Exploring the intricate relationship between noble metal ensembles and supporting materials stands as a captivating frontier in chemistry and catalysis. However, the precise control of noble metal ensembles on inert support remains challenging. We present here a facile strategy to construct noble metal ensembles, ranging from single atom to clusters or nanoparticles, on inert pure-silica zeolites. Typically, the internal silanol nests of silicalite-1 (S-1) platelike zeolites are identified as anchoring sites for Pt ensembles. The dynamic interplay between silanol nests and Pt precursors in solution phase are monitored to reveal the formation and evolution of Pt ensembles. Various Pt ensembles on S-1 zeolites show distinct catalytic behaviors in the model reaction of benzonitrile hydro-conversion. Thereinto, Pt1/S-1 catalyst containing exclusive mononuclear ionic Ptδ+ sites shows remarkable performance in the hydro-conversion of benzonitrile to di-benzylamine, which can be extended to the synthesis of secondary amines from the hydro-conversion of nitriles or nitriles/amines mixtures. The methodology developed herein offers the precise control of noble metal ensembles on inert zeolite support towards selective catalysis.
Introduction
Noble metal-based catalysts hold significant importance in heterogeneous catalysis, showing broad applications in various fields such as fine chemical engineering, energy transformation, and environmental protection.1 Among supported noble metal catalysts, single-atom catalysts (SACs) have drawn special attention owing to their superior atomic utilization efficiency and unique electronic configuration, thereby facilitating a wide range of chemical transformations including dehydrogenation, C–H bond activation, Suzuki coupling, and so on.2–5 However, the inherent high surface energy of noble metal species commonly results in their aggregation during catalyst preparation and reaction processes, compromising catalytic activity or even leading to catalyst deactivation.6–8 In this context, the precise control of stable noble metal ensembles, from single-atom to clusters or nanoparticles, over a specific support remains a key challenging topic in catalysis.1,8,9
Zeolites, characterized by adjustable framework composition, abundant acid-base sites, well-defined topology as well as distinctive local electronic field, stand out as versatile hosts for transition metal ensembles.10,11 Several strategies, including ligand-protect encapsulation,11,12 ion-exchange,13 and incipient wetness impregnation5 have been explored for constructing zeolite-supported SACs. Meanwhile, it has been reported that noble metal clusters can be confined in the structure of pure silica zeolites like silicalite-1 (S-1) via ligand-protect encapsulation followed by specific treatment, which show distinctive catalytic properties in several important reactions.14–17 However, all these processes might confront problems of complex procedures or poor reproducibility,13 which are unfortunately not suitable for production in large scale. On the other hand, diverse target product orientations necessitate different active metal species, ranging from single-atom5 to dual single atoms18,19 and metal clusters,14–16,20,21 or even multiple sites.22,23 A simple and general strategy to control stable noble metal ensembles over zeolite supports is highly desired to facilitate their catalytic applications with target product distributions.
Herein, we develop a straightforward postsynthesis strategy for the construction of noble metal ensembles, ranging from isolated atoms to clusters or nanoparticles, on pure silica platelike zeolites (S-1) utilizing their unique hydroxyl nests. The evolution of noble metal ensembles on zeolites are monitored to reveal the essence and general applicability of this strategy. The as-prepared Pt1/S-1 (containing single-atom Pt), Pt1+c/S-1 (containing single-atom Pt and Pt clusters), and Ptn/S-1 (containing Pt nanoparticles) samples show distinct catalytic behaviors in the hydro-conversion of benzonitrile. Notably, Pt1/S-1 catalyst containing hydroxyl nest-stabilized mononuclear Ptδ+ species demonstrates remarkable performance in the synthesis of secondary amines from the cascade hydro-conversion of nitriles.
Experimental Methods
Various characterization methods were applied to confirm the structure of Ptx/S-1, including powder X-ray diffraction (XRD), temperature-programmed reduction, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) shown in the Supporting Information. The catalytic performance was performed on a high-pressure autoclave. Subsequently, all the structures and simulation processes were performed based on the spin-polarized density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package.
