Catalytic cleavage of solid bonds including hydrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds is a highly desired yet difficult fundamental change for the manufacturing of chemicals and fuels. Transition metal-containing catalysts space employed, return accompanied with bad selectivity in hydrotreatment. Right here we report metal-free nitrogen-assembly carbons (NACs) with closely-placed graphitic nitrogen as energetic sites, achieving dihydrogen dissociation and also subsequent revolution of oxygenates. NACs exhibit high selectivity towards alkylarenes for hydrogenolysis of aryl ethers as version bio-oxygenates there is no over-hydrogeneration that arenes. Activities originate indigenous cooperating graphitic nitrogen dopants induced by the diamine precursors, as demonstrated in mechanistic and also computational studies. We further display that the NAC catalyst is flexible for dehydrogenation that ethylbenzene and also tetrahydroquinoline and for hydrogenation of typical unsaturated functionalities, including ketone, alkene, alkyne, and nitro groups. The exploration of nitrogen assembly as active sites have the right to open up wide opportunities because that rational architecture of brand-new metal-free catalysts for daunting chemical reactions.
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Hydrodeoxygenation that oxygenates through molecular hydrogen is critical for upgrading arising feedstocks prefer biomass1,2. Efficacious hydrodeoxygenation heavily relies top top heterogeneous catalysts based on transition metals because that hydrogenolysis and also hydrogenation3, although over-hydrogenation of functionalities choose arenes is often encountered4. An ext sustainable and also selective metal-free catalysts can overcome these limitations and also are extremely appealing, particularly when steel contamination in assets can it is in an issue5, however their design and also deployment remains a formidable job in both hydrogenolysis that C–O linkages and hydrogenation/dehydrogenation.
Metal-free catalysts have actually been reported primarily for hydrogenation6 and also oxidative dehydrogenation7,8 but not because that hydrogenolysis and non-oxidative dehydrogenation. For instance, irritable Lewis bag (FLPs) are qualified of straight reducing olefins or imines v H29,10; however, the majority of FLPs are easily deactivated in the presence of water and also alcohols11,12, preventing their use in industrial applications. Most other metal-free catalysts cannot straight employ H2 however instead use its surrogates favor hydrazine or NaBH413,14,15. Carbon materials catalyze the hydrogenation the the π bonds of ethylene and acetylene, yet not because that the breaking of stronger bonds (e.g., C–O and also C–H)16. Metal-free catalysts have actually shown activities in dehydrogenation however only v the assistance of oxidants.
Here, we present a carbon-based catalyst because that hydrogenolysis that C–O linkages in aromatic oxygenates utilizing molecular hydrogen. The unexpected, unique reactivity existing in this study mostly originates from the nitrogen assemblies presented by the diamine precursors right into the graphitic carbons, which also enables the non-oxidative dehydrogenation that ethylbenzene and tetrahydroquinoline (THQ) and also the selective hydrogenation the unsaturated functional groups in aromatics. Handling these metal-free catalysts walk not require oxygen- and also moisture-free conditions.
Synthesis and characterization
We synthesized N-assembly carbons (NACs) with closely placed nitrogen sites by condensing ethylenediamine (EDA) and carbon tetrachloride by a hard-template route17,18 (Fig. 1a), adhered to by carbonization under flow Ar at 600–900 °C and then etching to remove the silica template, giving the materials NAC-600–NAC-900. In the very first step, the diamine-carbon tetrachloride condensation outcomes in irreversible development of C–N bonds, as confirmed by the presence of secondary amines formed as crosslinked acyclic chains and imidazolidine, presented by dynamic atom polarization (DNP)-enhanced 15N1H cross-polarization magic-angle rotate (CPMAS) NMR (Fig. 1b and also Supplementary Fig. 1)19,20. Top top carbonization at 300 °C for 10 min, second broad signal showed up in the variety between −180 and −230 ppm in the 15N1H CPMAS spectrum (Fig. 1e). Fast CP accumulation suggests the the brand-new signal have the right to be assigned to the N–H-containing species, imidazolium19, in i beg your pardon the fragrant ring contains two nitrogen atoms (Supplementary Fig. 2). At higher temperatures (>600 °C), these carefully placed nitrogen atoms were further incorporated right into NACs. The graphitic centregalilee.com that NACs is shown in the matching powder X-ray diffraction (XRD) patterns and Raman spectra (Supplementary Figs. 3 and also 4). The thermal stability of the NACs is evidenced by thermogravimetric analysis (TGA) (Supplementary Fig. 5) and in situ diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS) examine (Supplementary Fig. 6a, b).
