catalysts Article Effects of Surface Oxygen-Containing Groups of the Flowerlike Carbon Nanosheets on Palladium Dispersion, Catalytic Activity and Stability in Hydrogenolytic Debenzylation of Tetraacetyldibenzylhexaazaisowurtzitane Yun Chen 1, Xinlei Ding 2, Wenge Qiu 2,* , Jianwei Song 3, Junping Nan 2, Guangmei Bai 2 and Siping Pang 1 1 School of Materials Science & Technology, Beijing Institute of Technology, Beijing 100081, China; [email protected] (Y.C.); [email protected] (S.P.) 2 Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China; [email protected] (X.D.); [email protected] (J.N.); [email protected] (G.B.) 3 Qing Yang Chemical Industry Corporation, Liaoyang 111001, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-13521382103 Abstract: The influence of the surface chemical properties of the carbon support on the Pd dispersion, activity and stability of Pd(OH)2/C catalyst for the hydrogenolytic debenzylation of tetraacetyldiben- zylhexaazaisowurtzitane (TADB) was studied in detail. The flowerlike nanosheet carbon material (NSC) was chosen as the pristine support, meanwhile chemical oxidation with nitric acid and physical Citation: Chen, Y.; Ding, X.; Qiu, W.; calcination at 600 ◦C treatments were used to modify its surface properties, which were denoted as Song, J.; Nan, J.; Bai, G.; Pang, S. NSCox-2 (treated with 20 wt% HNO3) and NSC-600, respectively. The three carbon supports and the Effects of Surface Oxygen-Containing corresponding catalysts of Pd/NSC, Pd/NSC-600, and Pd/NSCox-2 were characterized by scanning Groups of the Flowerlike Carbon electron microscope (SEM), transmission electron microscopy (TEM), nitrogen sorption isotherm Nanosheets on Palladium Dispersion, measurement (BET), powder X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectra Catalytic Activity and Stability in Hydrogenolytic Debenzylation of (XPS), temperature-programmed desorption (TPD), temperature-programmed reduction (H2-TPR), Tetraacetyldibenzylhexaazaisowurt- thermogravimetric analysis (TG), and element analysis. The debenzylation activities of Pd/NSC, zitane. Catalysts 2021, 11, 441. Pd/NSC-600, and Pd/NSCox-2, as well as the three catalysts after pre-reduction treatment were also https://doi.org/10.3390/ evaluated. It was found that the activity and stability of the Pd(OH)2/C catalysts in the debenzylation catal11040441 reaction highly depended on the content of surface oxygen-containing groups of the carbon support. Academic Editor: Alfonso Grassi Keywords: palladium catalyst; hydrogenolytic debenzylation; surface oxygen-containing groups; palladium dispersion Received: 5 March 2021 Accepted: 25 March 2021 Published: 30 March 2021 1. Introduction Publisher’s Note: MDPI stays neutral Carbon materials have been widely used as support to fabricate the heterogeneous with regard to jurisdictional claims in catalysts due to its high stability, large tunable specific surface area, chemical inertness, published maps and institutional affil- iations. easy tailorable surface chemical properties [1–3], especially for the precious metal cata- lysts. Palladium on carbon is an important class of catalyst, which can be used in several industrial chemical processes including hydrogenation, dehydrogenation, hydrogenolysis, hydro-dechlorination, nitroarenes reduction, and C–C coupling [4–6]. It is well known that the activity and selectivity of the Pd-based catalysts supported on carbons, either Copyright: © 2021 by the authors. Pd/C or Pd(OH)2/C, strongly depend on the preparation methods, the physicochemical Licensee MDPI, Basel, Switzerland. properties of Pd particles, such as size, morphology, and dispersion of Pd species, and the This article is an open access article nature and structure of carbon supports, involving porous structures and surface proper- distributed under the terms and conditions of the Creative Commons ties [2,7–9]. There are two main general routes for the preparation of Pd-based catalysts Attribution (CC BY) license (https:// on carbons: the ion exchange and the deposition-precipitation methods. In the case of creativecommons.org/licenses/by/ ion exchange, both anionic and cationic palladium precursors can be used [10–12], result- 4.0/). ing well dispersed Pd-based catalysts. In this process, the metal loading and dispersion Catalysts 2021, 11, 441. https://doi.org/10.3390/catal11040441 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 441 2 of 17 highly depend on the surface properties of the carbon support and the metal precursor [2]. The deposition-precipitation method is a more extensively employed route to elaborate heterogeneous precious metal catalysts, in which a compulsory experimental procedure and a well