Results and Discussion
Synthesis and characterization of Pt ensembles on S-1 zeolites
S-1 platelike zeolites were obtained through hydrothermal synthesis involving a mixed
gel comprising SiO2, tetrapropylammonium hydroxide, H2O, tetraethoxysilane, and H2SO4 and subsequently processed (detailed procedures in Supplementary Information).24 Then, various Pt ensembles, including Pt1, Pt1+c, and Ptn, were deposited on S-1 zeolites through wetness impregnation followed by evaporation
and calcination in flowing air ( Supporting Information Table S1). XRD patterns reveal the characteristic Mobil Five (MFI) topological structure of
all zeolite samples without any discernible diffraction peaks corresponding to Pt
species ( Supporting Information Figure S1). The type
To reveal the distribution and configuration of Pt species within Ptx/S-1, aberration-corrected high-angle annular dark-field scanning transmission electron
microscopy (AC-HAADF-STEM) analyses were conducted, capitalizing on distinct atomic
contrasts (78Pt, 14Si, and 8O) in samples.25 As depicted in Figure?1a, the as-synthesized S-1 zeolites exhibit good crystallinity with well-defined crystal
lattice, devoid of any discernible structural defects. The topologic structure of
S-1 sample remains intact along the [010] orientation, as evidenced by the corresponding
selected area electron diffraction pattern shown inset (Figure?1b,c). Particularly, the perspective along the [010] direction provides a clear view to
distinguish the 5,6,10-membered rings, while the [001] and [100] orientations showcase
the interlacing of various rings ( Supporting Information Figure S6).26,27 As shown in Figure?1d, Pt1/S-1 zeolite displays singular bright dots without any bright aggregates, indicative
of atomically dispersed Pt atoms. The contrast is accentuated in the high-magnification
AC-HAADF-STEM images along the [010] orientation (Figure?1e). The elemental mapping analysis of Pt1/S-1 zeolite further confirms the homogeneous distribution of Pt atoms throughout
the crystal of S-1 zeolites (Figure?1f). In contrast, the Ptn/S-1 sample displays a series of regular aggregates with apparent sizes of ～2?nm (Figure?1g–i), as labeled by blue squares. Expectedly, the Pt1+c/S-1 displays both single Pt atoms and aggregates of ～1?nm, the latter being denoted
as Pt clusters immobilized within zeolite structure (Figure?1j–l).28 Figure 1 | Electron microscopy analyses of Ptx/S-1 samples. (a–c) AC-HAADF-STEM images of S-1 zeolites along the orientation of
[010] as well as the schematic models on the same projection. Insets: view of single
S-1 zeolite crystal and selected area electron diffraction pattern; (d, e) AC-HAADF-STEM
images of Pt1/S-1 zeolite, indicating the presence of Pt single atoms, as marked by red circles;
(f)?Elemental mapping analysis of Pt1/S-1 zeolite; (g, h) AC-HAADF-STEM images of Ptn/S-1 zeolite, indicating the presence of Pt nanoparticles, as marked by blue squares;
(i)?Elemental mapping analysis of Ptn/S-1 zeolite; (j, k) AC-HAADF-STEM images of Pt1+c/S-1 zeolite, indicating the co-existence of Pt clusters (～1?nm) and single atoms,
as marked by blue squares and red circles; (l)?Elemental mapping analysis of Pt1+c/S-1 zeolite.
Diffuse reflectance infrared spectroscopy (DRIFTS) of CO adsorption was conducted to identify the geometric configuration of various Pt sites in Ptx/S-1. As shown in Supporting Information Figure S7, Pt1/S-1 manifests a distinct peak at 2092?cm?1, attributed to di-carbonyl on atomically dispersed Ptδ+ species ( Supporting Information Table S2).29–31 Ptn/S-1 containing nanosized Pt aggregates exhibits a distinctive peak at around 1850?cm?1, which can be attributed to bridge-bonded CO on Pt–Pt ensembles ( Supporting Information Figure S8).32–34 While for Pt1+c/S-1, two major peaks at 2078 and 1934?cm?1 can be captured, corresponding to linearly adsorbed CO on atomically dispersed Ptδ+ species and Pt0 clusters, respectively ( Supporting Information Figure S9).30,33,35 The abovementioned results are consistent with STEM observations (Figure?1), together revealing the growth of Pt species from single atoms to clusters and nanoparticles on S-1 zeolites with increasing Pt loadings from 0.05% to 0.3% ( Supporting Information Table S1).
