Synthesis and Catalytic Activity in the Hydrogenation Reaction of Palladium-Doped Metal-Organic Frameworks Based on Oxo-Centered Zirconium Complexes

: Metal-nanocluster-doped porous composite materials are attracting considerable research attention, due to their specific catalytic performance. Here we report a simple, cheap, and efficient strategy for the preparation of palladium hydrogenation catalysts based on metal-organic frameworks (MOFs). It has been shown that the synthesis of Pd/MOF results in the formation of palladium nanoparticles uniformly fixed on the surface. The composition and structure of the resulting composites were studied using elemental analysis, DSC, TGA, XRD, TEM, and IR spectroscopy. Pd nanoparticles with an average diameter of 8–12 nm were successfully confined in the UiO-type MOFs, and the obtained nanocomposites exhibited abundant porosity, high stability, and a large surface area. It has been shown that the resulting catalytic systems with high activity, selectivity, and stability reduce phenylacetylene and allyl alcohol to styrene and propanol, respectively, in liq-uid-phase hydrogenation reactions.


Introduction
Hydrogenation is a widely used reaction in oil refining and industrial organic synthesis; for example, about 75% of industrial hydrogenation processes are carried out in the presence of Pd/C catalysts [1]. The use of palladium on various carriers is preferable from the point of view of a significant decrease in the activation energy of H-H bond cleavage on the palladium surface and the mobility of chemisorbed hydrogen [2]. However, at low temperatures of the hydrogenation process, the catalyst is very sensitive to various impurities present, for example, in the feedstock for hydrogenation [3]. This problem of catalyst poisoning, the loss of catalytic activity and/or selectivity over time, is essential in the practice of industrial catalytic processes and is enormously time-consuming and costly because of catalyst replacement and process shutdown [4]. Therefore, it is important to create hydrogenation catalysts with high activity, selectivity, and stability.
Such catalytic systems can be obtained by using supports of a completely new type in the field of chemistry and materials science: metal-organic frameworks (MOFs) [5,6]. The possibility of careful selection of the nature of the structure-forming element: the type of metal and its chelate environment, the organic bridging linker, its size and functionality, is an undoubted advantage of MOF in comparison with other types of porous materials, since it allows the design of a nanoreactor space, the walls of which can be equipped with functional groups of various chemical natures in an optimal way for the selective transformation [7][8][9][10][11]. The most studied catalytic transformations involving MOFs are ligand, linker, and metal ion exchange reactions, substitution reactions, the functionalization of the organic ligand by postsynthetic methods, and the catalytic transformation of guest molecules [12][13][14]. The MOF-based catalyst systems demonstrated to date can be roughly divided into three types: (1) catalysts with catalytic domains on inorganic sites of MOFs, (2) catalysts whose catalyst sites are immobilized on organic linkers, and (3) catalysts whose catalytic sites are encapsulated in pores. Undoubtedly, MOF, as a new type of highly ordered porous material with a variable size, shape, and microenvironment of pores, is also a promising platform for creating heterogeneous catalysts by encapsulating noble metal nanoparticles (NPs) into pores, which would limit the migration and aggregation of metal clusters.
At present, the use of noble metal NPs introduced into the matrix of MOFs has been demonstrated for CO oxidation [15], the water-mediated Suzuki-Miyaura reaction [16], the Ullmann reaction [17], hydrogenation and isomerization of olefins [18], chemoselective reduction of the C=C bond in various α,β-unsaturated carbonyl compounds [19][20][21], hydrogenation of phenylacetylene [22,23], methoxycarbonylation of iodobenzene [24], hydrogenation of nitrophenol to aminophenol [25], etc. Thus, we can conclude that the possibility of using MOFs in catalysis today knows no bounds, and impressive progress has already been made in using the porous system of MOFs as heterogeneous catalysts. At the same time, a number of scientific problems remain, the solutions to which will make it possible to use heterogeneous catalysts based on MOFs on an industrial scale: (1) the search for and creation of thermally and hydrolytically stable MOFs; (2) the development of efficient scalable activation methods (for many MOFs, a huge remaining trend is structure collapse upon removal of solvent molecules, counterions, or additional cocrystallizing organic molecules from the cavity); (3) the development of approaches that make it possible to control the morphology, topography, and dispersity of NPs formed on the MOF surface. In this regard, the previously developed method for the preparation of hybrid heterogeneous catalysts, which are polymer-doped metal NPs uniformly fixed on the surface of an inorganic support, turned out to be convenient [26,27]. In addition, a promising strategy in the literature is the creation of composite materials based on carbon nitride [28] or graphene oxide [29] to obtain materials with a synergistic effect of the matrix and palladium NPs.
Therefore, this work is aimed both at solving the problems associated with the creation of catalytic hydrogenation systems based on a new type of porous materials that can work effectively under mild conditions, studying the stability of the MOF support structure and the composition of active sites and developing convenient scalable methods for obtaining such highly porous materials, and at assessing the competitiveness of the obtained heterogeneous MOF-based catalysts with the most well-known industrial analogs (Pd/C). As a promising platform for the support of catalytically active Pd, a series of MOFs of the UiO type was chosen as one of the most chemically and thermally stable MOFs known to the literature [30][31][32]. For the synthesis of a series of isoreticular MOFs, a lowtemperature rational synthesis concept was used, using presynthesized hexanuclear oxohydroxocarboxylate blocks of the [Zr6O4(OH)4(RCOO)12] type. Depending on the synthetic conditions, these nodes may possess Brønsted and Lewis acidic sites, which have been shown to catalyze various reactions, including hydrolysis, dehydration, and esterification, among others [18].

