NiO Composite: A Catalyst for One-Pot Cascade Conversion of

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Au/NiO Composite: A Catalyst for One-Pot Cascade Conversion of Furfural Qihua Fang, ZhaoXian Qin, Yuanyuan Shi, Fei Liu, Sami Barkaoui, Hadi Abroshan, and Gao Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00001 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Energy Materials

Au/NiO Composite: A Catalyst for One-Pot Cascade Conversion of Furfural Qihua Fang,†,& ZhaoXian Qin,†,& Yuanyuan Shi,† Fei Liu,*,† Sami Barkaoui,† Hadi Abroshan,*, ‡ Gao Li*,† †State

Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China. ‡School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA. ABSTRACT: Furfural is a promising renewable platform chemical that is widely produced from lignocellulosic biomass, and has received significant attention as a sustainable precursor for the production of chemicals and fuels. To date, one-pot conversion of furfural with cellulosic ethanol is mostly limited to the synthesis of C5-C7 hydrocarbons, poising a challenge for the production of high-energy density fuel from biomass-derived compounds. In this study, we present gold nanoparticles supported over nickel oxide (Au/NiO) as robust catalysts for selective conversion of furfural to hydrocarbons. The catalysts presents ~ 92% furfural conversion with ~ 81% selectivity towards the production of C7 and C9 hydrocarbons through a one-pot cascade reaction, viz., cross aldol condensations in the presence of ethanol, O2, and K2CO3, followed by a hydrogenolysis process using H2(g). Results indicate the unprecedented production of C9 from furfural and ethanol is yielded via in situ cross-aldol condensation of 3-(2-furyl)acrolein, an , β–unsaturated aldehyde that is evolved in the reaction medium. Analysis shows the promising catalytic performance of the Au/NiO composite for the furfural conversion can attribute with synergic effects at the interface of the Au nanoparticles and the NiO, offering potential active sites for the reaction. This study may provide new guidelines for design of efficient catalysts to transform bio-based platform compounds into biofuels with high-energy density. KEYWORDS: Gold nanoparticles, Nickel oxide, Biofuel, Bio-based furfural, Aldol condensation, Hydrogenolysis.

1. INTRODUCTION Liquid fuels are the dominant source of energy for the transportation sector. According to recent data analyses, petroleum products account for over 90% of the U.S. transportation sector, generating the largest share of greenhouse gas emissions of the country.1,2 Owing to the nonrenewable nature of fossil-based resources as well as environmental challenges associated with CO2 emissions, exploring alternative resources of energy is a subject of continuing importance and interest for many research groups worldwide. In this regard, biomass has gained overwhelming interest over the last decade as an abundant source of green energy to meet ever-increasing demand for the production of fuels and chemicals with environmental sustainability.2-8 A combine approach of mechanical, biological, thermal, and chemical processes can transform polysaccharides in lignocellulosic biomass into a series of simple sugars. The asproduced hydrocarbons can further undergo follow up operations to convert to several platform compounds, including furfural, 5-hydroxymethylfurfural (HMF), lactic acid, and levulinic acid.9-12 In particular, furfural is among the 30 topmost biomass-derived platform chemicals selected by the U.S. Department of Energy (DOE), and is extensively studied as a key building block for the production of not only liquid hydrocarbon suitable for diesel and jet fuel applications, but also fuel additives, polymers and valuable chemicals.13-17 Through a two-step cascade catalytic reaction, aldol condensations of biomass-based furfural with alcohols and ketones can induce the formation of new C—C bonds, resulting