On the premise of different configurations of Pt ensembles revealed by STEM and DRIFTS
of CO adsorption, XAS was conducted to reveal the electronic structure and coordination
environment of Pt species. As shown in the X-ray absorption near-edge structure (XANES)
spectra (Figure?2a), the Pt L3-edge white-line intensities of Ptx/S-1 zeolites are located between reference PtO2 and Pt foil, following the order of PtO2 > Pt1/S-1 > Pt1+c/S-1 > Ptn/S-1 > Pt foil. That is, positively charged Pt species in Pt1/S-1 exhibit stronger electronic interaction with oxygen atoms than those in Pt1+c/S-1 and Ptn/S-1. The oxidation states of Pt species in Ptx/S-1 were investigated by XPS. As shown in Supporting Information Figure S10, cationic Pt species are detected in Pt1/S-1 and metallic Pt species are observed in Ptn/S-1, while a mixture of ionic and metallic Pt species are present in Pt1+c/S-1. Obviously, most deposited Pt species in Ptn/S-1 and partial Pt species in Pt1+c/S-1 undergo auto-reduction to metallic Pt species upon calcination in flowing air,
accompanied by the formation of aggregates. These can be further confirmed by the
experiments of dihydrogen temperature-programmed reduction (H2-TPR). As shown in Supporting Information Figure S11, a distinct dihydrogen consumption peak at ～710?K is observed for Pt1/S-1 zeolite corresponding to the reduction of ionic Pt species (H/Pt?=?1.9). In contrast,
Ptn/S-1 shows a very small dihydrogen consumption peak at ～710?K (H/Pt?>?0.01) since
most Pt species already exist in the metallic form that will not be reduced by dihydrogen.
For Pt1+c/S-1 containing both ionic and metallic Pt species, a small dihydrogen consumption
peak with H/Pt ratio of 0.12 can be observed. Fourier-transformed k2-weighted extended X-ray absorption fine structure (EXAFS) spectroscopy was conducted
to provide detailed coordination information of Pt sites in Ptx/S-1 (Figure?2b,c and Supporting Information Table S3). For Pt1/S-1, a prominent signal assigned to the first Pt-O shell coordination (～1.6??, CN?≈?2.7)
is observed without any Pt–Pt scattering, demonstrating the atomically dispersed ionic
Pt species.36 For Pt1+c/S-1, two dominant peaks at 1.6 and 2.4??, assignable to Pt-O and Pt–Pt path, respectively,
are observed. While for Ptn/S-1, the Pt–Pt scattering signal at ～2.4?? becomes the prominent one with an average
coordination number of 5.6, indicating the presence of Pt aggregates rather than single
Pt atoms.19 The EXAFS data were further processed by wavelet transformation (WT) analyses to
reveal the fine structure of Pt species in both k and R space. As shown in Figure?2d, only one intensity maximum at ～6???1 is observed for Pt1/S-1 zeolite, confirming the presence of exclusive atomically-dispersed Pt sites and
verifying the assignment of peak at 1.6?? to Pt-O path. In contrast, the WT contour
plot of Ptn/S-1 zeolite shows a dominating maximum at ～10.7???1 associated with Pt–Pt scattering (Figure?2d), validating the presence of metallic Pt aggregates with high coordination number.
In short, the characterization results from XAS, XPS, and H2-TPR reveal the evolution of atomically dispersed ionic Pt species to metallic Pt
aggregates with increasing Pt loadings from 0.05% to 0.3% in Ptx/S-1.
Figure 2 | Pt L3-edge XAS analyses of Pt-containing zeolite samples. (a)?XANES spectra of PtO2, Pt foil, and Pt-containing zeolite samples; (b)?FT EXAFS spectra of PtO2, Pt foil, and Pt-containing zeolite samples; (c)?FT EXAFS fitting spectra of PtO2, Pt foil, and Pt-containing zeolite samples at R space; (d)?WT EXAFS of PtO2, Pt foil, and Pt-containing zeolite samples.
