Hybrid Polymer-Immobilized Nanosized Pd Catalysts for Hydrogenation Reaction Obtained via Frontal Polymerization

A new approach to the synthesis of mixed-type immobilized catalysts by frontal polymerization of metal-containing monomer in the presence of highly dispersed mineral support has been developed. Synthesis of the acrylamide Pd(II) nitrate complex, Pd(CH 2 =CHCONH 2 ) 2 (NO 3 ) 2 (PdAAm), on the SiO 2 (Al 2 O 3 , C) surface and its consequent frontal polymerization and reduction lead to the formation of organic-inorganic composites with polymer-stabilized Pd nanoparticles. e immobilized metal complexes and palladium nanoparticles were characterized by various physical and chemical methods. e synthesized hybrid nanocomposites are eﬃcient and selective catalysts for hydrogenation of cyclohexene, alkene, and acetylene alcohols, as well as di-and trinitrotoluene. Catalyst intermediates separated by nondestructive testing method have been described and changing in the palladium charge during the catalytic process has been identi�ed.


Introduction
In recent years materials containing metal nanoparticles were intensively studied specially as catalysts due to their unique physical and chemical properties [1], high ratio of surface atoms to the total number of atoms in a particle, and possibility to vary catalytic properties by controlling the size of particles [2][3][4].Zero-valent palladium complexes and nanoparticles are well known as efficient and selective catalysts for many organic reactions such as alkene arylation [5,6], cross-coupling [7], hydrogenation of dienes, ole�ns [8,9], and unsaturated alcohols [10].Liquid phase catalytic hydrogenation of aromatic nitro compounds is widely used to produce corresponding amino derivatives, which are intermediates in the production of plastics, pharmaceuticals, and so forth.Hydrogenation of trinitrotoluene (TNT) in recent years becomes a reaction of practical importance for utilization of nitroaromatic explosives to useful chemicals (dyes, amino compounds, etc.) [11,12].Platinum metals catalyze selective reduction of nitroaromatic compounds.Particular attention is paid to developing effective and selective palladium catalysts based on complexes and nanoparticles [13][14][15][16].Aggregation and agglomeration of nanoparticles limits their use as catalysts, so they are �xed on supports (metal oxides, zeolites, carbon, etc.) or stabilized with different types of ligands, including polymers.One of the promising methods to obtain metal polymers is a polymer-mediated synthesis based on in situ poly-and copolymerization of metal-containing monomers with subsequent controlled thermolysis of the resulting products.is approach allows one to combine the formation of metal nanoparticles and stabilizing polymer shells in one stage [17,18].e aim of this paper is to demonstrate a new approach to the design of catalysts by the frontal polymerization of acrylamide Pd(II) nitrate complex on inorganic supports and to characterize the features of their catalytic properties in reactions of hydrogenation of various unsaturated compounds (cyclohexene, ole�nic and acetylenic alcohols and selective reduction of di-and trinitrotoluene).F 2: XPS spectra of (a) acrylamide complex monomer with Pd(NO 3 ) 2 , (b) PdAAm aer polymerization, and (c) the same aer cyclohexene hydrogenation.
T 1: Elemental analysis of the acrylamide complexes of Pd(II) nitrate and the relative content of elements according to the XPS data.= 123 m 2 /g) were used as inorganic supports.e supports were preactivated (calcination and evacuation at 450 ∘ C) and cooled in an inert atmosphere. in a bath with a heat carrier (Wood's alloy) at 80-100 ∘ C for 10-15 seconds (Figure 1).e rate of reaction was evaluated from the migration of the front of a colored zone.Powdered polymer and hybrid nanocomposite were washed with methanol and ether, and dried in vacuum at room temperature to constant weight.

Hydrogenation Reactions.
Hydrogenation reaction was carried out in a non-�ow glass reactor under a constant atmospheric hydrogen pressure with vigorous stirring (300-400 rocking min −1 ).Hydrogen was fed into the reactor from a calibrated receiver with a water lock.e reaction rate was calculated graphically from the initial part of kinetic curves of hydrogen consumption over time.Discrepancies in parallel experiments did not exceed 5%.

