Ring-Opening Transformation of 5-Hydroxymethylfurfural Using a

Research Article Cite This: ACS Catal. 2019, 9, 6212−6222

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Ring-Opening Transformation of 5‑Hydroxymethylfurfural Using a Golden Single-Atomic-Site Palladium Catalyst Ming-Ming Zhu,† Xian-Long Du,‡ Yi Zhao,† Bing-Bao Mei,§,⊥ Qi Zhang,† Fan-Fei Sun,⊥ Zheng Jiang,⊥ Yong-Mei Liu,† He-Yong He,† and Yong Cao*,†

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†

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438, China ‡ Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China § University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China S Supporting Information *

ABSTRACT: The development of function-integrated catalysts to enable scalable and efficient biomass valorization is an important topic. Here, a stepwise methodology was developed to fabricate novel supported Au−Pd bimetallic catalysts featuring immobilized well-defined on-surface nanoarchitectures (Pd∧Au). This was achieved by an atomic decoration of a trace amount of Pd (Pd/Au molar ratio of 0.01−0.02) on the surface of 2 nm Au nanoparticle (NP, 0.86 wt %) preanchored to titania. Benefiting from their unique structural merits and the cooperative interplay among the selective hydrogenation activity furnished by the ultrasmall Pd∧Au NPs and prominent Lewis acidity endowed by TiO2, Pd0.02∧Au/TiO2 exhibits excellent activity with 100% conversion and over 90% selectivity and stability (turnover number up to 48 300) toward controlled ring-opening conversion of 5-hydroxymethylfurfural and related furanic compounds to the corresponding di- and triketone-type products under mild conditions (120 °C, 10 bar of H2). The results offer great promise for the future advancement of next-generation multifunctional solid catalysts by tuning the interface structure and precise decoration of active sites with required functionalities. KEYWORDS: 5-hydroxymethylfurfural, ring-opening, 1,4-diketone, gold−palladium, single-atom-site

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INTRODUCTION Ever-pressing global need to reduce dependence on fossil fuels has spurred tremendous interest in platform technologies for biorenewable chemical production.1−9 To this end, much effort has been devoted to finding green and affordable solutions for value-added processing of biobased feedstocks.10,11 5-Hydroxymethylfurfural (HMF) is a highly promising biosourced feedstock; its direct reductive conversion is extensively explored as a potential pathway to open new scenarios for sustainability.12,13 Notably, even-though a plethora of reported catalytic methods are available, the challenge of procuring the desired product in high selectivity still remains.14 This obstacle is largely due to the inherently labile nature of HMF molecule,15 which significantly hampers its potential in biobased industry.16 Therefore, it is imperative to develop new and potentially useful strategies for HMFbased conversions. Recent studies on HMF reduction have focused on the transformation of HMF into a host of furan-ring-retaining products such as 2,5-dimethylfuran, 5-methylfurfural, and 2,5bishydroxymethyl furan (BHMF).17−19 However, current © 2019 American Chemical Society

research has delved into the challenging hydrogenative ringopening (HRO) reactions offering complementary selectivities.20−22 In this regard, 1-hydroxy-2,5-hexanedione (HHD), bearing a 1,4-diketone motif, is an essential target molecule to access diverse value-added compounds of industrial and biological interest.23−26 However, the synthesis of HHD requires three consecutive steps (Scheme 1), i.e., carbonyl group reduction, furan ring hydrolytic ring-opening, and CC double-bond reduction, to be carried out in a highly sequential and balanced manner. This significantly increases the overall difficulty in attaining high selectivity at high conversion levels. To date, several efficient methods have been developed for the reductive transformation of HMF to HHD using homogeneous Ir or Ru catalysts.27−30 Despite these advances, the recovery of the costly and potentially toxic metal complexes remains an issue. Although the development of heterogeneous catalysts is the most desirable option, the Received: January 31, 2019 Revised: June 3, 2019 Published: June 5, 2019 6212

DOI: 10.1021/acscatal.9b00489 ACS Catal. 2019, 9, 6212−6222

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ACS Catalysis Scheme 1. Reaction Network for the Hydrogenative Ring-Opening (HRO) of HMF