Formation and evolution of Pt ensembles on S-1 zeolites
To reveal the formation and evolution of Pt ensembles, it is extremely important to
know the anchoring sites in zeolite support and their interaction with Pt species.
There are no anchoring sites in perfect S-1 zeolite crystals and the only possible
anchoring sites should come from the crystal defects, namely silanols. As shown in
Figure?3a, S-1 and Ptx/S-1 samples demonstrate two distinctive signatures between 3800–3600?cm?1 and 3600–3400?cm?1 in the DRIFT spectra, corresponding to external free silanols (non-H-bonded) and
internal silanol nests (H-bonded), respectively.37–39 The intensity of internal silanol nests in Pt1/S-1 shows a significant decline in comparison with that in S-1, indicating the consumption
of internal silanol nests upon interaction with atomically dispersed Pt species. It
is interesting to note that the intensity of H-bonded silanol nests recovers with
increasing Pt loadings from 0.05% to 0.3% (Figure?3b), which might be due to the formation of metallic Pt aggregates and the corresponding
reduction in the demand of silanol nest anchoring sites (vide infra). Two dimensional
1H-1H single-quantum/double-quantum magic angle spinning (SQ/DQ MAS NMR) experiments were
further conducted to reveal the nature of silanols in S-1 zeolite support and their
interaction with Pt species. As shown in Figure?3c, strong autocorrelation peaks at (2, 4) and (3, 6) ppm, corresponding to external
free silanols and internal silanol nests, respectively, are observed for S-1 platelike
zeolites.40 Upon the introduction of Pt ensembles, a significant consumption of internal silanol
nests can be observed (Figure?3d and Supporting Information Figure S12), revealing that the internal silanol nests act as the preferred anchoring sites
for Pt ensembles. That is actually very easy to understand since the silanol nests
can provide stronger interaction and better stabilizing effect for guest metal species
than the free silanols. On the other hand, it is strange to see that less silanol
nests are required to stabilize more Pt ensembles (Figure?3b and Supporting Information Figure S12). For Pt1/S-1, the atomically dispersed Ptδ+ species are exclusively stabilized by the silanol nests with the formation of Si-O-Ptδ+ linkages. While for Pt1+c/S-1 and Ptn/S-1, the interplay between silanol nests and Pt ensembles/precursors are much more
complicated, ultimately controlling the configuration of Pt ensembles.
Figure 3 | Characterization of anchoring sites in S-1 zeolites for Pt ensembles. (a)?DRIFT spectra
of S-1 and Ptx/S-1 zeolites; (b)?Relative intensity of internal silanol nests in S-1 and Ptx/S-1 zeolites; 1H-1H SQ/DQ MAS NMR spectra of S-1 (c)?and Pt1/S-1 (d)?zeolites.
The dynamic interplay between silanol nest anchoring sites and Pt ensembles/precursors
(PtL, L: ligand) is proposed, as shown in Scheme?1. With an abundance of internal hydroxyl nests as the anchoring sites (Figure?3), the concentration of Pt precursor emerges as a pivotal factor driving significant
alterations in the configurations of obtained Pt ensembles (Scheme?1). Typically, the preferential formation of [Si-O]x-Pt1 linkages (Pt1/S-1) occurs when the rate constant associated with the amalgamation of Pt precursor
and internal hydroxyl nests is much greater than that of the spontaneous collision
among Pt precursor, namely k1?k2. This is kinetically facile at low Pt precursor concentrations. In contrast, at higher
Pt precursor concentrations, Pt clusters or nanoparticles will be formed with accelerated
collision rate among Pt precursors (k2, k3?k1), which are then stabilized through bonding with silanol nests (k4, k5).
Scheme 1 | Formation of Pt ensembles ranging from single atom to aggregates on S-1 zeolites.
