metal species, namely Cr, Cr-Zn, and Cr-Ni, have been incorporated into the structure of SiO2/Al2O3-Fe3O4-PPP composites by wet impregnation method of corresponding metal acetyl acetonates followed by a thermal decomposition in a furnace tube at 350 °C. To elucidate the possible promoting effect of Fe3O4 molecules, the nanocomposites without magnetic NPs have been synthesized as well.

2.1. Structure and Morphology of the Nanocomposites

To identify the position of all species in the composites, STEM EDS mapping was performed. Figure 1 shows the STEM dark field image and EDS maps of SiO2-Fe3O4-PPP-Cr-Zn nanocomposite. The carbon map demonstrates a full coverage of silica by polymer layer. Fe map totally repeats the shape of Si map indicating the formation of Fe3O4 NPs in silica pores. Cr and Zn maps are similar to Fe map which may correspond to the incorporation of Cr and Zn species in magnetite structure. It is worth noting that deposition of catalytic species on the top of magnetic oxide with formation of heterogeneous hybrid nanocomposite is more preferable for effective catalysis rather than homogeneous nucleation of separately being Cr2O3 and ZnO2 nanoparticles [25,44].
STEM EDS images for SiO2-Fe3O4-PPP-Cr-Ni nanocomposite are given in Figure 2. Again, C, Si, Fe, Cr, and Ni maps show a good fit revealing the formation of magnetic and catalytic species in the silica covered by polymer. However, Ni species are distributed across the sample with some aggregation of NPs. This may be due to a poor intermixing of Ni species with a magnetite phase.
STEM EDS maps of Al2O3-Fe3O4-PPP-Cr-Zn, SiO2-PPP-Cr-Zn, and SiO2-Fe3O4-PPP-Cr are presented in Figures S2–S4, respectively. Replacement of SiO2 with Al2O3 did not influence the deposition of PPP on the support. The positions of metal species for all composites demonstrated their perfect distribution over the support-polymer area and a good correlation with Fe map.
To assess the chemical composition of the materials obtained, EDS spectra were recorded (Figure 3a,b for SiO2-Fe3O4-PPP-Cr-Zn, and SiO2-Fe3O4-PPP-Cr-Ni, respectively, and Figure S5a,b for SiO2-PPP-Cr-Zn and SiO2-Fe3O4-PPP-Cr, respectively). The spectra confirmed the presence of constituent elements together with successful deposition of a polymer layer which is evidenced by the presence of carbon in the composite. The elemental compositions for all samples are given in Table S1.
Figure 4 displays the XRD spectra of SiO2-Fe3O4-PPP (a), SiO2-Fe3O4-PPP-Cr-Zn (b), and SiO2-Fe3O4-PPP-Cr-Ni (c) nanocomposites, which are very similar to each other and show a polymer halo at ~10° 2θ and a broad signal at 22° corresponding to amorphous silica. A set of reflections characteristic for a spinel structure of magnetite is also observed for all samples. Surprisingly, no additional reflections are detected, which could be attributed to the formation of zinc or chromium oxides. This could be due to the formation of amorphous oxide structures or different ferrites whose reflections are overlapped with those of Fe3O4 or due to intermixing of Zn, Cr, and Ni species with a magnetite phase that do not disturb the Fe3O4 crystalline structure. The more careful inspection of XRD-spectra revealed a slight shift of (311) peak of Fe3O4 to smaller angles for Cr-Zn-containing composites. The enlarged depiction of this region is presented in Figure S6. This could indicate a partial penetration of Cr and Zn species in a magnetite structure as it was reported earlier [42,45,46]. The lattice constants calculated from XRD data showed an increase from 8.3801 for SiO2-Fe3O4-PPP to 8.3982 for SiO2-Fe3O4-PPP-Cr-Zn. The effect could be attributed to the substitution of some Fe ions with Zn2+ and Cr3+, and formation of ferrites. Another possible explanation is the penetration of Zn2+ and Cr3+ possessing the higher ionic radii into Fe3O4 NP.
In case of composite without Fe3O4 only the signals corresponding to PPP and SiO2 are presented in diffractogram (Figure S7a). This suggests the formation of amorphous Cr- and Zn-containing species. At the same time, for Al2O3-based composites containing Fe3O4, the diffraction pattern contains a set of peaks corresponding to Al2O3 and reflections attributed to Fe3O4 structure (Figure S7b,c). Similar to composites based on SiO2-, the reflections characteristic for Cr and Zn oxides are absent.
To achieve further insight into the structure of the composites obtained, XPS spectra were recorded. The survey spectra of the samples are given in Figure S8. The oxidation states of the metal species were analyzed by high resolution (HR) XPS. In all samples, Fe 2p spectrum contains a main peak at 711.0 eV which is typical for iron oxides [47] (Figure 5a for SiO2-Fe3O4-PPP-Cr-Zn, and SI for other Fe3O4-containig composites). A region between Fe 2p3/2 and Fe 2p1/2 displays a plateau indicating the formation of Fe3O4. In the opposite case (formation of Fe2O3), the prevalence of Fe3+ ions would result in a satellite peak with binding energy 8 eV higher than the main peak [47]. Importantly, impregnation by Cr, Zn, and Ni species do not disturb the Fe3+/Fe2+ ratio. If any substitution of Fe2+ by Zn2+ or Ni2+ would appear, which took place upon the formation of ferrites structures like ZnFe2O4, NiFe2O4, ZnCrFeO4, or chromite, this would influence the Fe3+/Fe2+ ratio and lead to the occurrence of additional signals, in particular the satellite peak, or peak shifting (See Figures) [48,49]. However, the Fe 2p spectra before and after incorporation of metal compounds are similar (See Figure 5a and Figure S9).