Scheme 2. Schematic Illustration of Pd∧Au/TiO2 Synthesis

nanostructures, much effort is still needed to precisely control the atomic structure and local chemistry of SACs. An alternative approach to SA-based Pd−Au catalysts is the use of supported Au NPs as substrates for anchoring single atomic Pd. This provides additional prospects for tuning or enhancing the chemical reactivity and selectivity of catalytic systems as well as introducing unique NP stability. Herein, we describe the synthesis of a supported Pd−Au-based singleatomic catalytic system and demonstrate a novel, facile, and clean process of diketones synthesis catalyzed by single-atomicsite Pd-on-Au NPs supported on commercial titania P25 (denoted as Pd∧Au/TiO2). Because of the beneficial interplay among Pd-on-Au NPs and moderately acidic TiO2−P25, Pd∧Au/TiO2 exhibits superior catalytic performance for the facile conversion of HMF to HHD in neat water. To the best of our knowledge, this study reports the first example of clean and efficient HHD synthesis using heterogeneous multifunctional catalysis.

existing systems are based on nanoparticulate Pt-group metals (PGMs), necessitating the addition of an external mineral acid to facilitate the desired ring-opening reaction.31,32 As a result, the occurrence of unwanted side reactions contributes to the rapid deterioration of the catalyst, inevitably hindering its effectiveness.33 From both technical and sustainable points of view, it is crucial to develop robust and effective catalytic systems that preclude additives and can produce HHD in a pure and facile manner from HMF under mild conditions. Single-atom catalysts (SACs) with atomically dispersed metal atoms as active entities are versatile platforms for efficient catalysis.34−36 Unfortunately, their synthesis and utilization remain a daunting challenge due to the very mobile and agglomerative nature of single atoms.37 The development of more robust SACs relies on bimetallic systems featuring well-defined on-surface nanoarchitectures to enhance the stability of single-site metal centers.38,39 A notable example is Pd-on-Au, which is comprised of unsupported Au nanoparticles (NPs) as the underlying hosts to bear single Pd sites.40,41 The common method used for the preparation of single-atomic Pd-on-Au NPs as well as others is colloidal synthesis followed by a postsynthetic modification.42−44 Even though it is synthetically straightforward to access diverse

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RESULTS AND DISCUSSION Synthesis and Characterization of Pd∧Au/TiO2. A facile two-stage sequential deposition procedure was developed for the preparation of Pd∧Au/TiO2 catalyst (Scheme 2). A 6213


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Figure 1. Characterization of Pd0.02∧Au/TiO2. (A) HRTEM image and particle size distribution. (B) AC-HRTEM image of an individual nanoparticle loaded on TiO2. (C) STEM-HAADF image of an individual metal nanoparticle of Pd0.02∧Au/TiO2. (D) STEM-EDX line scan and compositional line profiles of Au (black) and Pd (red) for the nanoparticles in Pd0.02∧Au/TiO2 recorded along the arrow. (E) Fourier transforms of k2-weighted Pd K-edge EXAFS experimental data for Pd foil and Pd0.02∧Au/TiO2; the quantified fitting results are shown in Table S1. (F) DRIFT spectra in the carbonyl region of Pd0.02∧Au/TiO2. It shows peaks of C−O adsorbed on Au (TOP-edAu) and Pd at the vertex (TOP-vtPd), edge (TOP-edPd), and facet sites (TOP-fcPd) of Au clusters. (G) XPS in the Au 4f region of Pd0.02∧Au/TiO2 with Au/TiO2 as reference.

To investigate the effect of Pd/Au ratio, a series of TiO2supported Pd∧Au NPs with varied metal compositions were synthesized and characterized (Figure S4). The resulting catalysts were denoted as Pdx∧Au/TiO2, where x is the molar ratio of Pd/Au. Figures 1A and 1B show the high-resolution transmission electron microscopy (HRTEM) and aberrationcorrected HRTEM (AC-HRTEM) images of Pd0.02∧Au/TiO2 catalyst. These images illustrate that the metal particles dispersed therein maintained an identical size of ∼2 nm as pristine Au NPs (Figure S1A), consistent with the expectation that the incorporation of a small amount of Pd would not cause noticeable changes in the characteristic features of Au/ TiO2. Figure 1C shows a typical aberration-corrected high-