Extension of noble metal ensembles on plate-like S-1 zeolites
Harnessing the distinctive interplay between the metal precursor and the hydroxyl
nest anchoring sites, a series of noble metal ensembles on S-1 zeolite support, including
Rh1/S-1, Rh1+c/S-1, Rhn/S-1, Pd1/S-1, Pd1+c/S-1, and Pdn/S-1 can be engineered. As shown in Figure?4a,d, isolated noble metal atoms, marked by red circles, on platelike S-1 zeolites can
be clearly illustrated for Rh1/S-1 and Pd1/S-1 through the AC-HAADF-STEM images. In contrast, Rhn/S-1 and Pdn/S-1 present uniform noble metal clusters of ～2?nm, as labeled by blue squares (Figure?4c,f). As for Rh1+c/S-1 and Pd1+c/S-1, both single atoms as well as tiny clusters can be observed (Figure?4b,e). These observations strongly confirm the generality of this facile strategy for
the construction of controlled noble metal ensembles on platelike S-1 zeolites. Subsequently,
Fourier transform infrared spectroscopy of CO adsorption was employed to identify
the configurations of noble metal sites on zeolite support. As shown in Figure?4g, two infrared (IR) bands at 2095 and 2020?cm?1 attributed to the symmetric and asymmetric stretch of CO from the positively charged
Rh(CO)2 species can be observed in Rh1/S-1 ( Supporting Information Table S2).12 As for Rhn/S-1 catalyst, two IR bands at 2050 and 1920?cm?1 ascribed to linearly adsorbed CO and bridge-CO on Rh clusters are captured. Meanwhile,
several IR bands at 2090, 2055, and 2015?cm?1 are observed with Rh1+c/S-1, indicating the co-existence of isolated Rh ions and Rh clusters.41,42 Similarly, Pd1/S-1 shows a typical band at 2050?cm?1 corresponding to linearly bonded CO on positively charged Pd species, while Pdn/S-1 shows an IR band at 1855?cm?1 ascribed to three-fold bridge bonded CO species (Figure?4h and Supporting Information Table S2).43 Expectedly, Pd1+c/S-1 containing both isolated Pd ions and clusters show IR bands at 2050 and 1930?cm?1, in line with AC-STEM observations. Overall, noble metal (Pt, Rh, and Pd) ensembles
ranging from single atom to clusters or nanoparticles, on inert S-1 zeolites can be
rationally constructed through the facile strategy developed in this study, offering
versatile candidates for selective catalysis.
Figure 4 | Characterization of noble metal species on S-1 zeolites. AC-HAADF-STEM images of Rh1/S-1 (a), Pd1/S-1 (d), Rh1+c/S-1 (b), Pd1+c/S-1 (e), Rhn/S-1 (c), and Pdn/S-1 (f), with topologic structure inset in (a)?and (d), scale bar?=?2?nm; DRIFT spectra
of CO adsorption on Rhx/S-1 (g)?and Pdx/S-1 (h)?(x?=?1, 1+c and n). Figure 5 | Catalytic behaviors of Pt1/S-1 (left), Pt1+c/S-1 (middle), and Ptn/S-1 (right) in the hydro-conversion of benzonitrile. Reaction conditions: catalyst?=?100?mg,
BN?=?1?mmol, isopropanol?=?5?mL, temperature?=?413?K (left), 393?K (middle and right),
tetradecane?=?40?μL, H2?=?1?MPa, stirring speed?=?1000?rpm.
Selective catalysis by Pt ensembles on S-1 zeolites
The hydro-conversion of benzonitrile (BN) (Scheme?2) was employed to clarify the ensemble effect of Ptx/S-1 catalysts. Generally, all Ptx/S-1 catalysts are active for the hydro-conversion of BN (full BN conversion achieved
within 1.5–6?h) while various Pt ensembles manifest totally distinct product distributions.
As shown in Figure?5, Pt1/S-1 shows high selectivity towards N-benzylidenebenzylamine (DBI) and di-benzylamine (DBA), accompanied by the formation
of trace benzylamine (BA) as by-product. The selectivity towards DBA gradually increases
to >95% with prolonged reaction time, in accordance with the hydrogenation of DBI
to DBA under employed conditions ( Supporting Information Figure S13). That is, the atomically dispersed ionic Pt sites on S-1 zeolites facilitate the
condensation deamination between BI and BA (
Scheme 2 | Reaction pathways of benzonitrile hydro-conversion.
