Novel aminophosphonate ligand for the preparation of catalytically active silica aerogels with finely dispersed palladium

Silica-based aerogels bearing a novel aminophosphonate-containing substituent were prepared. Pd2+ and Cu2+ ions were introduced into aerogel matrix to form chelated complexes with aminophosphonate moiety. Both complexes securely retain Me2+ after supercritical drying (SCD) in CO2. In silica matrix, Pd complex is reduced to metallic Pd after treatment by gaseous H2 at 120 °C. In lyogel matrix, Pd complex can also be reduced to metallic Pd upon SCD in isopropanol. Pd-containing aerogel showed excellent catalytic activity in the reaction of C=C and C=Cbonds hydrogenation.

Chemical modification of aerogel matrices with coordination compounds make it possible to obtain a wide variety of functional materials possessing important properties inherent in both aerogels and metal complexes [13]. This kind of modification can be achieved by the use of organic functional groups bound to aerogel matrix acting as the ligands to form complexes with transition metals. Such metals or their complexes immobilized on the surface of aerogels can act as efficient heterogeneous catalysts where catalytic centers are finely and homogeneously distributed over a large surface resulting in a decrease of metal consumption (that is mostly important for noble metals: Pd, Pt, etc.) and/or to an increase in a catalyst's activity [13][14][15].
The synthesis of Pd 2+ complexes secured in aminoaerogels and xerogels have recently been reported [17,18,21,22], along with their use as catalysts for Suzuki coupling reaction (conversion values up to 78%). Aminosilane salen-Cu 2+ or Co 2+ complexes were successfully immobilized in aerogels and demonstrated high catalytic activity in allylic oxidation of olefins (e.g. a 99.9% conversion of ethylbenzene was achieved) [19]. Fe + 3 complex immobilized in amino functionalized aerogel demonstrated excellent stability and high activity in amination of allyl alcohols (reaction yields 1 3 reached 95%) [14]. Ru complexes in aminated xerogels catalyzed unsaturated alcohols transformation to dimethylfuran (yields up to 89%) [23], Ru complexes were also mentioned as catalysts in hydrosylilation reactions, yet no details of catalytic experiments were disclosed [20].
Thus, aerogels functionalized with metal complexes attract a great deal of attention due to their high potential for the design of new active heterogeneous catalysts. At the same time, amine-or phosphine-containing ligands form very strong complexes with platinum-group metals and can limit the catalytic activity of the material. In metallocomplex catalysis, a free site at metal ion is often needed to coordinate substrate molecules, thus a certain lability of metals complexes is required to achieve high catalytic activity. In this view, for the binding of metal ions to aerogel surface, bi-functional chelating ligands are of primary interest. In such a ligand, one functional group (e.g., :NR 2 ) ensures strong binding of a metal ion and prevents its washing out from the aerogel matrix during its synthesis. The second "weak" functional group (e.g. P=O) imparts the lability to a complex to ensure the accessibility of a central atom to substrate [31][32][33][34][35].
In this paper, a new concept is suggested for the design of aerogel-based heterogeneous catalysts based on use of bi-functional ligands for the binding of metal ions to aerogels surface. A novel aminophosphonate chelating ligand was synthesized bearing NH and P = O moieties, (EtO) 2 P(O) CH 2 CH 2 NHCH 2 CH 2 CH 2 -Si(OMe) 3 (1), which readily formed complexes with transition metals (Cu, Pd). These complexes were cogelated with Si(OMe) 4 to form novel catalytically active aerogels. The resulting aerogels bearing immobilized Pd 2+ complexes effectively catalyzed industrially important hydrogenation of C-C double or triple bonds.

Preparation of aerogels
Ligand 1 was synthesized as follows: 2 mL of methanol were placed into a glass flask. Then 1.05 g (5.9 × 10 −3 mol) of APTMS and 1.05 g (6.5 × 10 −3 mol) of diethyl vinylphosphonate were added under stirring and the mixture was heated at 50 °C for 3 h. The product obtained was evacuated (25 °C, 6.7 kPa) to remove methanol. To prepare silylated complex 2, 0.103 g (3 × 10 −4 mol) of ligand 1 was mixed with 1.95 mL of methanol solution containing 0.027 g (1.5 × 10 −4 mol) of Li 2 PdCl 4 . The solution of complex 2 was mixed with 2.24 g (0.0147 mol) of TMOS (1:100 = Pd:Si molar ratio) and 1.5 mL of methanol. Then, 1.08 g (0.06 mol) of deionized water was added and the mixture was stirred for several minutes. The gelation occurred in 4 h.
The resulting gels were soaked in isopropanol for 24 h, to exchange the pore liquid for the solvent chosen. This procedure was repeated five times. The gels formed were placed into an autoclave for supercritical drying.
Metal-free aerogels (AG-MF) were prepared by the same protocol.
Supercritical drying of gels in isopropanol and CO 2 was performed according to the procedure described elsewhere [37].