Au/TiO2 precatalyst comprised of Au NPs (average size of ca. 2 nm, Figure S1A) uniformly dispersed on commercial P25 TiO2 was prepared using the deposition−precipitation (DP) method.45 The resultant 0.86 wt % Au/TiO2 was then subjected to a reaction-mediated deposition of Pd using an in situ [H] species, generated by Au−HCOOH reducing system (see the Supporting Information for further details).46 Additional Pd sites were selectively introduced onto the preformed Au NPs surface to construct well-defined Pd-on-Au nanoarchitectures, as confirmed from UV−vis spectra (Figure S2). Figure S3 shows that Pd species was not uptaken by the TiO2 support during a control experiment, as shown by no change in the intensity of peaks at 209 and 237 nm. 6214


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Figure 2. Characterization and catalytic behavors of various Pdx∧Au/TiO2 (x = 0.01, 0.02, 0.0, and 0.2). (A) DRIFT spectra in the carbonyl region. (B) XPS spectra in the Au 4f region. (C) Influence of Pd content in Pd∧Au/TiO2 catalyst on the turnover frequency (TOF) values and selectivity toward HHD during the HRO of HMF. Reaction conditions: HMF (1 mmol), H2O (4 mL), metal loading (0.1 mol %), H2 (10 bar), 120 °C for appropriate time. TOF was measured below 50% conversion. (D) HD exchange of surface OH groups of the catalysts at 25 °C in a flow of D2. Integrated intensity of OD band with respect to exposure time.

angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of Pd0.02∧Au/TiO2.47 Along with STEM measurements, line-scan analysis (Figure 1D) was performed, confirming that the Pd species are exclusively located on Au NPs in Pd0.02∧Au/TiO2.48 The existing form of Pd species and precise identification of essential nature of Pd-on-Au nanostructures were determined by X-ray absorption measurements. Figure 1E shows the nonphase-corrected Fourier transform of k2-weighted Pd Kedge extended X-ray absorption fine structure (EXAFS) data for Pd. Furthermore, the spectrum of simultaneously acquired bulk Pd foil reference is also included for comparison (black dotted lines). The Pd foil shows two distinct peaks at 2.06 and 2.49 Å with an intensity ratio of 1:2, typical of metallic Pd. In contrast, Pd0.02∧Au/TiO2 shows two split peaks centered at ca. 2.09 and 2.85 Å solely originating from Pd−Au contribution (Table S1), indicating the atomic dispersion of Pd species in this material.43 Moreover, the Pd atoms in Pd0.02∧Au/TiO2 were coordinated by seven Au atoms, which was in line with a structural model that Pd atom was incorporated on the surface of Au NPs. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic analysis further confirmed that Pd atoms solely existed as isolated single Pd sites surrounded by Au surface atoms in Pd0.02∧Au/TiO2 material, as no bands related to

bridging-type CO were observed. As shown in Figure 1F, the νC−O frequency ranges of C−O adsorbed on isolated Pd varied because of Pd atoms located on different coordination sites (vertex, edge, and facet) in Au matrix.49,50 In particular, the linear-CO feature of Pd0.02∧Au/TiO2 shows a substantial red shift in frequency compared to monometallic Pd/TiO2 as a control sample (Figure 2A). Even though this result may contradict the electron transfer tendency elucidated by X-ray photoelectron spectroscopy (XPS) (Figure 1G), it is rationalized by the nature of atomically dispersed Pd promoting a lower dipole−dipole coupling of adsorbed CO.51 To gain more insight into the interactions between Pd and Au, we systematically investigated Pd∧Au/TiO2 catalysts with different Pd/Au ratios. Figure 2B shows the XPS of Pdx∧Au/ TiO2, the progressive negative shifts in Au 4f binding energy (BE) with increasing amounts of Pd, up to 0.02 Pd/Au ratio, is consistent with the expected strong interaction between Au and atomically dispersed Pd species. Intriguingly, as Pd/Au ratio increased from 0.02 to 0.05, Au 4f BE reverted back to that of bulk Au as deposition progressed, reflecting the growth of Pd ensembles. This trend is consistent with the IR data (Figure 2A): The bands corresponding to bridge-bonded CO adsorbed on the Pd∧Au surface indicate the formation of small Pd ensembles, which were identified in catalysts with Pd/Au ratio >0.02. EXAFS analysis also supports this result, revealing 6215