organic molecules with higher carbon content. For example, previous studies showed that the C–C coupling reaction between furfural and acetone catalyzed by NaOH (a base catalyst) leads to the formation of oxygenated C8 species, e.g., 4-(2-furyl)-3-buten-2-one.18-20 The aldol condensation reactions can also be applied with cellulosic ethanol to transform furfural into oxygenated C7 chemicals, e.g., 3-(2-furyl)acrolein.21-23 The later reaction of furfural with ethanol includes two main steps. First, ethanol becomes selectively oxidized to form acetaldehyde that is often catalyzed over metal particles. Second, the as-formed acetaldehyde reacts with furfural catalyzed by a base to result 3-(2-furyl)acrolein. Through consecutive hydrogenation-dehydration reactions, the C7 and C8 aldols thus prepared can further transform into alkanes, i.e., C8H18 and C7H16. Despite extensive efforts, however, the synthesis of C≥9 molecules through a one-pot reaction using aldol condensation of furfural and ethanol remains elusive due to the complexity of the systems, posing a challenge for the production of alkanes with higher carbon content from biomassbased chemicals. Therefore, it is both valuable and worthwhile to search for alternative catalytic cascade reactions that facilitates the synthesis of C≥9 molecules, leading to an important development towards the production of bio-fuels with high-energy density. Recently, gold nanoparticles have emerged as a promising class of catalysts for organic transformation, such as selective oxidation, hydrogenations, and C—C coupling reactions.24-28 Catalytic efficiency of gold nanoparticles has been shown in several reactions, including alcohol oxidation, aldehyde hydrogenation, Ullmann, Sonogashira, and multicomponent

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coupling.29-33 To disperse and stabilize gold nanoparticles for the catalytic reactions, metal oxides are commonly used. The supported nanoparticles usually present a superior activity in catalysis owing to the presence of active sites at the catalyst— support interface. Among metal oxides supports, NiO is considered as a typical basic oxide. With this issue in mind and the general requirement of a base catalyst for aldol condensation, we speculate that the use of NiO as a support for gold catalysts can promote the in-situ oxidative aldol condensation process. Herein, we report one-pot cascade reaction over the Au/NiO composite catalysts to produce longchain hydrocarbons. For the first time, we found C9 (i.e., difurylmethane) can be generated though a cascade reaction using furfural, ethanol and O2. Results demonstrate ~ 92% furfural conversion with ~ 81% selectivity towards the formation of C7 and C9 hydrocarbons in the one-pot cascade reaction.

2. EXPERIMENTAL METHODS 2.1. Chemicals. All chemicals were used as received. HAuCl4·3H2O (99.9% metal basis) and sodium borohydride (NaBH4, 99.99%) were purchased from Aldrich. 5-methyl furfural (98%), 5-nitrofurfural (97%), benzaldehyde (99%), and 3-nitrobenzaldehyde (98%) were purchased from Adamas. Sodium hydroxide (NaOH), Potassium carbonate (K2CO3), ethanol, furfural, and polyvinyl alcohol (PVA) were purchased from Sinopharm Chemical Reagent Co. ltd. Nanopure water (resistivity 18.2 MΩ cm) was purified with a Barnstead NANOpure Diwater™ system. All the glassware was thoroughly cleaned with aqua regia (37% HCl:HNO3=3:1, v:v), rinsed with copious nanopure water, and then dried in an oven prior to use. 2.2. Synthesis of NiO oxides. The hydrothermal method is employed to synthesize NiO.34 Typically, NiSO4·6H2O (2.45 mmol) dissolved in 40 mL H2O, and mixed with a 40 mL aqueous NaOH solution (3.3 mmol). The solution was vigorously stirred for 30 min, and then transferred to a Teflonlined autoclave. The system thus prepared was heated in an oven at 120 °C for one day. The precipitates were washed with water and ethanol several times. The final NiO was calcined at 400 °C for 2 h in air to remove the protecting PVA ligands. 2.3. Preparation of Au/NiO catalysts. The Au/NiO catalysts are prepared via a wet-synthetic process of Au:PVA colloids (PVA: polyvinyl alcohol) and NiO oxides. The Au:PVP colloids were prepared using PVA as a protecting agent and NaBH4 as a reducing agent. Typically, 1 g PVA was dispersed in a 400 mL of distilled hot water (60-95 °C). After cooling to room temperature, 21 mg of HAuCl4·4H2O was added into the mixture and continuously stirred for 1 h. This is followed by rapid injection of an aqueous solution of NaBH4 (Au/NaBH4 1:5 mol⋅mol-1), leading to the formation of a dark orange-brown solution, indicating the formation of the gold colloids. The solution thus prepared was maintained under stirring for 4.5 h. Then, 1 g of the NiO support was added to the colloidal gold solution under stirring and kept under stirring for 3 h. After impregnation process, samples were washed with water and ethanol, and then dried at 50 °C overnight. The final products were calcined at 300 °C for 3 h in a flow of air to remove the capping surfactant on the Au NPs’ surface. ICP-MS revealed the exact amount of Au loadings of the Au/NiO was 0.91 wt%. 2.4. Characterization. Transmission electron microscopy (TEM) images were collected using a FEI Tecnai G2 Spirit