a Proposed formation mechanism of NAC catalysts within the mesoporous silica template. b DNP-enhanced 15N1H CPMAS spectrum of polymer prior to carbonization. c TEM picture of NAC-800, whereby 800 represents the sample preparation temperature. Scale bar, 40 nm. d HAADF-STEM image and EDS mapping of C and also N the NAC-800. Range bar, 40 nm. e DNP-enhanced 15N1H CPMAS spectrum the condensed polymers calcined because that 10 min at 300 °C. f N1s XPS spectra the NAC catalysts synthesized at various carbonization temperatures, showing pyridinic N (green), pyrrolic N (blue), graphitic N (magenta), and pyridine N-oxide (brown). g Pulsed H2 chemisorption the NAC-800 in ~ 60–360 °C.
NACs, isolated after ~ silica etching, own ordered mesopores (Fig. 1c and also Supplementary Fig. 7)21, uniform bit sizes (Supplementary Fig. 7), and also 9.0–19.8 at% N among carbon skeleton. Facet mapping (Fig. 1d) shows that the N atoms space homogeneously distributed over the bulk of the carbon, agreeing well v the X-ray photoelectron spectra (XPS) and also elemental analysis (Supplementary Table 1). High-resolution N1s XPS researches (Fig. 1f, Supplementary Fig. 8 and also Supplementary Tables 2,3) addressed pyridinic, pyrrolic, graphitic, and pyridine N-oxide species in NACs22. Upon carbonization, the signals in N1s XPS corresponding to pyridinic and also pyrrolic N content decrease considerably (Supplementary Tables 2 and 3). The trend of graphitic N complies with a volcano profile, the preferably of which reaches 4.8 at% among all atoms in NAC-800 (Supplementary Fig. 9). The NAC catalysts were all set using high-purity precursors from various sources to ensure the the carbocatalysts were entirely metal-free (Supplementary Table 4). Besides, signals for other elements, an especially common metal contaminants (e.g., Cu, Co, Ni, Ru, Pd, Pt, and also Rh), were listed below detection limits of ICP-MS, scanning transmission electron microscopic lense (STEM)-EDX, and also XPS (Supplementary Figs. 10 and 11).
Dissociative chemisorption of H2 is a prerequisite step in catalytic hydroprocessing. We demonstrated that NAC-800 dissociates H2 making use of ambient-pressure pulsed H2 chemisorption in ~ elevated temperature (60–360 °C), which reflects a amazing temperature dependence (Fig. 1g and Supplementary Fig. 12). A small, yet reproducible chemisorption worth of ~0.5-μmol H2 g−1 in ~ 60 °C increases almost 12-fold ~ above elevating the temperature come 240 °C come 5.9-μmol H2 g−1. The H2 chemisorption is significantly suppressed to 1.5-μmol H2 g−1 and ultimately zero in ~ 300 and 360 °C, respectively. In situ DRIFTS study confirmed the formation of C–H bonds quite than O–H or N–H in ~ 240 °C under the flow of H2 (Supplementary Fig. 6c, d).