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ACS Catalysis the existence of distinct Pd−Pd coordination in Pd0.05∧Au/ TiO2 (Figure S7, Table S1). Catalytic Behavior of Pd∧Au/TiO2. After the structure of catalysts was established, their catalytic ability toward the hydrogenative transformation of HMF in neat water was evaluated. Scheme 1 shows the possible side reactions during HHD formation, including (I) acid-catalyzed condensation of metastable intermediates to form oligomers, (II) further hydrogenation of BHMF into THBHMF, and (III) aldol addition of HHED or aldol condensation of HHD to yield HCPN. Importantly, no reusable solid catalyst has been reported for the effective elimination of these side reactions under additive-free conditions. However, it was discovered that 64% yield of HHD can be achieved using only 0.1 mol % Pd0.02∧Au/TiO2 catalyst, requiring specific H2 pressure and reaction temperature (Table S2, entries 1−5). Thus, with 10 bar H2 at 120 °C, Pd0.02∧Au/TiO2 can provide the desired product HHD in 87% yield in 3.5 h (Table S2, entry 6). Furthermore, an excellent carbon balance (CB, defined here as the percentage of the molar amount of carbon in all detected compounds to the molar amount of carbon in the HMF feed, section S4.4) of 90% was readily obtained under these conditions. To the best of our knowledge, this is the most effective outcome for raw-material-efficient HHD production from HMF, especially using a ligand-free solid catalyst system (Table S3). In the presence of pristine TiO2, excluding Pd∧Au, the reaction did not proceed, confirming that Pd∧Au NPs are essential (Table S2, entry 7). Preliminary time-dependent studies of HMF conversion under optimal conditions showed that Pd0.02∧Au/TiO2 exhibited a high selectivity toward HHD initially. As the reaction progressed, the conversion of HMF steadily increased without any decrease in HHD selectivity (Figure 3). The overall byproduct formation, including

appreciable side reactions occurred concurrently. Pd/TiO2 is a material in which 1 wt % Pd from PdCl2 has been deposited onto TiO2 using a traditional wet impregnation followed by reduction with H2/Ar at 300 °C. A typical TEM image (Figure S8A) shows the Pd particle size of about 2.2 nm average diameter, with only a small degree of particle aggregation. Because the specific nature of Pd species has been proven to be especially influential for supported Pd catalysts,52,53 an attempt was made to examine the efficacy of a set of TiO2-supported Pd catalysts with different Pd loadings and precursors. The procedure for preparation of these Pd/TiO2 references and relevant TEM images (Figure S8) are provided in the Supporting Information. These references, however, were not found to be more effective, even though in all situations, high to complete conversion of HMF was achieved (Table S4, entries 2−5). A deliberately prepared reference Pd/TiO2− OAc−HR catalyst exhibiting stronger metal−support interaction (SMSI) between Pd and the TiO2 support (Figure S9) also did not show the required control over selectivity (Table S4, entry 6), despite the reported promotional SMSI effect on Pd/TiO2 systems for diverse industrially important reactions.54−56 Consistent with the conclusion of many earlier studies,32,33,57,58 these results confirm that the selective aqueous-phase hydrogenation of HMF is very difficult over conventional monometallic Pd catalysts. Support Effect for Pd∧Au/TiO2. To elaborate the catalytic performance of Pd0.02∧Au/TiO2 in the HRO reaction, a series of control catalysts under identical conditions (4 mL of H2O, 10 bar H2, 120 °C, 1 h) were systematically tested. To probe the role of underlying support, a set of mineral oxides of various surface acidities, including SiO2, Al2O3, ZrO2, CeO2, and Nb2O5, were subjected to the above-mentioned two-stage procedure to deposit Pd0.02∧Au NPs. The TEM analysis confirmed the successful anchoring of Pd0.02∧Au NPs with an average particle size of ∼2 nm (Figure S1). Surface acidity assessment shows no observable changes in the acidic properties of the corresponding Pd0.02∧Au-based catalysts compared with their pristine counterparts (Table S5). Respecting the catalytic activity of different supported Pd0.02∧Au NPs (Figure 4A), the data in Figure 5 clearly show a significant impact of the nature and density of surface acid sites on the activity of these Pd0.02∧Au-containing materials. It is noticeable that the presence of Brønsted acid sites, even in very small amounts as found for ZrO2 or Al2O3, is unfavorable for achieving a high carbon yield of target product (Figure 5A). Although such a severe detrimental effect seems somewhat unexpected, it must be noted here that HMF is a notoriously labile compound highly sensitive to the pH of solvent environment, with degradation being more readily in water assisted by Brønsted acids. Of particular relevance in this connection is that, as opposed to the cases observed for Pd0.02∧Au/ZrO2 and Pd0.02∧Au/ Al2O3 catalysts, the Pd0.02∧Au/Nb2O5 sample bearing much more pronounced surface Brønsted acidity can deliver relatively high yield of HHD and high conversion of HMF, albeit with higher overall carbon loss in terms of HMF consumption. We note that this finding is consistent with a recent study by Ohyama et al.59 On investigating the ring rearrangement of HMF to HCPN over Au NPs supported on a series of metal oxide supports, they demonstrated that Au/ Nb2O5 in the presence of H3PO4 can attain similar level of performance in yielding HHD from HMF. In view of these observations, it could be inferred that other factors, apart from