Cyclohexene Hydrogenation. e reaction was carried
out at 20 ∘ C in isopropyl alcohol with substrate concentration of (4.72-14.5)× 10 −3 mol/L.Catalyst amount was 0.06-0.12g.Before introducing cyclohexene, the catalyst was treated with hydrogen for 15 min immediately in the reactor with stirring.

Hydrogenation of Allylic and Acetylenic
Alcohols.e reactions were carried out at 40 ∘ C in ethanol (20 mL) with substrate concentration of 2.2 × 10 −3 mol/L.Catalyst amount was 0.03 g.Catalyst was pretreated by hydrogen for 30 minutes before injection of the substrate into reactor.Catalyst selectivity was evaluated as the mass fraction of the desired product in the total content of the reaction products.

Hydrogenation of Di-and Trinitrotoluene . e reaction
was carried out in methanol with concentration of substrate of (4.72-14.5)× 10 −3 mol/L at 36 ∘ C. Catalyst was pretreated by hydrogen for 15 minutes before injection of the substrate into reactor.

Characterization. e speci�c surface area (𝑆𝑆 spec )
and pore size of inorganic supports and polymer hybrid nanocomposites were determined by static volumetric method for N 2 adsorption at 77 K (AUTOSORB-1, Quantachrome, USA).
Analysis of the products of allyl alcohol hydrogenation and isomerization was carried out on "Crystal-2000M" chromatograph (Russia) with a �ame ionization detector under isothermal conditions.e universal capillary column for organic compounds of 50 m length and 0.20 mm inner diameter was used.Helium was carrier gas.
Elemental analysis was carried out by organic microanalysis and �ameless atomic absorption method using spectrometer AAS3, Germany.
IR absorption spectra were recorded on a Specord 75 IR using the KBr disc method.
Electron microscopic studies were performed using JEM-3010 transmission electron microscope with an accelerating voltage of 100 kV.e samples were prepared by dispersing the diluted catalyst suspension in heptane onto copper grids.
XPS spectra were recorded on an ES-2401 spectrometer with a magnesium anode.X-ray tube power was 200 watts; a vacuum in the analyzer chamber was 10 −6 Pa.e analyzer energy was 50 eV.e Au4f 7/2 line (84 eV) was used to calibrate the spectrometer.e binding energy of C1s line of

Synthesis of Pd(CH
Acrylamide complex of Pd(II) nitrate was synthesized by substitution of crystallization water in crystalline hydrate of metal nitrate by acrylamide molecules (AAm).Elemental analysis and some spectral characteristics of the complex are given in Table 1.
e infrared spectroscopy data indicated coordination of metal atom with the oxygen atoms of carbonyl group of   AAm-ligand.Bands CO (1665 cm −1 ) are shied into longwave region as it was shown previously for similar complexes of transition metals [20].e valence vibration bands of anion-nitrate at 1384 cm −1 (NO 3 ) were also observed in the spectra of the complexes.In XPS spectra the shis of lines 8 C1s (bond energy  bond = 288.3eV), N1s ( bond = 399.8eV) and O1s ( bond = 531.6 eV) by 0.5, 0.7, and 0.5 eV to high-energy region correspondingly were observed.e appearance of a low-intensity line with  bond = 337.2eV in Pd3d 5/2 spectrum and the increase in the intensity of the line with  bond = 285.7 eV could be explained by additional -coordination of metal atom with the double bond of the ligand.Such examples for Pd-alkene complexes are known [21].For the surface layer the ratio of Pd/N atoms is equal to 0.11 and O/N = 1.4 (Table 1).
e line in the N1s spectrum with  bond = 407.2eV is 10% of the integral spectral intensity, correlating with the line intensity in the Pd3d 5/2 spectrum with  bond = 338.8eV and indicating that the Pd atoms are shielded by the AAm groups.e results of elemental analysis and study by physical and chemical methods con�rm the formation of a palladium complex of the following composition: Pd(CH 2 =CHCONH 2 ) 2 (NO 3 ) 2 (PdAAm).