microscope operated at 120 kV. Aberration-corrected STEM (AC-STEM) images were taken on a JEM–ARM200F at 200 kV. The specimen was prepared by ultrasonically dispersing the sample powder into ethanol, and the drops of the suspension were deposited on a carbon-coated copper grid and dried at room temperature in air. The lattice spacing and the morphology of Au nanoparticles were analyzed by a Digital Micrograph software. Power X-ray diffraction (XRD) patterns of the samples were studied on a D/Max-2500/PC diffractometer (Rigaku, Japan) using Cu Kα (λ = 0.154 nm) radiation operated at 40 kV and 200 mA in the range of 10 to 80o with a rate of 5o min-1. The BET surface area of NiO and Au/NiO was measured by N2 adsorption on a Micromeritics ASAP 2020 instrument. Each sample was degassed for 2 h at 300 °C before the test. X-ray photoelectron spectra (XPS) were measured using an ESCALAB MK-II spectrometer with an aluminum anode for Kα (hν = 1484.6 eV) radiation. The equipment base pressure was 1.7×10-10 bar, and all samples were characterized at room temperature. Detailed spectra were recorded for the region of Ni 2p, O 1s and Au 4f photoelectrons with a 0.1 eV step. Analysis was performed by the XPSPEAK41 software, and charging effects were corrected by adjusting binding energy (B.E.) of C1s to 284.6 eV. The spectra were deconvoluted using the XPSPEAK program by curvefitting with a mixed Gaussian-Lorentzian function. CO2temperature programmed desorption (CO2-TPD) measurements were performed on a conventional CO2-TPD instrument. 100 mg of the catalyst was loaded into a quartz reactor and heated in a flow of Helium at 300 °C for 1 h to remove adsorbed pollutants on the sample. Then the reactor was cooled to room temperature under the flowing He gas (40 mL·min−1). The sample was saturated with CO2 at room temperature for 0.5 h, and the excess adsorbates were removed by allowing the sample to remain in a flow of He until no significant amount of adsorbates could be detected. The temperature was ramped to 900 °C at a heating rate of 20 °C min-1. The gas chromatography-mass spectrometry (GC-MS) at m/z=44 is used to monitor the process of CO2-TPD. To perform O2temperature programmed desorption (O2-TPD) analysis, the catalyst was packed in a quartz reactor and heated under a He gas flow at 300 °C for 2 h. Then the reactor was cooled to 0 oC under the flowing He gas (40 mL·min−1). The sample was next saturated with 10 vol% O2 at 0 oC for 0.5 h, and the excess adsorbates were removed by allowing the sample to remain under the flow of He until no significant amount of adsorbates could be detected. The temperature was ramped to 900 °C at a heating rate of 20 °C min-1. This process was monitored by GCMS at m/z=16 for detection of oxygen species. The amount of furfural adsorbed on the catalysts was measured by thermogravimetric analysis (TGA). In a typical operation, 100 mg of furfural was added into 5 mL ethanol. Then 100 mg supports or catalysts was introduced into the solution. After 2 h, the suspension was filtrated and treated with a flow of N2 in 1 L/min at room temperature to remove ethanol until the sample become dried (∼1 h). TG experiments were performed on STARe System (METTLER TOLEDO) using temperature programming. 2.5. Catalytic measurements. Liquid-phase oxidative esterification and cross-aldol condensation reactions of furfural were conducted in a 10 ml continuous stirred-tank reactor. Typically, the reactions were performed at 130 °C under atmosphere of 1 MPa air, stirring at 600 rpm, for 1-6 h using ethanol, potassium carbonate, and a variety of aldehyde derivatives, including 5-methyl furfural, 5-nitrofurfural, benzaldehyde, and 3-nitrobenzaldehyde. The reactor was