The analysis of Cr 2p XPS spectra (Figure 5b) revealed that Cr solely exists in the form of Cr3+. The spin-orbit splitting energy (Cr 2p3/2−Cr 2p1/2 = 9.76 eV) coincides with that of Cr2O3 [50]. The positions of peaks at Zn 2p XPS spectra with binding energies of 1022.8 and 1045.9 eV for 2p3/2 and 2p1/2, respectively, correspond to Zn2+ typical for ZnO (Figure 5c) [51]. For Ni-containing composite, the positions of peaks at HR XPS spectrum match Ni2+ state in oxide-hydroxide form [49,52,53] (Figure 5d). The deconvolution parameters for all spectra are summarized in Tables S2–S7. HR XPS of Fe, Cr, and Zn for all nanocomposites are presented in SI, Figures S9–S13. The positions of the main peaks are consistent with those described in Figure 5.
Thus, one can conclude that formation of ferrites is not observed, and decomposition of Cr, Zn, and Ni precursors leads to formation of amorphous metal oxides. However, considering the peak shifting (Figure S6) and increase in the values of lattice constants revealed by XRD, we assumed the possible diffusion of Cr, Zn, and Ni species into magnetic NPs. This assumption is based on XPS data for 2p and 3p electrons. 2p electrons are known to possess considerably lower kinetic energy than that of 3p electrons. Thus, the contribution of 2p electrons in the spectrum reflects the surface concentration of the element, while 3p electrons characterize the deeper layers of the material. Data on 2p and 3p content of the metals in the nanocomposites are presented in Table 1. As one can see, the enrichment of the surface with metal is dependent on the metal type. For all samples, the composites surface was enriched with Cr3+ species since the chrome content for 2p electrons was higher than that of 3p electrons. However, small amount of Cr3+ species were able to penetrate the deeper magnetite NP level as well. For Ni-containing samples, XPS data show the surface accumulation of Ni species. Surprisingly, Zn content was higher for 3p electrons which means a higher fraction of Zn in the deeper layers rather than at the surface. This phenomenon could be explained by a better compatibility of Zn2+ with Fe3O4 in comparison with that of Cr3+ and Ni2+. Although, the possibility of solid solution and alloy formation is governed by Hume-Rothery rules, small metal clusters do not always follow the rules [54]. In addition, the thermal decomposition of zinc acetyl acetonate starts at 190 °C and ZnO NPs formation was reported to proceed at 195 °C [55,56]. For chromium acetyl acetonate, the decomposition temperature is much higher (250 °C) [57]. The differences in decomposition temperature and consequent earlier destruction of Zn precursor are another reason for deeper migration of Zn2+ into a magnetite structure in comparison with Cr3+.
To conclude, most probably the decomposition of Cr, Zn, and Ni acetyl acetonates, in the presence of magnetite-containing solid supports covered by PPP, results in the formation of thin metal oxide layers on SiO2(Al2O3)-Fe3O4-PPP composites together with a partial diffusion of metal oxides into magnetic NP. The diffusion was the most prominent for Zn2+ ions and slightly occurred for Cr3+. Such intermixing does not influence the Fe3+/Fe2+ ratio and the magnetite structure was preserved in all samples. Considering the structural data obtained, the formation of ferrite structures like ZnFe2O4, ZnCrFeO4, and NiFe2O4 were not established. Therefore, intermixing of ZnO and Cr2O3 with magnetite NP stands at the intermediate stage before the ferrite formation which has been reported elsewhere [48].
Since the adsorption of CO2 and H2 plays an important role for the CO2 hydrogenation process, the adsorption capacities of the composites have been estimated by CO2 and H2 temperature-programmed desorption (TPD) measurements. The results are presented in Figure 6 and Figure 7. To elucidate the polymer ability to enhance the CO2 adsorption, measurements were carried out for parent SiO2-Fe3O4 and after deposition of PPP-SiO2-Fe3O4-PPP (Figure 6a). Placement of a thin polymer layer on a solid support significantly increases the CO2 adsorption. While parent SiO2-Fe3O4 is characterized by two types of adsorption centers (moderate center at 266 °C and strong center at 478 °C) with the total amount of adsorbed CO2 equal to 0.015 mmol/g, the addition of PPP results in noticeable changes in adsorption centers. The appearance of new desorption peaks at 244 °C, 294 °C, 366 °C, and 562 °C is accompanied by increase in CO2 amount up to 0.035 mmol/g. The effect is undoubtedly attributed to the presence of basic pyridine centers in PPP structure. Since the CO2 adsorption is the first step of the catalytic reaction, introduction of PPP into the catalyst structure should enhance the catalysis rate due to advanced CO2 adsorption.
The incorporation of metal species further influenced the CO2 adsorption capacities of the composites (Figure 6b). Thus, monometallic SiO2-Fe3O4-PPP-Cr showed the adsorption capacity of 0.073 mmol/g exceeding that of SiO2-Fe3O4-PPP. It is worth noting that strong adsorption centers (at 562 °C) arising due to PPP structure are retained in composites with catalytically active metals. Formation of bimetallic composites leads to weakening of total basicity to 0.032 mmol/g. Nevertheless, the composites efficiently adsorb CO2 which is a prerequisite for effective catalysis.
The H2 adsorption capacities of the synthesized nanocomposites were also estimated (Figure S14a,b). Similarly, deposition of PPP leads to an increase in amount of adsorbed H2 (0.023 mmol/g for SiO2-Fe3O4 vs 0.03 mmol/g for SiO2-Fe3O4-PPP). Incorporation of metal species and formation of bimetallic nanocomposite increases the total amount of adsorbed H2 to 0.043 mmol/g.