Figure 3. Reaction profile for HMF conversion. Reaction conditions: HMF (1 mmol), H2O (4 mL), Pd0.02∧Au/TiO2 as catalyst (metal loading: 0.1 mol %), H2 (10 bar), 120 °C.

THBHMF and HCPN, remained consistently below 6%. It is interesting in this connection to observe that the CB decreases continuously over time, which could be most likely due to the increased formation of unknown products (as discussed below) as a consequence of unproductive degradation of HMF or related intermediates. It is worth mentioning here that, despite the remarkable activity of the Pd/TiO2 monometallic catalyst (Table S4, entry 1), only a moderate HHD yield (36%) was obtained, and 6216


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Figure 4. Catalytic performance of (A) Pd0.02∧Au/MOx, (B) Pd0.02∧Au/SiO2 + MOx (weight ratio 1:1), (C) Pd0.02∧Au/TiO2−SiO2-x (x = 5, 10, 20, 50, 100), (D) different bimetallic catalysts for HRO of HMF. Reaction conditions: HMF (1 mmol), H2O (4 mL), metal loading (0.1 mol %), H2 (10 bar), 120 °C, 1 h. We carried out three parallel reactions and show here the average activity (error bars represent the standard deviation).

Figure 5. Effect of the support acidity on the carbon balance and yield of HHD with various Pd0.02∧Au/MOx catalysts under the same reaction condition, where the error bars represent standard deviations from three replicate experiments. (Analysis based on the activity data in Figure 4A and characterization data in Table S5.)

surface acidity alone, related to the catalytic behavior of the supported Au catalysts, are important for the final outcome of the reaction. Another interesting observation is the need for sufficiently high levels of surface Lewis acidity to afford the desired HHD with minimal carbon loss (Figure 5B). In fact, the main characteristic of the most effective P25 TiO2 sample is that it

contains only weak Lewis acid sites with a prominent surface density (Table S5). This together with the fact that the use of other TiO2-based materials (TiO2−H and TiO2−P90) with comparable population of Lewis sites (Figure S10) as control supports does not significantly affect the performance of similarly prepared catalysts (Figure S12)60 indicates the necessity of a sufficient amount of surface Lewis acidity to 6217