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loaded with 3 ml of ethanol, 40 mg substrates, and 10 mg catalyst. To hydrogenate the oxygenated sugars produced in the reaction, the atmosphere of the reactor is changed to H2 (1 MPa). The catalysts were collected by filtration, washed with water and ethanol, dried at 100 °C overnight, and used to study the recyclability of the catalyst. A fresh reaction was run with furfural and ethanol under identical reaction conditions using the recycled catalyst. The supernatant was analyzed on a GC/MS 7890B-5977A gas chromatography-mass spectrometer equipped (Agilent Technologies) with MS-5977A, temperature programmer, and a capillary column HP-5MS (L = 30 m × I.D. = 0.25 mm × df = 0.25 μ m, made in USA of domestic and foreign components) with He as carrier gas. The conversion and product selectivity were analyzed and determined byGC-MS.

Intensity (a.u.)

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ACS Applied Energy Materials TEM images of the composite show that the NiO supports exhibit a uniform nanosheet-like structure with an average size of ~ 120 nm × 140 nm (Figure S1a in the Supporting Information). The Au nanoparticles (NPs) are uniformly dispersed on the surface of NiO oxides with an average size of ~ 3.2 nm (Figure S1b). For a second evidence, the BET of the NiO supports and the Au/NiO catalysts is measured, and found to be 222.5 and 138.8 m2 g-1, implying the gold NPs are indeed loaded over the NiO oxide. Further, aberration-corrected STEM (AC-STEM) technique is employed to characterize the catalyst with atomic resolution. Result shows the gold nanoparticles with the (111) facet and the lattice spacing of 0.23 nm are deposited onto the (200) plane of the NiO (Figure 2).

Au/NiO (200) (111) (220)

PDF: 47-1049

20

30

40

50

60

70

2 Theta (degree) Figure 1. XRD pattern of the Au/NiO composite.

3. RESULTS AND DISCUSSION

Figure 2. AC-STEM image of the Au/NiO composite.

3.1. Synthesis and characterization of the Au/NiO catalysts. The Au/NiO samples are prepared by immobilization of Au:PVA colloids (PVA: polyvinyl alcohol) on the surface of NiO oxides. The as-prepared Au/NiO are characterized using XRD to study the crystallinity and phase composition of the composite. As Figure 1 shows, three sharp and intense diffraction peaks at 2θ = 37.5, 43.6 and 63.2o are assigned to the (111), (200), and (220) crystal facets of the NiO (PDF 47-1049), respectively. It is worthy to note that no diffraction peaks are detected to be associated with the gold nanoparticles. This is mainly due to the low concentration (~ 0.91 wt%), and high dispersity of the gold nanoparticles.

The XPS spectrum of the composite shows the binding energies (BE) of Au 4f7/2 is 84.2 eV (Figure. 3a), slightly higher than that for neutral metallic Au (83.9 eV).35 This result indicates the formation of Auδ+ species due to electron transfer from the gold nanoclusters to the NiO support. Two predominant peaks located at BE of 873.7 and 855.8 eV correspond to Ni 2p1/2 and Ni 2p3/2 of the Ni(II)O, respectively (Figure 3b). There are also two weak peaks at 871.9 and 854.1 eV that are assigned to Ni 2p3/2, associated with the presence of Ni(OH)2 species at the surface of NiO.36 Further, we note that three O species can be identified with BE of 532.1, 531.3 and

Figure 3. XPS spectra of 1 wt% Au/NiO composite: (a) Au 4f, (b) O 1s, and (c) Ni 2p.