The metal-free NAC products catalyze hydrogenolysis of C–O bonds in 2-phenoxy-1-phenylethan-1-ol (PPE, 1) v 20-bar H2 (Fig. 2a), which frequently require change metal catalysts. PPE, containing both α-OH and also phenolic ether, is thought about a version compound for the β-O-4 linkage in lignin23. The NAC-800 catalyst provides the highest possible conversion (30%), contrasted with NAC-650 (7%), NAC-700 (15%), and also NAC-900 (25%) in experiments performed in 2-propanol (2-PrOH) for 8 h at 230 °C, if NAC-600 is inactive (Fig. 2b and Supplementary Table 5). Quantitative conversion of PPE is completed after 80 h, with phenol and ethylbenzene developed as the significant final products (Supplementary Fig. 13). No PPE conversion to be observed in the manage experiments with regulate catalysts containing common metal contaminants (such as Fe, Cu, and also Ni in Supplementary Table 6).
a Reaction network. b yields of assets (except phenol). The PPE conversions and yields because that 1-phenylethanol (purple), ethylbenzene (yellow), styrene (orange), and also PEB (blue) are displayed in Supplementary Table 5. Reaction conditions: 14-mmol L−1 PPE in 2-PrOH (1.50 mL), NAC catalyst (5.0 mg), 20-bar H2, 8 h. The error bars are standard deviation (s.d.) of repeated tests. c measure of rate orders because that the NAC-800 catalyst (black), PPE (blue), and H2 (red). d Plots of initial rates of PPE conversion against the absolute amount the graphitic N in NAC catalysts. The error bars of absolute content of graphitic N were estimated and also given in Supplementary Table 3 and also the s.d. Of the initial prices are ~3% of the mean values, acquired after repetitive runs.
Three intermediates—styrene, 1-phenylethanol, and acetophenone—are detected in experiments employing the NAC-800 catalyst. The reaction network because that PPE conversion end NAC-800 catalyst is displayed in Fig. 2a. The yield of styrene peaks in ~ 4 h (13%), vice versa, the 1-phenylethanol and acetophenone boosts until 16 (7%) and 40 h (5%) of reaction time, dong (Supplementary Fig. 13). Elevation experiments present that NAC-800 catalyzes conversion of both styrene and 1-phenylethanol come ethylbenzene. The intermediate styrene is current in the reaction mixture until all PPE room consumed. Specifically, styrene and 1-phenylethanol are formed straight from PPE, conversely, ethylbenzene and also acetophenone room daughters. The acetophenone most likely kind through equilibrium through 1-phenylethanol, saying that the catalyst is active for both hydrogenation and dehydrogenation in ketone–alcohol conversion. At much longer reaction times, acetophenone experience hydrogenation come 1-phenylethanol and also then hydrogenolysis come ethylbenzene. In addition, phenethoxybenzene (PEB), resulting from direct hydrogenolysis that Cα–OH in PPE, is a terminal product the continuously rises over the reaction (Supplementary Fig. 13). Markedly, the development of styrene under problems for hydrogenation and hydrogenolysis is exceptionally unusual, and contrasts the assets observed with common heterogeneous steel catalysts, such as Ni, Pd, and Ru24,25. Importantly, this NAC catalysts are the an initial metal-free catalysts qualified of both C–O activation and hydrogenation with H2.
Further kinetic researches with PPE proved that the apparent rate law is zero-order on the PPE and 1.3-order on H2 (Fig. 2c and also Supplementary Fig. 14). Hence, under the reaction kinetic regime, the rate-determining step is mainly associated to the surface dissociation that H2 yet not come PPE C–O cleavage. A 1.1-order dependence on the catalyst was also observed, describe no internal mass transfer limitation26. The turn-over frequency (TOF) because that PPE switch can additionally be estimated, assuming that the thickness of energetic sites equates to to the maximal H2 uptake quantified by pulsed chemisorption study (at 240 °C, Fig. 1g). Thus, the TOF that NAC-800 for PPE hydrogenolysis (230 °C, 20-bar H2) to be calculated to be 32 h−1 utilizing initial prices (Supplementary Table 7). Compare of TOF that catalysts in this job-related is also made with literature values by metal-based catalysts for comparable reactions (Supplementary Table 7).