Characterization Techniques
The content of C, H, and N was determined on a VarioMicrocube elemental analyzer (ElementarGmbH, Germany). The specific surface area of the samples, the average radius, and the pore size distribution were determined by low-temperature nitrogen adsorption (at 77 K) on an AUTOSORB-1 sorption analyzer (Quantachrome, United States) using a static volumetric method. Curves of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were recorded, using a METTLER TOLEDO DSC822e differential scanning calorimeter (Mettler Toledo, Switzerland) and a TGA/SDTA851e METTLER TOLEDO thermogravimetric analyzer (Mettler Toledo, Switzerland). The samples were heated in a nitrogen atmosphere at a heating rate of 10 K min -1 in the temperature range of 25-600 °C. Identification of functional groups and additional information on the type of coordination was carried out on an ALPHA IR spectrometer (Bruker Optik GmbH, Germany) with a spectral resolution of 2 cm −1 for a wavenumber range of 4000-350 cm −1 . An X-ray diffraction analysis (XRD) of the samples was performed on an Aeris Benchtop powder diffractometer in the range of scattering angles from 5 to 90° (scanning step in 2θ is 0.02°, radiation is CuKα, λ = 1.5405 Å). Transmission electron microscopy (TEM) was performed using a transmission electron microscope (Tecnai Osiris FEI, Hillsboro, OR, USA). The Digital Micrograph, JEMS, TIA, and Image Scope programs were used to process and analyze images obtained with an electron microscope.

Synthesis of MOF of the UiO Type
In a typical synthesis, the calculated amount of the organic ligand, namely 1,4-benzenedicarboxylic, naphthalenedicarboxylic and 2-amino terephthalic acid, was dissolved in an ethanol solution with the addition of the calculated amount of potassium hydroxide. Zirconium oxoacetate was prepared in a separate reaction vessel. To do this, concentrated acetic acid was added dropwise to an aqueous solution of zirconium chloride. The pH was adjusted to 1.5 with 1 M NaOH solution. Since there is only a relatively small pH window where the Zr acetate complex predominates as the hexanuclear secondary structural unit [Zr6O4(OH)4(CH3COO)12], the mixture was kept at 55 °C for 180 min with constant stirring until the color changed from light pink to light yellow. The organic ligand solution was added to the resulting solution by stirring. The resulting precipitate was isolated by centrifugation at 4500 rpm, washed several times with absolute ethanol, and dried in a vacuum (10 -3 Torr, 80 °C, 10 h).