Reaction mechanism of benzonitrile hydro-conversion over Pt ensembles on S-1 zeolites
For insight into dihydrogen activation over Pt1/S-1 catalyst, hydrogen-deuterium pulse experiments were performed. As shown in Supporting Information Figure S15, H-D signal (m/z=3) emerges immediately upon the injection of D2 molecules into H2 stream over Pt1/S-1 no matter with BN preadsorption or not, indicating the cleavage of H2/D2 and the subsequent formation of HD. The facile dissociation of dihydrogen over Pt1/S-1 catalyst can be therefore illustrated, facilitating the subsequent hydro-conversion
reactions. Then in situ DRIFT spectroscopy was employed to monitor the surface species
during BN hydro-conversion over Pt1/S-1. As shown in Supporting Information Figure S16, Pt1/S-1 displays two distinctive IR bands at 3725 and 3460?cm?1, corresponding to residual terminal free silanol and internal silanol nests in S-1
zeolites, respectively. The introduction of dihydrogen does not induce any discernible
changes in the silanol groups (Figure?6a). Upon the subsequent feeding of BN, notable IR bands appear at 3100–3000?cm?1, along with a series of IR bands at 2230, 1600, 1490, 1448, and 756?cm?1, attributable to the stretching vibrations of aryl C–H, C≡N, and C–C bonds of BN
molecules ( Supporting Information Figures S17–S19 and Table S4).45–52 The vibrational frequency of silanol nest exhibits a significant red shift from 3460
to 3400?cm?1 upon BN introduction, indicating the strong interaction between the –C≡N functional
group and the silanol nest within Pt1/S-1 zeolite.53 This red shift intensifies with inverse feeding of reactants (first BN and then dihydrogen),
demonstrating the preferential adsorption of BN in Pt1/S-1 (Figure?6b). These observations disclose that the hydro-conversion of BN over Pt1/S-1 catalyst is characterized by the preferential adsorption of BN followed by dihydrogen
activation. To confirm the mode of dihydrogen activation, in situ DRIFT spectra of
H2 and D2 activation were recorded. As shown in Figure?6c, a distinct IR band at 3743?cm?1, corresponding to free silanol group, can be captured on Pt1/S-1 only after the adsorption of BN molecules37 ( Supporting Information Figure S20). Compared to the results from hydrogen-deuterium pulse experiments ( Supporting Information Figure S15), the DRIFT spectra provide strong evidence of BN-assisted dihydrogen heterolytic
activation mechanism. Although dihydrogen activation can proceed on bare Pt1/S-1, the formed Pt-H and O-H species are extremely unstable and hardly detectable.
In contrast, the Pt-H and O-H species remain stable on Pt1/S-1 with preadsorbed BN molecules. It is therefore proposed that BN adsorption induces
the heterolytic dissociation of dihydrogen via the associative mechanism and subsequently
produces additional silanol groups (H bonded to framework oxygen). While in the DRIFT
spectra of D2 activation (Figure?6d and Supporting Information Figure S21), BN can induce the heterolytic dissociation of D2 and the subsequent H-D isotope exchange with OH bonds (3743, 3725, and 3460?cm?1; Figure?3a), which will produce a series of O–D bonds (2761, 2685, and 2546?cm?1, νO?H/νO?D≈1.35)54–57 ( Supporting Information Table S4).
Figure 6 | In situ spectroscopy of benzonitrile hydro-conversion over Pt1/S-1 catalyst. (a)?In situ DRIFT spectra of Pt1/S-1 recorded after the successive feeding of H2 and BN at 403?K; (b)?In situ DRIFT spectra of Pt1/S-1 recorded after the successive feeding of BN and H2 at 403?K; (c)?In situ DRIFT spectra of H2 activation over Pt1/S-1 after saturation with BN at 403?K; and (d)?In situ DRIFT spectra of D2 activation over Pt1/S-1 after saturation with BN at 403?K.