decorated carbon support are desirable in order to get well-dispersed metal particles [13]. Several strategies have been reported to fabricate highly dispersed Pd/C catalysts through an effective control of the adsorption, deposition, and reduction process of palladium species, such as using homogeneous precipitation [14], simple liquid-phase precipitation-reduction [15], functional ion pre-adsorption on support [16], photochemical route [17], etc. Besides adjusting the preparation method, adding surfactant agents [18] or polymer molecules [19,20] during synthesis also had been employed to control the Pd particles size and dispersion. The textural and surface chemical properties of the carbon support also showed much more effects on the catalytic performance of precious metals supported on carbon. The textural properties of carbon support, such as specific surface area and porosity, influenced not only the Pd loading and dispersion [7,21], but also the diffusion of substrate and product molecules in the catalyst framework. Therefore, mesoporous carbon materials are supposed to be proper supports of palladium [22], particular for the large size substrate molecules [23,24]. The contribution of surface functional groups of carbon supports to the catalytic performance may reflect in many ways, such as affecting the size of Pd particles, the electronic state of palladium species, the stability of catalyst, and the adsorption of reactant molecules, so enormous efforts have been devoted toward modifying the surface chemical properties of carbon supports. It was found that the introduction of nitrogen atom [25–27] or other component, such as phosphomolybdic acid [28], could enhance chemical, electrical, and functional properties of the carbon surface and increase the interaction between the metal and carbon surface. The oxidation treatment was a convenient approach to introduce oxygen functional groups into the carbon support surface, which was also beneficial to improve the dispersion of active metal species and to enhance the stability of catalysts. Commonly used oxidant included nitric acid [29], mixtures of H2SO4/HNO3 [30], hydrogen peroxide [31], O3 [32], potassium permanganate [33], etc. Gu found that the pretreatment of coal-based commercial granular activated carbon using nitric acid at different temperature changed its surface groups and surface total acidity, resulting in catalysts with higher dispersion and activity on rosin disproportionation [34]. Tang reported that modification porous carbon spheres by UV-O3 increased their surface oxygen content and defects, leading to the improvement of the Pd loading and the activity of Pd-Ce/PCSs [32]. Benzyl group is one of the most commonly used protecting groups for O- and N-based functionalities and have been broadly employed in fine chemical synthesis and organic transformations. Palladium on carbon has been almost the first choice for hydrodeben- zylation reaction [35]. Although much attention has been paid to gain more insight into the influences of the support properties on the catalytic performance of palladium-carbon catalytic systems for cinnamaldehyde hydrogenation [7], furfural hydrogenation [36], cyclododecatriene hydrogenation [21], rosin disproportionation [34], liquid-phase hydro- dechlorination of chlorobenzene [5], etc. Few reports exist to clarify the correlation of the surface functional groups of support with the performance of Pd/C catalyst for the hydrogenolytic debenzylation reaction [37]. Therefore, an investigation of the catalytic performance of Pd-based hydrogenolytic debenzylation catalyst as a function of surface chemical properties of carbon support becomes desirable. As an outstanding representative of high energy density materials (HEDM), hex- anitrohexaazaisowurtzitane (CL-20) has attracted much attention in the past decades. Hexabenzylhexaazaisowurtzitane (HBIW) has been the main precursor for the fabrication of CL-20 [38], but it cannot be nitrated directly to afford CL-20 due to its low stability of the cage framework. So the debenzylation and transformation of HBIW became the key step for the synthesis of CL-20. Usually, HBIW is firstly converted to TADB in the presence of acylation agent, and subsequently TADB can be further transformed to tetraacetylhex- Catalysts 2021, 11, x FOR PEER REVIEW 3 of 18 dium-carbon catalytic systems for cinnamaldehyde hydrogenation [7], furfural hydro- genation [36], cyclododecatriene hydrogenation [21], rosin disproportionation [34], liq- uid-phase hydro-dechlorination of chlorobenzene [5], etc. Few reports exist to clarify the correlation of the surface functional groups of support with the performance of Pd/C catalyst for