PdAAm Frontal Polymerization with and without
Inorganic Support.As shown earlier [20,22], the acrylamide complexes of metal nitrates in the condensed state can efficiently be polymerized in the frontal mode, that is, under the conditions when the monomer is converted to the polymer in the localized reaction zone and is propagated by layers over the whole volume.e reaction occurs under mildest conditions known for processes of this type, at atmospheric pressure and thermal initiation without chemical initiators and activators.In the mode of stationary propagation of the heat wave, polymerization occurs in a narrow temperature interval.e heat evolved in the reaction zone is transmitted to the heating zone, where the substance is heated due to heat conductivity and the temperature increases from the initial value to the temperature at which the reaction starts, that is, the polymerization front is propagated.e heat wave is initiated upon the short-term (∼15 s) introduction of thermal perturbation into the terminal part of the PdAAm monomer sample molded as a cylinder of the system with the inorganic support.e appearance and propagation of the melt zone (phase transition of the �rst order) (Figure 1) and the color change (the reaction rate was monitored by the migration of the color boundary) from light brown to dark brown and black were visually observed.Kinetic studies showed the high rate of frontal polymerization of Pd(II) acrylamide complex (w = 0.038 cm/s) for which the "ignition" temperature (80-100 ∘ C) was much lower than for AAM complexes with Co(II), Ni(II) nitrates and other metal ions (170-180 ∘ C) [20].
At higher ignition temperatures (150-170 ∘ C), we observed the release of the reaction mass from a glass tube and the combustion mode (!), that probably were caused by the formation of �ne particles of pyrophoric palladium.According to XPS data, the basic line in Pd3d5/2-spectrum of polymerization product (Figure 2) is characterized by  bond = 336.5 eV, which is higher than  bond in Pd0 (33�.0 eV) because of formation of �ne particles of Pd 0 and Pd  [9,23].
Increase in  bond of basic line in the N1s spectrum from 399.8 to 401.7 eV is probably caused by the reaction of cyclization of polymer chains and formation of imide groups [24] (see Scheme 1).
Previously developed approach to obtaining nanocomposite materials by the frontal polymerization [25] is of interest for the design of polymer-immobilized catalysts of mixed (hybrid) type: metal nanoparticles-polymer-inorganic support.Indeed, synthesis of Pd(II) acrylamide complex on the surface of the mineral support and its subsequent polymerization lead to formation of polymer-inorganic composite (e.g., poly-PdAAm/SiO 2 ) (see Scheme 2).
According to electron microscopy data, the resulting composite includes Pd nanoparticles with a diameter of 10-20 nm, stabilized with a polymer matrix (Figure 3).As expected, the stabilizing effect of the �llers-SiO 2 (Al 2 O 3 , C) on autowave mode of polymerization⋅ (  0.024 cm/s) shows that the polymerization front becomes space-time stable: thermophysical properties of the system are such that frontal mode is carried out with a high (75 wt.%) degree of �lling.�ide diffraction peaks at 2Θ  36-85 degrees corresponding to crystal Pd 0 were registered on Xray diagram for synthesized composites (Figure 4).e diffuse diffraction maxima and line broadening indicate that the sample contains small particles (Table 2).e prepared nanocomposites are characterized by a microporous structure with pore sizes from several nm to 20 nm and uniform size distribution (Figure 5).It should be noted that the speci�c surface of the mixed-type supports decreases aer PdAAm frontal polymerization on their surface, although its value is higher than  sp of PdAAm polymerization product prepared without inorganic support (Table 3).us, the synthesized hybrid nanocomposites have sufficiently developed surface and a porous structure that provides access of reactants to active sites of the catalyst and their high activity in the studied catalytic reactions.