2.2. Catalytic Properties of Nanocomposites in CO2 Hydrogenation

Cr-, Cr-Zn-, and Cr-Ni-containing nanocomposites have been tested in CO2 hydrogenation to methanol. The process is challenging due to the high thermodynamic stability of CO2 molecule. The formation of methanol from CO2 is energetically undesired, implying the high temperature and pressure along with structural promotion of the catalyst should be applied to obtain the sufficient methanol productivity. Table 2 displays the results of the catalyst performance. The widely accepted approach to assess the catalyst activity in this reaction is to calculate the methanol productivity rate per the catalyst amount and per the metal content. The activity of the catalysts was compared with commercial catalyst MegaMax 800. To elucidate the optimal ratio of the metals, nanocomposites with different metal loadings have also been synthesized. Here we focused on the best catalytic results obtained, while the activity of the composites differed in metal ratio is given in SI, Table S8.
The magnetic silica with deposited PPP expectedly shows no catalytic activity. The monometallic composite SiO2-Fe3O4-PPP-Cr provides the methanol productivity comparable with that of a commercial catalyst. The result confirms the ability of Cr2O3 to catalyze the methanol synthesis from CO2 and H2. The incorporation of additional metals leads to an increase in the catalyst activity with the highest methanol productivity observed for the Cr-Zn-containing composite. Notably, the SiO2-based, Cr-Zn-containing composite was in two orders of magnitude more active than Al2O3-based system prepared with the same metal loadings. The catalytic activity of the composite without magnetic NPs-SiO2-PPP-Cr-Zn-was considerably lower than that of the Fe3O4-containing sample, revealing the promoting effect provided by Fe3O4 NPs. It also should be noted that a decrease in metal loading leads to a decrease in catalytic activity (Table S8).
To analyze the enhanced catalytic activity of SiO2-based systems, the nitrogen adsorption-desorption measurements were carried out. The adsorption isotherms are presented in Figure S15. The analysis revealed the distinct textural properties of SiO2 vs Al2O3 composites with the smaller BET surface area and pore sizes of the latter. This could have a detrimental effect on the catalyst activity due to worse accessibility of active catalytic species and diffusion limitations. Several studies also report a positive influence of SiO2 on catalytic activity in CO2 hydrogenation reaction. The addition of SiO2 as itself to convenient Cu-ZnO-Al2O3 catalyst increased the activity and enhanced CuO dispersion [58]. The similar promoting effect has been observed in another study [59]. SiO2-modified Cu-ZnO-ZrO2 catalyst exhibits a better catalytic activity and higher long-term stability in comparison with a non-modified system due to a better intermixing of metal oxide components [59]. SiO2 has also been shown to accelerate the selective hydrogenation of CO2 to methanol for Co-based catalysts [60]. The effect was attributed to efficient stabilization of methoxy *CH3O species being intermediates in the catalytic process by interface formed between SiO2 and catalytic metal—Me-O-SiOn. Our results support the data on the promotional effect of silica on methanol synthesis from CO2, and highlight the importance of careful adjustment and optimization of catalyst composition by the appropriate choice of support.
Our results demonstrated that the introduction of magnetic NPs in catalyst structure along with convenience and simplicity of magnetic separation provided the promoting effect on methanol productivity. We assume that Fe3O4 NPs serve as a reservoir for a better intermixing and dispersion of ZnO and Cr2O3 components. Such assumption is supported by XPS data. This led to a closer contact between catalytic species being a prerequisite for effective catalytic performance. The positive effect of spinel structures of Fe3O4 on methanol synthesis has also been reported previously [42,61]. The addition of Fe3O4 to Cu-Zn-based catalyst results in improved methanol selectivity and enhanced catalyst activity. Magnetic NPs have also been shown to induce a strong metal support interaction (SMSI) effect due to formation of oxygen vacancies that stabilize the reaction intermediates and activate CO2 molecule [42,62,63].