ACS Applied Energy Materials can potentially employed for efficient conversion of furfural. Further, the O2-TPD profile of the composite shows a relatively weak peak at 441 oC, which is assigned to adsorbed [O] species at the surface of the Au/NiO (Figure S2 in the Supporting Information). This result reveals the potential of the Au/NiO composite to activate O2 for the oxidation reaction. We now proceed to examine the catalytic effectiveness of Au/NiO composite for the conversion of furfural via aldol condensation reaction. 3.2. Cascade catalytic reaction of furfural. The results for the catalytic conversion of furfural in the presence of ethanol, O2, and K2CO3 over NiO-supported gold nanoparticles are compared in Table 1. Several aspects are noteworthy. First, in the absence of a base (K2CO3), the Au/NiO catalyst presents a very low furfural conversion, i.e., 25% at 130 °C (Table 1, entry 1). The reaction yields 2-furaldehyde diethyl acetal (FDEA) as the main product with 84% selectivity, and 3-(2-furyl)acrolein (FA) as a byproduct. Previous studies show that FDEA can be synthesized via aldol condensation of furfural with ethanol and the help of a base catalyst.38,39 These results indicate the NiO support exposes active sites with base property, in good agreement with the CO2-TPD results presented above. Secondly, in the presence of an inorganic base (K2CO3), the furfural conversion improves significantly to 83% and 90% at 110 and 130 °C, respectively (Table 1, entries 2 and 3). We further note that FDEA is not yielded in the presence of K2CO3, which can be due to a reverse reaction that transforms FDEA to furfural. Using gas chromatography–mass spectrometry (GCMS), four products of furfural conversion are identified, including 3-(2-furyl)acrolein (FA, the main product), ethyl furoate (EF), ethyl-2-furanacrylate (EFA), and 2,2'difurylmethane (DFM). The amount of formed acetaldehyde (CH3CHO) and ethyl acetate (EtOAc) from the ethanol conversion in the reactor is ca. 0.10 wt% and 0.25 wt%, respectively. The selectivity towards the production of FA is enhanced when the catalysis is carried out under 1 MPa of air

529.4 eV, which correspond to H2O adsorbates, hydroxyl (OH), and lattice oxygen (O2-), respectively, Figure 3c.

Intensity of CO2 mass signal (a.u.)

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385

589 701

200 300 400 500 600 700 800 900 o Temperature ( C)

Figure 4. CO2-temperature-programmed desorption of the Au/NiO composite. To investigate the acid-base properties of the Au/NiO composite, we apply the CO2-temperature-programmed desorption (CO2-TPD) method, which is shown to be a powerful tool to shed light on the concentration and strength of basic and acidic sites of catalysts.37 Figure 4 shows that there are three main peaks for the CO2 desorption in range of 200-900 oC. The peak at 385 oC can be assigned to weak basic sites of the NiO.37 The broad peak centered at 589 oC and the shoulder peak at 701 oC correspond to medium and strong base sites of the sample, respectively.37 These results demonstrate the presence of active basic sites at the surface of Au/NiO, which

Table 1. Conversion of furfural in the presence of ethanol catalyzed by the Au/NiO.a O

O

Au/NiO, air O

K2CO3, EtOH

O OEt +

O

CHO +

FA

EF

OEt

O

OEt

+

OEt

O

+

O

O

O

FDEA

DFM

EFA

Entry

Catalyst

Gas

base

T (oC)

Conversion (%)

Selectivity (%) EF

FA

FDEA

EFA

DFM

1 2 3 4 5 6 7b 8c 9d 10e 11f

Au/NiO Au/NiO Au/NiO Au/NiO NiO Au/NiO Au/NiO Au/NiO Au/NiO Au/NiO

O2 O2 O2 air air air air air air air air

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

130 110 130 130 130 130 130 130 130 130 130

25 83 90 91 14 n.r. 36 35 90 92 92

n.d. 11 23 13 n.d. n.d. 9 14 14 13

16 71 43 62 100 79 78 60 59 60

84 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. 3 10 5 n.d. n.d. 3 6 6 7