Catalytic Properties of PdAAm/SiO 2 (Al 2 O 3 , C) in the
Reaction of Cyclohexene Hydrogenation.e analyzed systems showed sufficiently high activity in the model reaction of cyclohexene hydrogenation.us, under comparable conditions, the initial reaction rate in the presence of poly-PdAAm/Al 2 O 3 catalyst nearly 2-times higher than that for the standard Pd/C (Table 4, Figure 6).
It is important that the studied nanocomposites keep high catalytic activity in the recycle, and the immobilized form is easily separated from the reaction medium and can be reused.Even more importantly, it provides an opportunity to explore catalytic intermediates by various physical and chemical methods of nondestructive testing.In some cases, there is increased activity with repeated use, so-called "developing"  of the catalysts, which is typical for many of immobilized systems.e phenomenon recently was demonstrated for polymer rhodium clusters [26].
Conditions for formation of Pd nanoparticles also effect the catalytic properties of the composites, for example, different modes of frontal polymerization in an inert environment or aerheat treatment at 100-150 ∘ C in a reduction atmosphere (H 2 ) (Figure 7).us, the rate of hydrogenation on the nanocomposites obtained by this method decreases with increasing temperature of their reduction.Similar effect is observed for catalysts with both SiO 2 and Al 2 O 3 (Table 4).Probable reason for the decreasing reaction rate is agglomeration of Pd particles at higher temperatures of treatment in the process of nanocomposite preparation.
Aer hydrogenation the basic part of Pd (90%) in poly-PdAAm is in zero valence state (the line of Pd3d 5/2 spectrum with  bond = 335.9eV) (see Figure 2) and its shielding by the polymer matrix is decreased, as it is evidenced by increasing Pd content in the surface layer (from 1.5 at.% in the initial polymer complex to 4.3 at.%).Polymer matrix is also changed, which is con�rmed by the broadening of �1s spectrum.
us, the hybrid polymer-immobilized Pd nanoparticles show high stable activity in the hydrogenation of cyclohexene.e catalysts preserve their activity in repeated cycles.e catalytic properties depend on the conditions of nanocomposite synthesis, which is probably associated with the formation of Pd nanoparticles of different sizes.