To conclude, each structural element in the designed nanocomposites influences the catalytic activity and the synergistic effect between catalyst constituents such as: support, polymer layer, magnetic NPs, and metal species ensures high catalytic activity and stability. The type of support determines distribution of metal species, accessibility of active centers, and occurrence of strong metal-support interactions which were shown to positively influence the catalysis. Here we demonstrated the advantages of SiO2 support over Al2O3. Magnetic NPs boost the catalyst activity due to metal intermixing and migration of catalytic species onto Fe3O4 surface, as was observed by XPS data on 2p/3p electron content and XRD analysis. The precious miscibility of metal species allows for better catalytic performance, as was evidenced by the higher activity of Cr-Zn-containing catalyst over Cr-Ni- one. Bimetallic Cr-containing systems were shown to outperform monometallic ones. The results are consistent with the data on the activity of other heterogeneous catalysts for CO2 hydrogenation, and bimetallic systems typically demonstrate a better catalytic performance [4,10,13,14]. The formation of bimetallic interfaces notably lowers the activation barrier of CO2 to methanol hydrogenation compared to monometallic systems [64]. For industrial Cu-ZnO-Al2O3 catalyst, Cu-ZnO interface is believed to be the active site [65]. Moreover, the formation of bimetallic interfaces is usually followed by the formation of oxygen vacancies which are crucial for CO2 activation and intermediate stabilization [14,63]. In particular, chromium doping with metals has been shown to induce the oxygen vacancy formation [41,66].
Comparison of the catalytic results with the literature data undoubtedly demonstrated the advantages of the proposed approach to creation of effective catalysts for the synthesis of methanol from CO2 [67,68,69,70,71,72,73,74,75]. The results of recycling experiments revealed the exceptional stability of the catalysts after six repetitive uses. The methanol productivity rate was preserved at nearly 98% for SiO2-based catalysts, while 5% drop of activity was observed for Al2O3-containing systems starting from the fifth use (Figure 7). We believe that superior catalyst stability is due to the protective role of the hydrophobic polymer layer. In contrast to the PPP-covered SiO2/Al2O3-catalysts, the conventional catalytic systems show 11% drop of activity after 36 h of catalytic experiment [30]. The deactivation was induced by agglomeration of ZnO species and sintering of Cu particles for CuO-ZnO-Al2O3 system [30]. Sintering and NP aggregation have also been proved to be the main reason of deactivation for other catalytic systems [29].
To elucidate the polymer layer role, the catalysts after catalysis have been separated from the reaction mixture with external magnet and analyzed by STEM EDS. SiO2-Fe3O4-PPP-Cr-Zn and Al2O3-Fe3O4-PPP-Cr-Zn have been chosen for analysis. The results are presented in Figures S16 and S17. For both samples, Cr- and Zn-species remained evenly distributed over all samples with no visible bulk NPs which could be due to the aggregation of metal species during catalysis. The results confirm the outstanding stability of the catalytic systems. Therefore, the deposition of PPP not only facilitates the catalytic reaction through the enhanced adsorption of CO2, but also ensures the structural separation of NPs against sintering.

3. Materials and Methods

3.1. Materials

Iron (III) nitrate nonahydrate (ABCR, 98%), mesoporous silica gels (Sigma-Aldrich, Darmstadt, Germany, 6 nm, 200–425 mesh), aluminum oxide (Sigma-Aldrich, 5.8 nm, 150 mesh), zinc acetylacetonate hydrate (Sigma-Aldrich), chromium (III) acetylacetonate (Sigma-Aldrich, 97%), nickel (II) acetylacetonate (ABCR, 98%), ethylene glycol (Sigma-Aldrich, 99%), and diphenyl ether (Sigma-Aldrich, 99%) were used as received. Acetone (99.5%) and ethanol (96%) were purchased from “Component-reactive” and used as received. Magnetic NPs in the pores of mesoporous silica gel and aluminum oxide were synthesized according to procedures published in [42,43], respectively. Deposition of polymer PPP layer on the surface of magnetic silica gel was performed as described previously [36]. Monomers for the synthesis of PPP in the presence of solid support (see Figure S1) were synthesized according to [76].