n.d. 15 24 20 n.d. n.d. 10 20 21 20

a Reaction conditions: 10 mg Au/NiO catalysts, 40 mg furfural, 10 mg K CO , 3 mL ethanol, 6 h, in the presence 2 3 of either O2 or air (1 MPa gas). b 0.11 MPa air. Of note, for this entry, 21% selectivity is found to produce 3furanmethanol. c 4 mg Au/NiO catalysts, 40 mg furfural, and 10 mg K2CO3. d 40 mg Au/NiO catalysts, 160 mg 3-furaldehyde, and 40 mg K2CO3. e Second cycle. f Third cycle. The conversion of furfural and the selectivity are determined by GC-MS analyses; n.r. = no reaction, n.d. = not detected.


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(Table 1, entry 4). A lower concentration of the catalyst (~ 10 wt%) is shown to decrease the furfural conversion to 35%, and increase the selectivity for FA formation to 78% (Table 1, entry 8). The reaction under a lower air pressure (0.11 MPa) is found to decrease the conversion as low as 36% (Table 1, entry 7). No reaction conversion is detected in a blank experiment (i.e. in the absence of Au/NiO and NiO, Table 1, entry 6), indicating the active sites for the reaction are located at the surface of the Au/NiO or NiO. It is worthy to note that the plain NiO oxide presents a very low furfural conversion of 14% with ~ 100% selectivity for the syntheses of FA (Table 1, entry 5), implying the crucial role of the gold nanoparticles in the oxidative esterification to yield EF and EFA. To assess the recyclability of the Au/NiO catalyst, we collected the catalyst by filtration after the reaction. Next, the catalyst was washed with water and ethanol, dried at 100 °C to remove any complexes. The recycled catalyst thus prepared was used in the one-pot cascade reaction under identical conditions. The recycled catalysts exhibit similar activity and selectivity in comparison with the fresh Au/NiO catalyst; it yields 92% furfural conversion for the 2nd and the 3rd cycles, respectively (Table 1, entries 8 and 9). These results demonstrate no appreciable loss of activity and selectivity of the composite for the reaction after three cycles (higher cycles were not examined), indicating that the NiO-supported gold nanoparticles present good recyclability in the furfural conversion process. TEM images of the recycled catalysts show that, after the third reaction cycle, some of the Au NPs aggregated to form particles with size of 5-7 nm (Figure S3 in the Supporting Information).

Table 2. The catalytic activity of the Au/NiO for the conversion of ethyl furoate (EF) and 3-(2-furyl)acrolein (FA). Reaction conditions: 10 mg Au/NiO catalyst, 0.42 mmol EF or FA, 10 mg K2CO3, 3 mL ethanol, 130 oC for 6 h, under 1 MP air. Entry

45

60

30

40 DFM EF

20 0 FDEA 0 1

Substrate O

1

Selectivity (%)

EFA

DFM

0

-

-

55

35

65

O OEt

O

Conversion (%)

CHO

60

FA

Selectivity (%)

80

selectivity for oxidative esterification product (EF) is 27% at 1 h, and found to rapidly drop to 15% at 2 h as the conversion is increased. The selectivity towards DFM gradually improves during the reaction process, i.e., 6%, 20%, 22%, 28% at 1, 2, 4, and 6 h of the reaction time. Of note, FDEA is not detected at any time of the catalysis process (Figure 5). 3.3. Pathway of the cascade reaction. In order to investigate the possible pathway that result in the formation of EFA and DFM, we first use EF as reactants for the reaction under identical conditions as noted for entry 4 of Table 1. No conversion is detected using such reactant, strongly indicates that the EFA and DFM do not form from ester EF (Table 2, entry 1). Next, we employed FA (cinnamaldehyde) to perform as reactants. Results show FA is transformed to EFA and DFM with conversion rate of 55% (Table 2, entry 2). These results indicate the C9 products (EFA and DFM) are yielded form the direct conversion of the , β–unsaturated aldehyde (FA) that are formed during the furfural conversion.