Catalytic Properties of Poly
composites in the Hydrogenation of Allyl Alcohol.It is known that hydrogenation of allyl alcohol is oen accompanied by undesirable side reaction of substrate isomerization, which sometimes may even prevail over the main process.erefore minimization of isomerization is essential for increasing the yield of the target hydrogenation products.For example, catalysts based on Pd nanoparticles encapsulated in polyelectrolyte multilayers [27] or dendrimers [28] were used for this purpose.
e synthesized Poly-PdAAm/SiO 2 (Al 2 O 3 ) catalysts show different activity and high selectivity in the hydrogenation of allyl alcohol (Table 5).e main product of allyl alcohol hydrogenation is propanol-1, propionaldehyde-a product of the allyl alcohol isomerization is identi�ed in small amount (up to 2% on poly-PdAAm/Al 2 O 3 , 5%-in the presence of poly-PdAAm/SiO 2 ).No other products were detected.Higher reaction rate over PdAAm/SiO 2 catalyst is apparently explained by more developed surface area of the catalyst and the speci�c adsorption of the substrate molecule on the surface of catalysts (see Table 3).
e stability of the catalysts was evaluated in hydrogenation of successive portions of allyl alcohol on the same catalyst loading (Figure 8).Poly-PdAAm/SiO 2 catalyst shows high activity in repeated cycles; the rate has decreased only aer 11 reaction runs.Interrupting the process for several hours (portions 7 and 10) apparently leads to the increasing of the reaction rate due to equalization of the concentrations of all process components (solvent, products, and substrate) near the active centers.A steric effect on the selectivity of the catalytic reaction is conveniently demonstrated by hydrogenation of hindered long chain acetylene alcohol.While the overall trend in activity and selectivity of the studied catalysts are similar, the rate of hydrogenation to compare with one for allyl alcohol is signi�cantly decreased (Figure 9, Table 5), which may be due to diffusion restrictions of access of the reagent to the active centers of the nanoparticles.
According to chromatographic analysis (Figure 10) the triple bond of C 20 acetylene alcohol is reduced with high rate to a double one.e reaction rate reaches a maximum at the 10th minute (see Figure 9) and then drastically decreases to (0.05-0.1)⋅10 −5 mol/L.During this time, 100% conversion of acetylene alcohol to reaction products is achieved on poly-PdAAm/SiO 2 catalyst.However, the process is not selective.Hydrogenation of the triple bond of acetylene alcohol occurs simultaneously with the reduction of forming double bond: both saturated 3,7,11,15-tetramethylhexadecane-1-ol-3 and ole�nic alcohol are detected in the reaction products.
Somewhat different results were obtained for the hydrogenation of C20 acetylenic alcohol in the presence of the poly-PdAAm/Al 2 O 3 .Initially, acetylene alcohol is mainly converted to ole�nic one.And only aer 20 minutes when the content of the initial substrate in the reaction mixture is about 10%, 3,7,11,15-tetramethylhexadecene-1-ol-3 formed is hydrogenated to aliphatic alcohol.Selectivity for ole�n alcohol is 70% (Table 5).Comparison of the data of chromatographic analysis and the reaction rates makes it possible to conclude that acetylene alcohol with high rate is converted to alkenol, but the formation of a saturated alcohol is 7-10times slower (w = 0.1 ⋅10 −5 mol/L).Currently, the mechanism of selective action of hybrid polymer-immobilized catalysts obtained is not completely clear and requires further study, as well as optimization of the catalytic process.It is known that nitro groups of polynitrocompounds can be hydrogenated stepwise to the corresponding polyamines [29].e kinetic curves indicate that the rate of formation of monoamine derivatives in the presence of the polymerimmobilized catalyst is signi�cantly higher than for Pd/C.In addition, hydrogenation of the second and third (for TNT) nitro groups is carried out at lower rates than the �rst one.
To identify the main routes of nitro groups' reduction in the studied processes, we have analyzed the composition of the reaction mixtures by 1 H NMR. Indeed, the  , or NHOH).Some unidenti�ed peaks in the spectrum probably refer to products of condensation and oxidation, which can be formed during the sampling at NMR analysis.ese results con�rm stepwise hydrogenation of 2,4-DNT in the presence of poly-PdAAm/SiO 2 .According to the composition of the reaction mixture, the scheme of major transformations in the reaction system can be represented as follows shown in see Scheme 3. e presence of 4A2NT among intermediate compounds indicates the possibility of parallel reaction routes that is consistent with references [29][30][31].As the authors of the paper [29] have rightly noted the �nal mechanism of main conversions of toluene nitroderivatives has not been yet elucidated.Probably, the use of the considered hybrid nanocomposite catalysts will help to solve this problem.On the one hand, the steric factors, including those caused by the presence of the polymer matrix, can lead to strong differentiation in coordination of substrate nitro groups.Consequently, the adsorption of the second and third nitro group will be much weaker.On the other hand, this type of contacts [2] allows to use methods of nondestructive testing for intermediate aromatic nitroamine; their different reactivity is demonstrated in many systems [30][31][32].
us, due to the immobilized form of the studied catalysts, it was possible to separate catalytic intermediate aer reduction of the �rst nitro group of 2,4-DNT and analyze it by XPS, as well as to compare the resulting spectrum with the Pd3d 5/2 spectrum of the catalyst, separated aer complete hydrogenation of 2,4-dinitrotoluene (Figure 12).e analysis showed that the intermediate contains, along with Pd 0 (335.5 eV), palladium atoms with partially positively charged Pd  (337.0 eV), promoting coordination of substrate molecules that affect the activity of the catalyst and, probably, a preferential hydrogenation of one nitro group.e shi to higher binding energy region (337.7 eV) is observed for the intermediate separated aer hydrogenation of the second nitrogroup.In this case, the polymer matrix is signi�cantly changed, as evidenced by changes in the N1s spectrum.

Conclusions
Effective approach to obtain mixed-type catalysts based on polymer-immobilized Pd nanoparticles on the support surface has been developed for selective hydrogenation of unsaturated compounds.In the case of unsaturated alcohols, hydrogenation process prevails over isomerization.e studied nanocomposites showed stable catalytic activity in repeated cycles.e circumstance that they occurred in an immobilized form facilitated their separation from the reaction medium and repeated use and made it possible to study the transformations of catalytic intermediates, as well as to study changes in catalyst intermediates by methods of nondestructive testing.e highest activity in the hydrogenation of both double bonds and nitrotoluene compounds showed the catalysts with Pd  form where the palladium atoms are activated by the appearance of positive charge on them.
Results of the XPS, XRD, and TEM analysis of polymer hybrid nanocomposites based on PdAAm.