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Pd0.02Au/TiO2, Figure S16B). Pd0.02Au/TiO2 showed strikingly low levels of performance (Figure 4D, HHD yield 37%; the major byproduct is THBHMF) compared with Pd0.02∧Au/ TiO2. The location of Pd in these two control samples and Pd0.02∧Au/TiO2 was confirmed by the TEM elemental mapping studies, where it was inferred that Pd0.02−Au/TiO2 contains a large fraction of Pd deposited on the surface of TiO2 support, (Figure S17) in contrast to the cases with Pd0.02Au/ TiO2 and Pd0.02∧Au/TiO2 (Figures S18 and S19). Given the fact that all these three catalysts share very similar structural and compositional features, these results highlight the vital significance of precisely controlling the Pd location for desired catalysis. The effect of Pd/Au ratios within Pd∧Au/TiO2 multifunctional platforms on HMF conversion was further evaluated. Figure 2C shows that pure Au, bereft of Pd, is only moderately effective in the reaction. Interestingly, Pd∧Au catalysts were always much more active, producing HHD as the major product, but the selectivity of HHD rapidly decreased when the Pd/Au ratio was >0.02. Theoretically, this effect reflects the fact that the surface Pd decoration leads to a decrease in H2 dissociation barrier and hence improved kinetics. Upon monitoring of H/D exchange between D2 and OH groups using DRIFTS (Figure 2D), it was found that Pd incorporation significantly enhanced the H2 dissociation efficiency. Therefore, as a result of an optimal interplay between the Lewis acid and metal sites on the conversion of HMF to HHD, a Pd/Au ratio of ∼0.02, i.e., Pd0.02∧Au/TiO2, is by far the most efficient catalyst for this reaction. The preceding results highlight the distinct and critical potency of ultradispersed Pd∧Au coupled with TiO2 support in enabling selective HHD formation from HMF. In this regard, two recent elegant studies by Cargnello et al.44 and Zhang et al.63 represent important contributions to the rational construction of well-defined Pd-on-Au or Pt-on-Au nanostructures based on nearly monodispersive small (2−10 nm) Au NPs. However, in both cases, the preparation of these nanostructures is complicated by the need for oleylamine as a stabilizer and capping agent. Moreover, it should be emphasized that these studies do not address the use of such nanostructures to construct function-integrated active sites by taking advantage of synergy between the metal NPs and underlying support. Within the scope of the interplay between metal−support interaction for supported single-metal alloy clusters, our results highlight new possibilities obtained with supported single-atomic-site catalysts. Given the importance of function integration as a tool in heterogeneous catalysis design and in particular its significance for the development of technologies for streamlined and affordable chemical synthesis,64−69 we anticipate that this SA-based approach may open up new avenues for establishing future-oriented sustainable chemical processes featuring biorenewable resources as main feedstocks. Mechanism and Scalability Study. To gain greater insight into the mechanism for direct HMF-to-HHD conversion, several model compounds were subjected to Pd0.02∧Au/TiO2-mediated reactions under optimal reaction conditions (Table S7). Because of the unavailability of HHED, 4-(5-methyl-2-furyl)-3-butene-2-one (MFBTO) was used as the corresponding model compound. The production rate for the hydrogenation of MFBTO is ∼10 times higher than that of HMF, explaining why this intermediate was not detected by GC. BHMF and 5-methylfurfuryl alcohol (MFOL) show

facilitate the desired result. To better understand this phenomenon and to assess the enabling role of TiO2-based materials, a separate set of hybrid samples composed of a physical mixture of chemically inert SiO2-supported Pd0.02∧Au (Pd0.02∧Au/SiO2) and various simple oxides (Pd0.02∧Au/SiO2 + MOx, weight ratio of 1:1) were evaluated (Figure 4B), where the combination of TiO2 and Pd0.02∧Au/SiO2 is critical for the production of desired HRO with minimal carbon loss on HMF-obtaining products (for Pd 0.02∧Au/SiO2 + TiO2, HHD yield 17%, CB 92%). It should be noted here that, in terms of the overall HHD yield, a physical mixture of P25 TiO2 and Pd0.02∧Au/SiO2 is significantly much less efficient than Pd0.02∧Au/TiO2 (Figure 4A, for Pd 0.02∧Au/TiO2, HHD yield 64%, CB 96%), emphasizing the importance of metal− Lewis acid cooperation and precise control of the proximity between different active sites for achieving high yields of HHD. To gain a deeper insight into how Lewis acidity of the support affects the performance of Pd0.02∧Au/TiO2, an additional set of control samples bearing different levels of surface Lewis acidity were prepared; a systematic approach was used to gradually decrease the population of Lewis acid sites on the surface-silated TiO2 materials. This was achieved by subjecting the as-received commercial P25 TiO2 powders to a controlled surface passivation by treating with tetraethyl orthosilicate (TEOS) (for details, see the Supporting Information). In such a manner, a submonolayer coating of SiO2 could be introduced onto the P25 TiO2 surface to quantitatively modulate the surface population of Lewis acid sites, as corroborated by pyridine-IR and NH3-TPD analyses (Figures S13 and S14; Table S6). Thus, we were able to confirm that in spite of their overall weak nature, the total number of Lewis acid sites decreased significantly with increasing the amount of added TEOS during the passivation processing of P25 TiO2. Thus, Figure 4C shows that the inferior performance of these surface-silated Pd0.02∧Au-based catalysts can be attributed to their decreased surface Lewis acidity compared with Pd0.02∧Au/TiO2. It should also be mentioned that these Pd0.02∧Au/TiO2−SiO2-x catalysts possess similar NP size (Figure S15), thus pointing to the fact that a minimum surface acidity density with at least ca. 66 μmol/g is essential to achieve a reasonable yield of HHD (Pd0.02∧Au/TiO2−SiO2-50, HHD yield 17%). All these factors strongly indicate that the suitable Lewis acidity of underlying support is one of the key factors in contributing to the observed high efficiency. Bimetallic Synergistic Effects for Pd∧Au/TiO2. Bimetallic synergistic effects are often key factors in tuning Pd- or Au-based catalysis.61,62 To explore the beneficial effect rendered by Pd∧Au nanoarchitectures, identically formulated Au−Pd species were deposited on TiO2 via two preparation routesPd-impregnation of Au/TiO2 and loading preformed Au−Pd bimetallic alloy NPs on TiO2, serving as a comparison to investigate the effect of Pd location on catalytic performance. Using a simple impregnation−reduction method with NaBH4 as the reductant, Pd was homogeneously deposited on the Au/TiO2 surface (denoted as Pd0.02−Au/TiO2), and the TEM image of Pd0.02−Au/TiO2 shows that the NP average size is 2.08 nm (Figure S16A). This is comparable to Au/TiO2. Pd0.02−Au/TiO2 showed an inferior performance, as indicated by a significant decrease in HHD yield, compared with Pd0.02∧Au/TiO2 (Figure 4D). PVA-capped Pd0.02Au NPs (ca. 2.0 nm), prepared ex situ using a NaBH4 reduction method, were also deposited onto the surface of TiO2 (termed 6218