2

100 Conversion of furfural (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

15

EFA

0 2

3

4

5

6

Reaction time (h) Figure 5. Conversion of furfural (black ■) and selectivity for ethyl furoate (EF, orange ♦), 3-(2-furyl)acrolein(FA, red ▲), ethyl-2-furanacrylate (EFA, green ●), 2,2'-difurylmethane (DFM, blue ▼) and 2-furaldehyde diethyl acetal (FDEA, pink *) as a function of reaction time over the Au/NiO catalysts for furfural conversion in the presence of ethanol. Further, the kinetics of the reaction is studied by monitoring the furfural conversion and product selectivity as a function of time under the same reaction conditions as listed for entry 4 of Table 1. The conversion is found to increase rapidly during the first few hours, reaching ca. 90% of conversion at 2 h, and finally converging to 92% with no appreciable loss of activity to the end of experiment (Figure 5). With respect to product selectivity, FA remains as the main product throughout the entire range of reaction time (61-63% during 1-4 h of the reaction time, and gradually decreases to 54% at 6 h). The

According to catalysis results presented above, we speculate that the catalytic conversion of furfural over the Au/NiO consists of two competing reactions: (I) oxidative esterification, and (II) cross-aldol condensation. As the reaction begins, furfural converts to either EF through oxidative esterification, or FA via cross aldol condensation (Scheme 1), evidenced by the product distribution after 1 h (Figure 5). Next, the as-formed FA reacts with ethanol to result in EFA and DFM, in good agreement with the constant selectivity for FA during the reaction time of 1-3 h, while DFM concentration increases after one hour. It is worthy of note that DFM is yielded over the Au/NiO catalysts via two steps: (i) Robinson annulation, and (ii) rearrangement of intermediate 3-(furan-2-yl)pentanedial which is formed by Michael addition of FA (Scheme S1 in the Supporting Information). Finally, FA and DFM thus produced can be fully (i.e., 100%) converted to C7H16 and C9H20 hydrocarbons when the atmosphere of the reaction changes to H2 (1 MPa).41 Of note, esters EF and EFA are reduced to saturated esters (i.e., C4H9COOC2H5 and C6H13COOC2H5 with ~ 19% selectivity, Scheme 1) after the completion of hydrogenolysis process, implying the esters cannot be fully hydrogenated over the Au/NiO catalysts to result alkane hydrocarbons. To investigate the possibility of the catalyst reduction during the hydrogenolysis step, the chemical state of Ni of the catalysts is studied using XPS. Figure S4 in the Supporting Information shows no appreciable change in the chemical state of Ni after the reaction.



Scheme 1. The conversion pathway of furfural to C7 and C9 hydrocarbons via the oxidative esterification and cross-aldol condensation with ethanol catalyzed over the Au/NiO catalysts in the presence of O2 and K2CO3, followed by a hydrogenolysis process using H2.

3.4. Scope of aldehydes. For completeness, we examine the catalytic performance of the Au/NiO composite for some other aldehydes, including 5-methyl furfural, 5-nitrofurfural, benzaldehyde, and 3-nitrobenzaldehyde. The reaction conditions are the same as that noted for entry 4 of Table1. The catalytic results are compiled in Table 3. The substitution of hydrogen at position 5 of furfural with a methyl or a nitro group exerts a considerable influence on the aldehyde conversion. The conversion of 5-methyl furfural and 5-nitrofurfural is 73% and 54%, respectively (Table 3, entries 2 and 3). We note 5-methyl furfural prefers to convert to , β–unsaturated aldehyde (Table 3, product B with 74% selectivity), while 5-methyl furfural is mostly transferred to a saturated ester (Table 3, product A) and product B with 50% and 46% selectivity. With respect to the benzaldehyde derivatives, the major products are found to be ester A (55% and 54% selectivity for ethyl benzoate and ethyl 3-nitrobenzoate, respectively, Table 3, entries 4 and 5). It is worthy to note that product D (a derivative of furan) is not yielded when the aldehydes with a nitro group (a strong electron withdrawing group) is used as reactants, i.e., 3nitrobenzaldehyde and 5-nitrofurfural, Table 2, entries 3 and 5. Table 3. The Au/NiO catalysts for aldehydes conversion. Reaction conditions: 10 mg Au/NiO catalysts, 0.42 mmol aldehydes, 10 mg K2CO3, 3 mL ethanol, 130 oC for 6 h, under 1 MP of air. n.d. = not detected. Au/NiO, air O R K2CO3, EtOH