Synthesis of Pd Doped Heterogeneous Catalysts
The synthesis of a heterogeneous catalyst was carried out based on the content of Pd 0.5 wt.%/g of support. In a typical procedure, PdCl2 (0.094 mol) and KCl (0.2068 mol) in the presence of PEG-1500 (0.1 mmol) were dissolved in 100 mL of absolute ethanol. After complete dissolution by stirring on a magnetic stirrer, the calculated amount of MOF was added, and the mixture was refluxed until a dark color, characteristic of palladium reduction, formed. The resulting precipitate was isolated by centrifugation at 4500 rpm, washed several times with ethanol, and dried in a vacuum. The water adsorbed in the MOF pores was replaced with absolute ethanol using a Soxhlet apparatus (16 h, 100 °C), and ethanol was then removed from the pores by vacuum activation (16 h, 150 °C).

Hydrogenation of Cyclohexene
The hydrogenation reaction of cyclohexene was chosen as a model reaction for the rapid determination of the activity of catalysts. The experiments were carried out in a nonflowing glass gradient-free thermostatic reactor of "the duck" type, in isopropyl alcohol at a cyclohexene concentration of 7 × 10 -3 mol/L, a constant hydrogen atmospheric pressure (0.1 MPa), and a temperature of 40 °C, under conditions of intensive shaking of the reactor (180-240 swings per minute). The catalyst weights were 0.10 ± 0.05 g. Hydrogen was supplied to the reactor from a calibrated receiver with a water seal. To determine the natural pressure of the gas, the partial pressure of the water vapor was subtracted from the total atmospheric pressure, and the gas volume was then brought to normal conditions. Before the introduction of cyclohexene, the catalyst was subjected to 15 min of treatment with hydrogen directly in the reactor, by stirring. The reaction rate was calculated graphically from the slope of the initial portions of the hydrogen consumption kinetic curves. In parallel experiments, the discrepancy did not exceed 5%.

Hydrogenation of Phenylacetylene and Allyl Alcohol
Hydrogenation was carried out in a non-flowing glass gradient-free thermostatic reactor of "the duck" type in ethanol (25 mL) at an atmospheric pressure of hydrogen and a temperature of 40 °C by intensive shaking of the reactor (600-700 swings per minute). The catalyst (0.05 g) was preliminarily treated with hydrogen directly in the reactor for 30 min with vigorous stirring, and then 0.25 mL of the substrate was introduced. The amount of substrate was taken, based on the absorption of 100 mL of hydrogen. The reaction rate was calculated from the change in hydrogen uptake per unit time. To assess the stability, hydrogenation was carried out cyclically, up to 5 times.