3.2. Synthesis of Al2O3-Fe3O4-PPP

Al2O3-Fe3O4 (0.2027 g) was heated in a Schlenk reaction flask under flowing argon atmosphere at 120 °C for 2 h to remove moisture. Monomer A6 (50 mg, 0.037 mmol) was dissolved in 3 mL of dichloromethane. This solution was added to Al2O3-Fe3O4 and sonicated for 20 min to allow the monomer to adsorb on the Al2O3-Fe3O4 surface. The flask was placed in a rotary evaporator and the solvent was evacuated. B2 (0.087 g, 0.111 mmol) was dissolved in 3 mL of diphenyl ether and added to the reaction flask containing A6 deposited on Al2O3-Fe3O4. The reaction flask was filled with argon and heated at 160 °C for 10 h upon stirring. The final material (Al2O3-Fe3O4-PPP) was collected from the suspension using a rare earth magnet, washed with dichloromethane (8 × 10 mL), and dried at room temperature overnight in vacuo. The elemental analysis data: N 1.23%, C 25.45%, H 1.89%.

3.3. Synthesis of Catalytically Active Nanocomposites

In a typical experiment, 0.23 g of zinc acetylacetonate were dissolved in 10 mL of acetone, followed by a dropwise addition to 2.57 g of Fe3O4-SiO2-PPP (or Al2O3-Fe3O4-PPP). After acetone evaporation, the solution containing 0.70 g of chromium (III) acetylacetonate dissolved in 10 mL of acetone was added to Fe3O4-SiO2-PPP (or Al2O3-Fe3O4-PPP) coated with zinc acetylacetonate. The mixture was allowed to stir for 3 h in air for acetone evaporation. The sample was then dried in a vacuum oven at room temperature for 12 h. Then the sample was heated in a tube furnace under argon with 7% of H2 to 350 °C with a heating rate of 2 °C/min. The temperature was held at 350 °C for 3 h and then the sample was cooled to room temperature. For the Cr-Ni-containing sample (SiO2-Fe3O4-PPP- Cr-Ni), 0.22 g of nickel acetyl acetonate was added instead of zinc precursor. For SiO2-Fe3O4-PPP-Cr nanocomposite, only 0.70 g of chromium (III) acetylacetonate in 10 mL of acetone was used. To assess the metal content in the resulted nanocomposites, X-ray fluorescence (XRF) measurements were applied.

3.4. Characterization

Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray (EDX) microanalysis was carried out in an Osiris TEM/STEM (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a high angle annular dark field detector (HAADF) (Fischione, Export, PA, USA) and an X-ray energy dispersive spectrometer Super X (ChemiSTEM, Bruker, Bradford County, FL, USA) at an accelerating voltage of 200 kV. Specimens for TEM, STEM, and EDXS studies were prepared by placement of the Lacey carbon film on Cu grid into the vial with the suspension of the nanocomposite in CH2Cl2.
Powder X-ray diffraction patterns were recorded using Proto AXRD Θ-2Θ diffractometer (Detroit, MI, USA) with copper anode (Kα = 1.541874 Å, Ni-Kß filter) and 1D-detector Dectris Mythen 1K in the angular range 2θ = 5–100°. A scanning step was set to be 0.02° and the speed was 0.5°/min. Identification was performed with the PDXL software (Rigaku Corporation, Tokyo, Japan) using the ICDD PDF-2 database (2017).
X-ray photoelectron spectroscopy (XPS) data were obtained using Axis Ultra DLD (Kratos) spectrometer (Kyoto, Japan) with a monochromatic Al Kα radiation. All the data were acquired at X-ray power of 150 W. Survey spectra were recorded at an energy step of 1 eV with an analyzer pass energy 160 eV, and high-resolution spectra were recorded at an energy step of 0.1 eV with an analyzer pass energy 40 eV. Samples were out-gassed for 180 min before analysis. The data analysis was performed by CasaXPS.
Carbon dioxide temperature programmed desorption experiments were made using AutoChem HP chemisorption analyzer (Norcross, GA, USA). For carbon dioxide desorption experiments synthesized samples were placed in quartz cuvette and placed in analyzer module. The sample was heated in helium atmosphere up to 700 °C, then cooled down to 105 °C, and flashed with carbon dioxide for one hour followed by flashing with pure helium for one hour. Afterwards, the sample was heated to 700 °C with a temperature gradient of 10 °C/min and carbon dioxide desorption curve was recorded. Quantity of basic sites were calculated according to quantity of chemosorbed carbon dioxide using preliminary made calibration curve.
Hydrogen temperature programmed desorption experiments were made using AutoChem HP chemosorption analyzer. For hydrogen desorption experiments synthesized samples were placed in quartz cuvette and placed in analyzer module. The sample was heated in argon atmosphere up to 700 °C, then cooled down to ambient temperature, and flashed with mixture of 10 v.% of hydrogen in argon for one hour followed by flashing with pure argon for one hour. Afterwards, the sample was heated to 700 °C with a temperature gradient of 10 °C/min and hydrogen desorption curves were recorded. Quantity of hydrogen-capable adsorption sites were calculated according to quantity of chemosorbed hydrogen using preliminary made calibration curve.
Zn, Cr, and Ni content of the composites was obtained from X-ray fluorescence (XRF) measurements using a Zeiss Jena VRA-30 spectrometer (Oberkochen, Germany) equipped with a Mo anode, a LiF200 crystal analyzer, and a SD detector.