Characterization of aerogels
Mass spectra were obtained using a Finnigan MAT INCOS™ 50 mass spectrometer at 70 eV EI. High resolution 1 H and 31 P NMR spectra were obtained in CDCl 3 using a Bruker Avance III 500 spectrometer at the Larmor precession frequencies of 500 and 202 MHz, relative to TMS and 85% H 3 PO 4 , respectively. The specific surface area of aerogels was determined by low-temperature nitrogen adsorption measurements with a Katakon ATX-06 analyzer, using a 5-point BET method. Energy dispersive X-ray analysis (EDX) was performed using a Carl Zeiss NVision 40 high resolution scanning electron microscope equipped with an Oxford Instruments X-MAX analyzer operating at 20 kV accelerating voltage. X-ray powder diffraction (XRD) patterns were recorded with a Bruker D8 Advance diffractometer (CuKα radiation). The low-temperature nitrogen adsorption measurements and EDX studies were carried out using shared experimental facilities of the IGIC RAS. TEM measurements were performed on an 80-300 TITAN transmission electron microscope operating at 300 kV and equipped with Tridium GIF. The sample for the TEM study was prepared using a VERSA dual beam facility.

Catalytic hydrogenation reactions
The reactions of catalytic hydrogenation were carried out in a gas phase (hydrogenation of benzene, hexene-1 and propargyl alcohol) or in 1,4-dioxane (hydrogenation of acetophenon).

Gas-phase hydrogenation
The powdered catalyst (V ≈ 0.25 cm 3 ) in a glass tubular reactor was placed in a furnace. The flow of hydrogen (1.2-1.6 L/h) was passed through the selected unsaturated compound (benzene, hexene-1 or propargyl alcohol) and saturated gas mixture directed to a reactor heated to a certain temperature. The reaction product was collected in a trap cooled by ice-water. A gas phase hydrogenation of hexene-1 was performed over one portion of the catalyst but using 5-7 loadings of the olefin. The analysis of the product was performed using NMR.
For the preparation of a gaseous mixture of hydrogen and propargyl alcohol, the azeotropic mixture of the latter with benzene was used to lower the boiling point.

Liquid-phase hydrogenation
A flask with a thermostatic jacket was charged with 2 mL of a solvent (dioxane), 30 mg of aerogel, and 0.2 mL of the test substance (acetophenone). The installation was purged with hydrogen for 1 h, followed by heating under constant stirring for 6 h. The resulted solution was separated from the catalyst by centrifugation and evaporated in vacuo (25 °C, 50 mmHg) to remove the solvent. The analysis of the product was performed by NMR.
For the study of hexene-1 hydrogenation reaction, no solvent was added and 3 mL of hexene-1 was loaded in the reaction vessel.
Thus prepared AG-Pd or AG-Cu aerogels were nontransparent monoliths (Fig. 1). Their textural characteristics together with metal-free sample are presented in Table 1 SEM images of aerogel samples are presented in Fig. 2 EDX analysis revealed metal to phosphorus atomic ratio in AG-Pd or AG-Cu aerogels to be 0.55 or 0.40, respectively, being close enough to the theoretical value (0.50, see composition of complexes 2 and 3 in Scheme 1) and Si:Pd = 100:1.2 (theoretical value Si:Pd = 100:1). This indicates that the bonding of Pd 2+ and Cu 2+ ions is strong enough to withstand the harsh conditions of supercritical drying. Quantitative estimation shows the 1.8% Pd mass loading in AG-Pd.
Upon SCD in CO 2 AG-Pd or AG-Cu aerogels retained the color inherent in the compounds of the corresponding metal ions -beige (ochreous) for Pd 2+ or blue for Cu 2+ . High-temperature SCD in IPA at 250 °C did not change the color of AG-Cu aerogel, while AG-Pd aerogel became black.
We assume that the color change of the AG-Pd sample upon SCD in IPA is due to the reduction of Pd 2+ to Pd 0 . To prove this assumption, an XRD study of the material was conducted (see Fig. 3).
The X-ray diffraction pattern of the obtained sample (Fig. 3) shows the presence of an amorphous halo corresponding to the SiO 2 matrix (in the 2θ range from 10° to ~ 32°), as well as the only well-resolved peak at 2θ = 39.4°. This diffraction maximum can be attributed to both palladium metal phase (PDF-2 46-1043) and the hydrogenloaded palladium phase (PDF-2 87-0641). It is well-known that the metals of the platinum group are capable of catalyzing both the hydrogenation and the reverse dehydrogenation processes. Palladium is known as a catalyst for the dehydrogenation of aliphatic alcohols [38][39][40][41][42] resulting in formation of gaseous hydrogen. Catalytic evolution of hydrogen can lead to the formation of a hydrogen-loaded palladium. In any case, the presence of either palladium or hydrogen-loaded palladium in the material proves the reduction of Pd 2+ species to palladium metal in supercritical isopropanol. The homogeneous distribution of palladium SCD CO 2 SCD IPA  nanoparticles in this sample was confirmed by EDX-mapping (Fig. 4). Upon treatment of AG-Pd aerogel at 120 °C in hydrogen atmosphere, it also changes its color to dark grey revealing the reduction of palladium(II) species to metallic palladium (Pd 2+ → Pd 0 ). Nevertheless, no free metal was detected by XRD indicating that the metal is present in the material in the form of ultra-small clusters having the size beyond the limits of XRD (approximately 1 nm). TEM also failed to find palladium clusters in the sample (Fig. 5).