Strikingly, species resulting from arene hydrogenation of, because that example, PPE, 1-phenylethanol, and phenol, were listed below detection limits. In contrast, sustained Pd or Ni catalysts convey PPE switch (under the same H2 pressure) to generate far-ranging amounts of hydrogenated arenes together side products (Supplementary Fig. 15). This drastically different selectivities strongly show that NAC catalysts are useful over change metals in keeping aromaticity of products27. Besides, NAC-800 was durable at 230 °C and showed neither evident deactivation nor transforms to selectivities ~ seven repeated cycles at ca. 28% PPE conversion (Supplementary Fig. 16). Analysis of recycle NAC-800 suggested no significant changes come catalyst morphology or chemistry composition compared with the fresh catalyst (Supplementary Fig. 17 and also Supplementary Tables 1, 2, 3, 8, and also 9).
The capacity in H2 activation also permits NACs together dehydrogenation catalysts in revolution of hydrocarbons. Because that instance, catalytic non-oxidative dehydrogenation of ethylbenzene over NAC-800 deserve to be completed to produce styrene in gas phase at 550 °C, affording 20% conversion during a 9-h time-on-stream check (Supplementary Fig. 18). A possible mechanism (Supplementary Fig. 19) is hypothesized based on an intermediate determined in the mechanistic examine with styrene (vide infra). Similarly, the NAC catalyst likewise demonstrated task in dehydrogenation the 1,2,3,4-THQ to productivity quinoline at 140 °C, and also H2 to be detected in the headspace (Supplementary Figs. 20 and 21). The dehydrogenation reaction of THQ are much slower under high push of H2 (Supplementary Table 10). Under 10-bar H2, a 6% conversion come quinoline to be observed at 140 °C in 2 h, if the conversion boosted slightly come 10% in ~ 230 °C. The turning back reaction, quinoline hydrogenation, deserve to be additionally catalyzed by NAC-800, showing a 5% conversion in ~ 140 °C v 10-bar H2. However, enhancing the temperature to 230 °C resulted in the conversion come drop to 2. Thus, the hydrogenation reaction of quinoline to THQ over NAC-800 is substantially temperature-dependent and also can it is in shut turn off at greater temperatures.
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The NAC-800 catalyst is efficient in hydrogenolysis reaction of connected compounds through C–O linkages, providing high conversions and selectivities (Fig. 3). The arene-methoxylated PPE derivative, 2, is converted quicker than PPE, delivering guaiacol and also 4-ethyl-1,2-dimethoxybenzene in 100% selectivity. Besides, 2-phenoxy-1-phenylpropane-1,3-diol (PPDE, 3) is also hydrogenolyzed in 48 h with complete cleavage of fragrant Cα–OH and also Cβ–O ether, producing propylbenzene, 1-phenyl-propene, and also 3-phenylpropanol together with phenol. Quantitative and rapid counter of arene-methoxylated PPDE derivative, 4, results in cleavage of all three C–O bonds in 48 h, including the Cγ–OH. Compound 5, containing α-O-4 lignin linkage23, can achieve 92% conversion in 12 h and completion in 16 h, developing toluene and also phenol with a TOF that 61 h−1 (Supplementary Table 7). Among Compounds 1–5 and also 1-phenylethanol, Cα–O is completely converted; only for 1 and also 3, there is a minor content of uncleaved Cβ–O and also Cγ–O, respectively. The NAC-800 catalyst is also versatile toward hydrogenation of many typical unsaturated functionalities15,28,29, consisting of –C=O, –C=C–, –C≡C–, and also –NO2 (Fig. 3), affording the matching reduced commodities with high yields and no arene over-hydrogenation.