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ACS Catalysis Scheme 3. HRO of HMF Derivatives

over the course of HMF reaction (Figure 3) indicates that the accumulation of this transient intermediate could contribute to byproduct formation such as humins and coke as indicated by the developed brown color of reaction mixture. Consistent with this observation, a separate control experiment using BHMF as the starting material showed a strong concentrationdependent behavior for BHMF degradation in both the presence and absence of TiO2 (Figure S23). This highlights the necessity of maintaining a sufficiently low level of BHMF in the reaction medium to minimize the formation of byproducts arising from BHMF degradation. Hence, by designing a catalyst that hinders BHMF accumulation, with the use of superior HMF, the efficiency of HHD production can be optimized. In this context, an increase in the relative number of Lewis acid sites on TiO2 surface should generate such an enhancement. Gratifyingly, it was found that the initial transient BHMF buildup was significantly inhibited by decreasing the Au loading in Au/ TiO2 precatalyst to 0.30 wt % (Figure S24A; Table S2, entry 8, HHD final yield 91%). Coupled with similar HHD evolution kinetics in association with a physical mixture of 0.86 wt % Pd0.02∧Au/TiO2 and bare TiO2 (Figure S24B), it was shown that the optimal combination of Pd0.02∧Au-support cooperation is essential in furnishing the desired product in an efficient and target-specific manner. Accordingly, even at ultralow catalyst loading (0.01 mol %), the 0.30 wt % Pd0.02∧Au/TiO2 catalyst was found to be efficient and afforded HHD in an excellent yield (ca. 87%) (Table S2, entry 9). By using this catalyst, the reaction also proceeded efficiently on a 10 mmol scale, producing HHD in 85% yield (Figure S25). To validate the general applicability of this new Pd0.02∧Au-catalyzed HRO methodology, other HMF derivatives were evaluated, producing the corresponding ketones (Scheme 3). A selection of functionalized furans was effectively converted smoothly and selectively in good-toexcellent yields (>85%). In the case of HMF/ketone aldol adducts, 2,5,8-triketones were produced, which have recently been advocated as feedstocks for the production of semiconducting or conducting polymers.73 One of the major drawbacks with reported catalytic systems is their lack of reusability; fortunately, with 0.30 wt % Pd0.02∧Au/TiO2 catalyst it could be reused up to five times while retaining the optimal catalytic performance (Figure S25), producing a total turnover number (TON) of 39 300. Our TON value is 17 times higher than that of previously reported heterogeneous catalyst (Table S9). If only surface metal atoms are considered, more flattering TON value (48 300) would be