3.5. Catalytic Study

In a typical experiment, 50 mg of the catalyst and 15 mL of dodecane were loaded in the stainles-steel reactor (7) (internal volume of 50 mL) equipped with a propeller mixer (stirring rate 250 rpm). Then the hydrogen pressure was set to 5.0 MPa, the reactor was purged with hydrogen for 3 times, and the mixture was heated up to 250 °C. When the above temperature was achieved, the catalyst was reduced for 1 h, then the hydrogen was substituted for the gas mixture (H2/CO2 = 4/1) and stirring rate was increased up to 750 rpm. Reaction was provided for six hours followed by cooling to ambient temperature. Gas phase was directed into the chromatographic system of on-line analysis along the heated line through the return pressure valve and removed from the system through a flowmeter. Liquid phase was analyzed using GS-MS Shimadzu 2010 (Kyotocity, Japan) gas chromatomass spectrometer using preliminary made calibration curves. Methanol accumulation rate was calculated taking into account mass of formed methanol in reaction media, catalysts mass and reaction time.
For repeated catalytic experiments, the catalyst after reaction was collected with an external magnet in a vial, washed thoroughly with ethanol, and dried in vacuo until the constant weight. The reaction conditions for recycling experiments were similar to those described above.

4. Conclusions

In this work, novel SiO2/Al2O3-Fe3O4-PPP-Cr-Me (Me = Zn, Ni) catalysts possessing high activity and exceptional stability in CO2 hydrogenation reaction to methanol have been synthesized. The developed approach to synthesis of the catalysts offers several possibilities for structural adoption that boost the catalytic activity due to the synergetic effect Synthesized SiO2/Al2O3-Fe3O4-PPP composites providing a carrier for deposition and dispersion of catalytically active metal species as well as a structural barrier against sintering. Presence of a hydrophobic heteroaromatic polymer layer on the silica and alumina supports prevents the aggregation of catalytically active species, elevates the stability of the catalyst, and facilitates CO2 hydrogenation through enhanced adsorption of CO2 and H2. Magnetic NPs facilitate the reaction rate through the structural promotion, probably due to formation of oxygen vacancies, and allow for fast and simple magnetic separation of the catalyst for repeated uses as well. Creation of bimetallic interface by combination of two metals (Cr-Zn and Cr-Ni) enhances the catalytic activity with the highest methanol productivity achieved for the Cr-Zn-containing composite. Considering the robust synthetic procedures, high activity attained for non-noble metals, easy catalyst recovery, and excellent stability over six consecutive catalytic cycles, the proposed approach can be considered as a promising strategy for improving the performance of the catalysts in production of methanol from CO2 + H2.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13010001/s1, Figure S1: Schematic presentation of deposition of a thin layer of pyridylphenylene polymer on a solid support; Figure S2: STEM EDS maps of Al2O3-Fe3O4-PPP-Cr-Zn; Figure S3: STEM EDS maps of SiO2-PPP-Cr-Zn. Figure S4: STEM EDS maps of SiO2-Fe3O4-PPP-Cr. Figure S5: EDS spectra of SiO2-PPP-Cr-Zn (a) and SiO2-Fe3O4-PPP-Cr (b) nanocomposites; Figure S6: XRD patterns of the samples in the area of the (311) spinel peak; Figure S7: XRD patterns of SiO2-PPP-Cr-Zn (a), Al2O3-Fe3O4-PPP (b), Al2O3-Fe3O4-PPP-Cr-Zn (c); Figure S8: XPS survey spectra of the SiO2-Fe3O4-PPP-Cr-Zn (a), SiO2-Fe3O4-PPP-Cr (b), SiO2-PPP-Cr-Zn (c), SiO2-Fe3O4-PPP-Cr-Ni (d), Al2O3-Fe3O4-PPP-Cr-Zn (e) nanocomposites; Figure S9: HR XPS of Fe 2p of SiO2-Fe3O4-PPP before incorporation of metal oxides in the nanocomposite structure; Figure S10: HR XPS of Fe 2p (a) and Cr 2p (b) regions of SiO2-Fe3O4-PPP-Cr; Figure S11: HR XPS of Cr 2p (a) and Zn 2p (b) regions of SiO2-PPP-Cr-Zn; Figure S12: HR XPS of Fe 2p (a), Cr 2p (b) and Ni 2p (c) regions of SiO2-Fe3O4-PPP-Cr-Ni; Figure S13: HR XPS of Fe 2p (a), Cr 2p (b) and Zn 2p (c) regions of Al2O3-Fe3O4-PPP-Cr-Zn; Figure S14. H2-TPD results of the examined catalysts: (a) SiO2-Fe3O4 (red line) and SiO2-Fe3O4-PPP (black line) and (b): SiO2-Fe3O4-PPP-Cr (red line) and SiO2-Fe3O4-PPP-Cr-Zn (black line). Figure S15: N2 adsorption-desorption isotherms (a,c) and pore sizes distributions (b,d) of Al2O3-Fe3O4-PPP (a,c) and SiO2-Fe3O4-PPP (c,d). SBET = 164 m2/g (Al2O3-Fe3O4-PPP) and SBET = 246 m2/g (SiO2-Fe3O4-PPP); Figure S16: STEM dark field image (a) EDS maps of Si (b), Cr (c), C (d), Fe (e) and Zn (f) of SiO2-Fe3O4-PPP-Cr-Zn nanocomposite after catalysis; Figure S17: STEM dark field image (a) EDS maps of Al (b), Zn (c), C (d), Fe (e) and Cr (f) of Al2O3-Fe3O4-PPP-Cr-Zn nanocomposite after catalysis. Table S1: Atomic percentage of elements in samples; Table S2: Fitting parameters for HR XPS of Fe 2p, Cr 2p and Zn 2p of SiO2-Fe3O4-PPP-Cr-Zn; Table S3: Fitting parameters for HR XPS of Fe 2p of SiO2-Fe3O4-PPP; Table S4: Fitting parameters for HR XPS of Fe 2p and Cr 2p of SiO2-Fe3O4-PPP-Cr; Table S5: Fitting parameters for HR XPS Cr 2p and Zn 2p of SiO2-PPP-Cr-Zn; Table S6: Fitting parameters for HR XPS of Fe 2p, Cr 2p and Ni 2p of SiO2-Fe3O4-PPP-Cr-Ni; Table S7: Fitting parameters for HR XPS of Fe 2p, Cr 2p and Zn 2p of Al2O3-Fe3O4-PPP-Cr-Zn. Table S8. Catalytic properties of the nanocomposites in methanol synthesis from CO2 + H2.