Synthesis of Pd-Doped Catalysts
The synthesis of target Pd-doped MOFs was carried out using exchange reactions of monotopic ligands in the in situ formed oxohydroxocarboxylate block of the [Zr6O4(OH)4(CH3COO)12] type for dicarboxylate (1,4-benzenedicarboxylic (C8H6O4), 2aminobenzene-1,4-dicarboxylic (C8H6O4-NH2) and 2,6-naphthalenedicarboxylic (C12H8O4)) with the formation of the required structure. This process can be described by the system of the following reaction equations: where H2L is dicarboxylic acid.
Since there is only a relatively small pH window where the zirconium acetate complex predominates as a hexanuclear secondary structural unit [33], synthesis of the Zr6O4(OH)4(L)6 cluster was carried out at pH = 1.5.
Palladium was introduced into the MOF matrix by the chemical reduction of Pd(II) ions with PEG-1500. Polyethylene glycol in this case serves as both a reducing agent and a stabilizer for Pd(0) NPs, as shown by F.A. Harraz et al. [34]. Thus, the in situ formation of Pd(0) NPs can be schematically represented as follows (Scheme 1): Scheme 1. Schematic representation of the proposed strategy for obtaining catalysts based on UiO-66.
Based on the results of elemental analysis (Table 1), thermogravimetric analysis and DSC studies (Table 2, Figure 1), empirical formulas of the obtained MOFs are proposed.   According to the TGA curves, all obtained MOFs demonstrate three main stages of decomposition; the maximum decomposition rates for the second stage, according to the DTG curve, are observed above 300 °C, which indicates their high thermal stability. The first stage of the MOF decomposition, which proceeds up to 200 °C, is associated with the removal of physically adsorbed solvents. Such an elevated temperature indicates strong coordination interactions. The second stage of decomposition is accompanied by decarboxylation, according to the mass spectra of gaseous products (m/e 44 and m/e 28). The third stage of decomposition, which occurs above 380 °C, is accompanied by a deeper decomposition of the MOF framework with the formation of zirconium oxides.
In accordance with the IR spectroscopy data presented in Figure Table 3 and Figure 3 show the results of low-temperature nitrogen adsorption of the samples. It can be seen that the obtained MOFs have segments of the S-shaped adsorption isotherm, indicating the occurrence of polymolecular adsorption in mesoporous systems. Interestingly, the introduction of amino groups into the organic linker reduces the pore volume. The observed hysteresis loops are found in materials with slit-shaped pores.  The observed XRD patterns of the obtained MOF powders are shown in Figure 4. As can be seen from the presented XRD patterns, the narrow diffraction maxima indicate a predominantly crystalline phase state of the obtained MOFs, and the introduction of Pd NPs does not affect the XRD curve profile and the thermal stability of the carrier matrix. The Bragg reflection centered around 7° corresponds to the (111) reflection plane of the triclinic grating type [35]. Peaks at 16.37, 24.23, 21.77° for Zr6O4(OH)6(C8H4O4)6(H2O)4 correspond to unreacted 2,6-naphthalenedicarboxylic acid ligand (CCDC 1422388). This may be due to the entrapment of ligand molecules in the MOF pores, and therefore it is difficult to purify, which also occurs for terephthalic acid [36]. The microstructure of the Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 and Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 catalysts under study was studied by TEM. For these types of catalytic systems, due to the good catalytic activity, it was interesting to see the distribution of components relative to each other; for this, surface areas were mapped for the presence of Pd, C, Zr elements, using TEM. The results of the studies are shown in Figure 5. It can be seen that catalytically active Pd NPs are located on the surface of the Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 catalyst, mainly in the MOF pores only; it would seem that this is due precisely to the formation of an amino complex with Pd at the amino group of the organic linker, which was previously confirmed by IR spectroscopy. Subsequent reduction using a strong reducing agent such as sodium borohydride led to the construction of a nanoreactor space, the walls of which are equipped with a functional amino group and NPs of the catalytically active Pd, which in turn can be useful for the selective hydrogenation transformations. NPs of the catalytically active Pd are uniformly distributed over the surface of the Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 catalyst, have a spherical shape, and dimensions of 8-12 nm. In the case of Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12, the particle sizes range from 2 to 10 nm. Comparison of micrographs of the TEM images of the surface of the catalysts shows that the surface morphology of the Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 catalyst has a looser structure. In the intervals between blocks of aggregates, one can see systems of pore channels ( Figure 6).