similar conversion rates, further indicating that the conversion of HMF to HHD proceeds via the hydrolytic ring-opening reaction of furanic group of BHMF. Interestingly, the transformation rate of MFOL to 2,5-hexanedione (HDN) was slower than those of other two steps, highlighting the ratecontrolling nature of hydrolytic ring-opening step (Scheme S1). In contrast to previous studies utilizing Brønsted acid catalysis for hydrolytic ring-opening reactions,70,71 this study demonstrates the importance of Lewis acid sites on TiO2 for this particular purpose. Furthermore, the Lewis acid properties of TiO2 did not significantly change upon the immobilization of Pd0.02∧Au NPs (Figure S20, Table S5), which is an important increment in the high catalytic efficiency toward HRO conversions. In the case of hydrolytic ring-opening of MFOL to HDN, TiO2 was less efficient compared to its Brønsted acid counterpart (Figure S21). Nevertheless, the fact that Brønsted acid promotes severe HMF degradation72 prohibits the possibility of any performance optimization by the cooperative combination of Brønsted acid and metal sites. The central role of TiO2 in promoting this HRO process is further corroborated by inspecting a separate reduction of HMF with H2 in ethanol as well as a separate hydrolytic ring opening of MFOL to HDN and separate reduction of MFBTO with H2 in water for Pd0.02∧Au NPs deposited on different Lewis acidic supports, wherein it is clarified that all associated reactions, especially the hydrolytic ring opening of MFOL (Table S8, Figure S22) over Pd0.02∧Au/TiO2, proceeded at a much faster rate than that over all other tested catalysts. Note that, with increasing amount of SiO2 passivation, the rate of hydrogenation also decreases with decreasing Lewis acidity, pointing to the negative influence of the creation of new Au/ TiO2−SiO2 interface on the activation of H2. Taken together, all these results point to the critical and previously unappreciated role of Lewis acidic oxide TiO2 in facilitating the essential hydrolytic ring-opening step and consistent with the fact that the present Pd0.02∧Au/TiO2-catalyzed HRO of HMF may proceed via a sequential hydrogenation/hydrolytic ring-opeing/hydrogenation pathway (Figure S22) in which the generation of transient HHED formed by Lewis acid-assisted ring opening is the rate-determining step. Throughout our investigation, the most peculiar and interesting finding was the dramatic decrease in the overall carbon yield during BHMF transformation, even within 0.5 h under conditions otherwise identical to that for selective HMF conversion (Table S7, entries 1 and 2). This observation together with the consistently low BHMF concentration level 6219


ACS Catalysis

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attained (see section S5.2). The TEM (Figure S26), XPS (Figure S27), and inductively coupled plasma mass spectrometry (ICP) analyses confirmed almost no change in the content, dispersion, and metallic state of Au species before and after reuse. These results are consistent with the excellent retention of catalytic activity. Finally, considering that both HMF and BHMF can undergo undesirable degradation in the reaction system, a semicontinuous procedure was developed, allowing the processintensified conversion of highly concentrated HMF in neat water. In the procedure, it was possible to obtain a solution containing ∼5.46 g of HHD in 40 mL of H2O (∼1.05 M) by subjecting 65 mg 0.30 wt % Pd0.02∧Au/TiO2 catalyst to a fivestage-repetitive injection of 10 mmol pure HMF during the reaction (Figure 6). Compared with conventional one-stage

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00489.

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Detailed description and data of materials synthesis and characterization and catalytic evaluation, kinetic experiments, and literature survey for HRO of HMF (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zheng Jiang: 0000-0002-0132-0319 He-Yong He: 0000-0002-1781-6255 Yong Cao: 0000-0002-8333-0181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSF of China (21773033, 91645201, 21473035, 91545108), Science & Technology Commission of Shanghai Municipality (16ZR1440-400), and SINOPEC (X514005) is acknowledged.

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REFERENCES

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Figure 6. Schematic illustration of the steps and results of the stepwise-batch approach of HHD enrichment in water using 0.30% Pd0.02∧Au/TiO2 as catalyst. Note: detailed results are shown in Figure S28.

reaction mode (Table S10), the process-intensified mode demonstrated high HHD yield (84%) and excellent overall carbon balance during the reaction. One of the major limitations in large-scale production is the lack of technologically feasible methods;74 however, our semicontinuous reaction constitutes an important step toward the development of more viable upgrading technologies for future biorefineries.

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CONCLUSIONS In conclusion, we developed a clean, efficient, and facile synthetic method for producing highly value-added di- and triketones from biomass-derived furans in aqueous media. The utilization of a unique single atomic Pd∧Au−TiO2 cooperative catalytic system was crucial in catalyst design, producing di/ triketones via a well-defined three-step tandem HRO process. The multifunctional catalyst system showed the facile preparation of desired products, efficient suppression of unwanted side reactions, as well as high durability and scalability, which are key factors in the development of advanced biorefinery processes that are essential for achieving an entirely renewable resource supply. 6220


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Ring-Opening Transformation of 5-Hydroxymethylfurfural Using a

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