Author Contributions

S.A.S., methodology, writing—original draft preparation, N.V.K., methodology, visualization, M.E.G., investigation, A.V.B., resources, A.K.R., visualization, investigation, V.Y.D., investigation, M.G.S., conceptualization, Z.B.S., conceptualization, writing—review and editing, supervision, data curation, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Russian Science Foundation, grant number 22-43-02025.

Data Availability Statement

Not applicable.

Acknowledgments

The assistance of A.L. Vasiliev, Federal Scientific Research Centre, Shubnikov Institute of Crystallography, in recording of STEM EDS images is gratefully acknowledged. The contribution of the Centre for Molecular Composition Studies of the INEOS RAS (elemental analysis) with financial support from Ministry of Science and Higher Education of the Russian Federation is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. STEM dark field image (a) and EDS maps of Si (b), Cr (c), C (d), Fe (e) and Zn (f) of SiO2-Fe3O4-PPP-Cr-Zn nanocomposite.
Figure 1. STEM dark field image (a) and EDS maps of Si (b), Cr (c), C (d), Fe (e) and Zn (f) of SiO2-Fe3O4-PPP-Cr-Zn nanocomposite.
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Figure 2. STEM dark field image (a) and EDS maps of Si (b), Cr (c), C (d), Fe (e) and Ni (f) of SiO2-Fe3O4-PPP-Cr-Ni nanocomposite.
Figure 2. STEM dark field image (a) and EDS maps of Si (b), Cr (c), C (d), Fe (e) and Ni (f) of SiO2-Fe3O4-PPP-Cr-Ni nanocomposite.
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Figure 3. EDS spectra of SiO2-Fe3O4-PPP-Cr-Zn (a) and SiO2-Fe3O4-PPP-Cr-Ni (b) nanocomposites. Cu-Kα signals are presented due to Cu-grid used for recording of the spectra.
Figure 3. EDS spectra of SiO2-Fe3O4-PPP-Cr-Zn (a) and SiO2-Fe3O4-PPP-Cr-Ni (b) nanocomposites. Cu-Kα signals are presented due to Cu-grid used for recording of the spectra.
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Figure 4. XRD patterns of SiO2-Fe3O4-PPP (a); SiO2-Fe3O4-PPP-Cr-Zn (b); SiO2-Fe3O4-PPP-Cr-Ni (c).
Figure 4. XRD patterns of SiO2-Fe3O4-PPP (a); SiO2-Fe3O4-PPP-Cr-Zn (b); SiO2-Fe3O4-PPP-Cr-Ni (c).
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Figure 5. HR XPS spectra of SiO2-Fe3O4-PPP-Cr-Zn in the Fe 2p (a); Cr 2p (b); Zn 2p (c) regions; and Ni 2p (d) of SiO2-Fe3O4-PPP-Cr-Ni.
Figure 5. HR XPS spectra of SiO2-Fe3O4-PPP-Cr-Zn in the Fe 2p (a); Cr 2p (b); Zn 2p (c) regions; and Ni 2p (d) of SiO2-Fe3O4-PPP-Cr-Ni.
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Figure 6. CO2-TPD results of the examined catalysts: (a) SiO2-Fe3O4 (red line) and SiO2-Fe3O4-PPP (black line); and (b): SiO2-Fe3O4-PPP-Cr (red line) and SiO2-Fe3O4-PPP-Cr-Zn (black line).
Figure 6. CO2-TPD results of the examined catalysts: (a) SiO2-Fe3O4 (red line) and SiO2-Fe3O4-PPP (black line); and (b): SiO2-Fe3O4-PPP-Cr (red line) and SiO2-Fe3O4-PPP-Cr-Zn (black line).