The reaction mechanism of BN hydro-conversion over distinct Pt ensembles was finally
interpreted by DFT calculations. According to characterization results, Pt1/S-1 should feature mononuclear Pt motifs embedded into the internal silanol nests
([Si-O]3-Pt1···[Si-OH]), while Ptn/S-1 is simplified by Pt4 clusters positioned adjacent to silanol nests.58 As shown in Figure?7a, the adsorption of BN molecules on mononuclear Ptδ+ proceeds smoothly, highly exothermic by 1.85?eV (
Figure 7 | Reaction pathways of benzonitrile hydro-conversion over (a)?mononuclear ionic Pt1 and (b)?metallic Pt4 clusters stabilized by the internal hydroxyl nests of S-1 zeolites at 0?K with optimized
structures shown inset.
Dihydrogen molecule can undergo facile dissociation over metallic Pt4 cluster (
Synthesis of secondary amines over Pt1/S-1 catalyst
Inspired by the unique catalytic behaviors of Pt1/S-1 in the hydro-conversion of BN, Pt1/S-1 was also tested for the synthesis of secondary amines from a wide array of both
aromatic and aliphatic nitriles. As shown in Table?1, Pt1/S-1 exhibits good tolerance to electron-withdrawing groups (
| Entry | Substrate | Product | Conversion (%) | Selectivity to Secondary Amines (%) |
|---|---|---|---|---|
| 1 |  |
 |
>99 | 95 |
| 2 |  |
 |
>99 | 94 |
| 3 |  |
 |
>99 | 91 |
| 4 |  |
 |
>99 | 92 |
| 5 |  |
 |
>99 | 93 |
| 6 |  |
 |
>99 | 92 |
| 7 |  |
 |
>99 | 92 |
| 8 |  |
 |
>99 | 95 |
| 9 |  |
 |
>99 | 97 |
| 10 |  |
 |
>99 | 96 |
| 11 |  |
 |
>99 | 91 |
| 12 |  |
 |
>99 | 93 |
| 13 |  |
 |
>99 | 92 |
| 14 |  |
 |
>99 | 96 |
| 15 |  |
 |
>99 | 95 |
| 16 |  |
 |
>99 | 94 |
Conclusion
We develop herein a facile strategy for the construction of various noble metal ensembles ranging from single atom to clusters or nanoparticles on pure silica zeolites. Typically, the internal silanol nests of S-1 zeolites are established as efficient anchoring sites for Pt ensembles. The dynamic interplay between silanol nest anchoring sites and Pt precursors in solution phase ultimately control the Pt ensembles on S-1 zeolites. This synthesis strategy is applicable to the large-scale production of various noble metal ensemble on pure silica zeolites containing internal silanol nests.
The ensemble effect of Pt catalysts in the hydro-conversion of benzonitrile is disclosed. Various Pt ensembles show quite different catalytic properties in these two reactions. Pt1/S-1 containing atomically dispersed ionic Pt sites stabilized by hydroxyl nests can efficiently catalyze the hydro-conversion of benzonitrile to DBA, following the cascade steps of selective hydrogenation, condensation deamination and selective hydrogenation. The substrate-assisted heterolytic dissociation of dihydrogen on Pt1/S-1 is identified as a key factor in the reaction. Pt1/S-1 is proved to be a versatile catalyst for the synthesis of various amines from the hydro-conversion of nitriles or nitriles/amines mixtures with good yields of >90%. Our work provides an example of controlling noble metal ensembles on pure silica zeolite support for the large-scale production of SACs toward complex chemical transformations.
Supporting Information
Supporting Information is available and includes experimental details, material and method, characterization results including XRD, scanning electron microscopy, TEM, XPS, XAS, more catalytic data, and theoretical calculations.
Conflict of Interest
There is no conflict of interest to report.
Funding Information
This work was supported by the National Natural Science Fund of China (grant nos. 22025203 and 22302099), China Postdoctoral Science Fund (grant nos. 2023M731797 and 2024T170438), and the Fundamental Research Funds for the Central Universities (Nankai University).
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