AG-Cu
The aerogels bearing ultra-small metallic nanoclusters is of a special interest for their possible applications in catalysis, but until now they were rarely reported. One of the available examples presents the material comprising copper clusters in highly porous silica as recently reported by Kristiansen et al. [43,44]. Copper clusters were formed upon gentle heating (lower than 300 °C) of silica aerogels loaded with divalent copper in a 5% hydrogen flow, the size of clusters was even beyond the limits of TEM. Our data indicate that the heating of AG-Cu aerogel did not change its appearance upon heating at 180 °C in H 2 atmosphere and thus corroborates the reports of Kristiansen et al.
AG-Pd aerogel (2) proved to be an effective catalyst for hydrogenation of terminal C =C or C=C bonds in selected organic compounds (Scheme 1).
In the presence of AG-Pd aerogel, catalytic hydrogenation of gaseous hexene-1 at 100 °С proceeded with a 100%  The unexpected formation of propanal is probably due to the acetylene-allene isomerization of propargyl alcohol and further allene enol isomerization to acrolein followed by hydrogenation to propionic aldehyde (Scheme 2).
AG-Pd catalyst failed to reduce acetophenone or aromatic compounds. It did not demonstrate any activity in the reaction of benzene hydrogenation even at 200 °C, whereas the same reaction in the presence of an industrial catalyst Pd/Al 2 O 3 (Engelhard) yielded 80% cyclohexane at 100 °C. Acetophenone hydrogenation using AG-Pd catalyst resulted in 2% only yield of 1-phenylethanol (which is in agreement with the presence of propionic aldehyde during propargyl alcohol hydrogenation) whereas Pd/ Al 2 O 3 (Engelhard) demonstrated a 100% conversion under the same experimental conditions. Thus, the palladium catalyst immobilized on an aminophosphonate-substituted aerogel exhibited high catalytic activity in the hydrogenation of terminal aliphatic C=C and C≡C bonds, but did not reduce aromatic hydrocarbons and possessed low activity in acetophenone reduction. AG-Pd aerogel has higher selectivity compared to the commercial Pd/Al 2 O 3 catalyst-for example C-C double and triple bonds can easily be hydrogenated in presence of carbonyls or aromatic compounds.
Our research was not intended on the creation of a catalyst with a superior catalytic activity. We focused our efforts on the chemical modification of aerogels surface with a bi-functional ligand bearing two different functionalities, a strong Pd 2+ bonding amino group and a weak Pd 2+ coordinating phosphoryl group. The latter ensures high lability of a metal complex and facilitates the accessibility of a metal to substrate molecules necessary for a number of catalytic processes (e.g. cross-coupling like Sonogashira, Suzuki or Heck reactions). Modification of an aerogel with such ligands can open new possibilities in creation of advanced metal-loaded ultraporous materials which can be regarded as heterogeneous-homogeneous catalysts. In our case each chelating substituent on the surface binds only one Pd ion giving the access to all catalytically active metal atoms which is impossible in usual heterogeneous Pd-metal catalysts.
Note, that as a reference material, we chose Engelhard's Pd/C or Pd/Al 2 O 3 , an unrivaled catalytically active material. Despite metal loaded aerogels failed to surpass Engelhard's conversion values, they showed higher selectivity in olefins/ arenes/carbonyl group hydrogenation.

Conclusion
A novel silylated aminophosphonate ligand (MeO) 3 Si(CH 2 ) 3 NH(CH 2 ) 2 P(O)(OEt) 2 was quantitatively synthesized by a fast nucleophilic addition reaction of APTMS and diethylvinylphosphonate. The ligand was successfully introduced into SiO 2 -based aerogel by co-gelation with Si(OMe) 4 . The similar procedure allowed for chemical immobilization of Pd 2+ and Cu 2+ chelate complexes with the aminophosphonate ligand into aerogel matrix. In aerogel matrix, Pd 2+ complex can be reduced to metallic Pd upon supercritical drying in isopropanol or upon heating of Pd 2+ -loaded aerogel in hydrogen atmosphere at 120 °C. On the contrary, supercritical drying in CO 2 does not result in palladium reduction. Pd-aerogel loaded with Pd 2+ chelate showed high catalytical activity in hexene-1 hydrogenation reaction (100% yield). Propargyl alcohol was quantitatively converted into a mixture of propanol-1 and propanal. The catalyst failed to reduced benzene and acetophenone thus showing higher selectivity in comparison with industrial catalyst Pd/Al 2 O 3 (Engelhard). Author contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by NAS, ANM, EAS and LLY. The first draft of the manuscript was written by NAS, AEB, VKI and SAL and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding This work was supported by the Russian Science Foundation (project no. 19-73-20125) and performed using the equipment of the JRC PMR IGIC RAS. Supercritical drying of the aerogels was conducted using the experimental facilities of IPAC RAS (theme no. 0090-2019-0002).