Catalytic Tests
The hydrogenation reaction of cyclohexene was chosen as a model reaction for rapid determination of the activity of catalysts. To assess the stability of the obtained catalysts, hydrogenation was carried out cyclically. For clarity, the dependences of the rate of hydrogenation of cyclohexene, reduced to g-atm of Pd, on time, for different types of the obtained catalytic systems, are shown in comparison with the industrial analogue of Pd/C (Figure 7). It has been shown that catalytic hydrogenation systems supported by mesoporous MOFs of the Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 and Pd/Zr6O4(OH)4(C12H6O4)6(H2O)18 types demonstrate initial cyclohexene hydrogenation rates higher than the industrial analog of Pd/C. The activity of Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 is comparable with those of the industrial analog of Pd/C. The next step in the analysis of the catalytic properties of the obtained systems was the study of the hydrogenation of phenylacetylene and allyl alcohol. It should be noted that the selective hydrogenation of phenylacetylene contained in the styrene monomer raw material is a necessary process for obtaining high quality polystyrene. During the hydrogenation of phenylacetylene (Figure 8a), the lowest hydrogenation rates are observed for Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12, which is apparently due to the localization of Pd NPs mainly in MOF pores, i.e., the lowest availability of Pd active sites. Interestingly, when the reaction is carried out in repeated cycles, Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 works stably without a decrease in the hydrogenation rate, while the hydrogenation rates of Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 and Pd/Zr6O4(OH)4(C12H6O4)6(H2O)18 decrease with each subsequent cycle, so that in the fifth cycle of phenylacetylene hydrogenation the rate of hydrogenation of these systems decreased by 50 and 40% of cycle 1, respectively.
It should be noted that hydrogenation at low temperatures of organic compounds, such as allyl alcohol or its derivatives, is also interesting from the point of view of evaluating the ability of the catalyst to carry out selective conversion, since several directions of the reaction are possible: (1) reduction to the corresponding alcohols, (2) isomerization to saturated or unsaturated aldehydes, (3) hydrodeoxygenation to hydrocarbons [37]. As a result of the hydrogenation experiments (Figures 8 and 9), it was found that the propanol selectivity of the catalysts under consideration was 72-78% at an allyl alcohol conversion of 100%, and the styrene selectivity was approximately 94% at a phenylacetylene conversion of 75-86%. It has been shown that the hydrogenation rates of allyl alcohol for Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 are two times higher than those of other systems; in addition, the rate in subsequent cycles increases somewhat, while the rate of hydrogenation for Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4 and Pd/Zr6O4(OH)4(C12H6O4)6(H2O)18 decreases with each subsequent cycle, so that in the fifth cycle of allyl alcohol hydrogenation the rate of hydrogenation of these systems decreased by 50 and 35% of cycle 1, respectively.  (c,d)-Pd/Zr6O4(OH)6(C8H4O4)6(H2O)4; (e,f)-Pd/Zr6O4(OH)4(C12H6O4)6(H2O)18.

Conclusions
As part of the research, a low-temperature rational approach was proposed for the preparation of UiO-type mesoporous MOFs using non-toxic solvents and presynthesized polynuclear zirconium complexes as sources of secondary building blocks. MOFs exhibit permanent porosity, mesoporous structure, and high thermal stability. The MOFs thus obtained have been shown to be suitable for use as a hydrogenation catalyst platform. Thus, palladium-supported catalytic systems based on MOFs constructed from molecules of benzenedicarboxylic acids and their derivatives demonstrate stable catalytic activity in the hydrogenation reaction in repeated cycles, and can operate effectively under mild conditions with comparable hydrogenation rates in comparison with the most well-known industrial analog Pd/C. It was found that the rates of hydrogenation of allyl alcohol in the presence of Pd/Zr6O4(OH)4(C8H4O4-NH2)6(H2O)12 are two times higher than those of other systems; in addition, the rate even slightly increases in subsequent cycles. We hope that the results of this study will contribute to the further development of low-temperature efficient heterogeneous hydrogenation catalysts, which are supported by objects of a completely new type in the field of chemistry and materials science: MOFs, capable of operating efficiently in mild conditions, not inferior to the most well-known industrial analogues of Pd/C.

Data Availability Statement:
The data presented in this study are available in the article.