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Figure 7. Recycling experiments for SiO2-Fe3O4-PPP-Cr-Zn (green column) and Al2O3-Fe3O4-PPP-Cr-Zn (blue column) in CO2 hydrogenation. Reaction conditions are: 250 °C, 5 MPa, 50 mg of the catalyst, CO2:H2 = 1:4, 15 mL of dodecane, 6 h.
Figure 7. Recycling experiments for SiO2-Fe3O4-PPP-Cr-Zn (green column) and Al2O3-Fe3O4-PPP-Cr-Zn (blue column) in CO2 hydrogenation. Reaction conditions are: 250 °C, 5 MPa, 50 mg of the catalyst, CO2:H2 = 1:4, 15 mL of dodecane, 6 h.
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Table
Table 1. Atomic concentrations of metals for Cr-, Zn-, and Ni-containing magnetic nanocomposites determined by HR XPS for 2p and 3p transitions.
Table 1. Atomic concentrations of metals for Cr-, Zn-, and Ni-containing magnetic nanocomposites determined by HR XPS for 2p and 3p transitions.
SampleRelative Content by XPS, Atomic %
Fe 2p/3pCr 2p/3pZn 2p/3pNi 2p/3p
SiO2-Fe3O4-PPP-Cr-Zn0.50/0.300.41/0.070.23/0.32-
SiO2-Fe3O4-PPP-Cr-Ni0.55/0.260.54/0.09-0.45/0
Al2O3-Fe3O4-PPP-Cr-Zn0.52/0.310.41/0.080.25/0.31-
Table
Table 2. Catalytic properties of the nanocomposites in methanol synthesis from CO2 + H2.
Table 2. Catalytic properties of the nanocomposites in methanol synthesis from CO2 + H2.
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Sample NotationContent by Elemental Analysis, wt%Conversion, %Selectivity, %Methanol Productivity, g Methanol/kg Me × h
FeCrZn Ni
SiO2-Fe3O4-PPP9.1------
SiO2-Fe3O4-PPP-Cr6.82.1--4.298.3115
SiO2-Fe3O4-PPP-Cr-Ni 4.32.2-2.36.098.5172
SiO2-Fe3O4-PPP-Cr-Zn4.12.02.1-9.499.1350
Al2O3-Fe3O4-PPP-Cr-Zn4.32.22.1-6.298.6195
SiO2-PPP-Cr-Zn-2.12.0-4.199.0120
MegaMax 800 4.887.578
Reaction conditions: 250 °C, 5 MPa, CO2:H2 = 1:4, 15 mL of dodecane, 50 mg of the catalyst, 6 h.
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Sorokina, S.A.; Kuchkina, N.V.; Grigoriev, M.E.; Bykov, A.V.; Ratnikov, A.K.; Doluda, V.Y.; Sulman, M.G.; Shifrina, Z.B. Cr-Zn/Ni-Containing Nanocomposites as Effective Magnetically Recoverable Catalysts for CO2 Hydrogenation to Methanol: The Role of Metal Doping and Polymer Co-Support. Catalysts 2023, 13, 1. https://doi.org/10.3390/catal13010001

AMA Style

Sorokina SA, Kuchkina NV, Grigoriev ME, Bykov AV, Ratnikov AK, Doluda VY, Sulman MG, Shifrina ZB. Cr-Zn/Ni-Containing Nanocomposites as Effective Magnetically Recoverable Catalysts for CO2 Hydrogenation to Methanol: The Role of Metal Doping and Polymer Co-Support. Catalysts. 2023; 13(1):1. https://doi.org/10.3390/catal13010001

Chicago/Turabian Style

Sorokina, Svetlana A., Nina V. Kuchkina, Maxim E. Grigoriev, Alexey V. Bykov, Andrey K. Ratnikov, Valentin Yu. Doluda, Mikhail G. Sulman, and Zinaida B. Shifrina. 2023. "Cr-Zn/Ni-Containing Nanocomposites as Effective Magnetically Recoverable Catalysts for CO2 Hydrogenation to Methanol: The Role of Metal Doping and Polymer Co-Support" Catalysts 13, no. 1: 1. https://doi.org/10.3390/catal13010001

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