R

O

Entry

1

3

CHO

O

2

O2N

4

O

CHO

CHO

CHO

CHO

5 O2N

B Conversion (%)

Aldehyde O

CHO+ R

OEt + R

A

OEt +R O

O

D

C Selectivity (%)

A

B

C

D

96

7

55

4

34

73

9

74

1

16

54

50

46

2

n.d.

65

55

28

6

11

93

54

29

17

n.d.

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The activity of a catalyst generally scales with its affinity to adsorb reactants, which is often deemed as the first step in catalytic reactions.37 To shed light on this, thermogravimetric analysis (TGA) method was employed to measure the amount of several aldehydes that are adsorbed at the surface of the Au/NiO catalyst. We choose boiling point of aldehydes to remove any physisorbed aldehydes from the reaction medium. Next, the temperature is increased to measure the amount of the aldehydes that are chemisorbed at the surface of the catalysts. Table 4 shows that the Au/NiO adsorbs more aldehydes in comparison to the NiO. This result indicates the active sites at the interface of the Au particles and the NiO are more active towards the adsorption of reactants. We further note the amount of chemisorbed aldehydes on the Au/NiO decreases in the order furfural > 5-methyl furfural > benzaldehyde, which is similar to the order found for the aldehydes conversion rate discussed above. Therefore, we may conclude that there is a correlation between the aldehydes conversion and their adsorption affinity on the Au/NiO catalyst. Table 4. Absorption of aldehydes on the NiO and Au/NiO measured by TGA. Aldehyde

O

Sample

Aldehyde adsorbed on sample (wt.%)a

Surface concentration of aldehyde (mmol m-2 cat. × 105)b

NiO

2.79

1.30

Au/NiO

4.20

3.15

CHO

O

NiO

2.08

0.90

Au/NiO

4.28

2.80

NiO

1.66

0.70

Au/NiO

3.92

2.66

CHO

CHO

The amount of chemisorbed aldehydes on the samples were calculated based on the weight loss above the boiling point of aldehydes using TGA method. bThe adsorbates (mmol) per square meter of the NiO or the Au/NiO. a

■ CONCLUSIONS In summary, Au nanoparticles with average size of ~3.2 nm are deposited at the surface of the NiO. The catalytic effectiveness of the as-prepared Au/NiO composite is examined in one-pot cascade reaction to convert furfural to hydrocarbons C7 and C9 via cross-aldol condensation with ethanol, followed by a hydrogenolysis process. The Au/NiO is further tested to catalyze cross-aldol condensations for a range of aldehyde substrates. The CO2 and O2 temperature-programmed desorption, transmission electron microscopies, and X-ray photoelectron spectroscopy (XPS) are employed to characterize the Au/NiO composite, revealing the promising catalytic efficiency is associated with the nature active sites at the interface of the NiO and Au nanoparticles. This study may provide a insights for development of efficient nanogold catalysts for other cascade reactions.

■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/*******.


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ACS Applied Energy Materials TEM images of the fresh and spent Au/NiO composites, O2-TPD analysis of the Au/NiO catalysts, Ni 2p XPS spectrum of the spent Au/NiO after hydrogenolysis, and tentative reaction mechanism for the cascade conversion of furfural (PDF).

■ AUTHOR INFORMATION Corresponding Author *Email: [email protected] (G.L.)

[email protected] (H.A.) [email protected] (F.L.) &Q.F.

and Z.Q. contributed equally to this work. ORCID Hadi Abroshan: 0000-0003-1046-5170 Gao Li: 0000-0001-6649-5796 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT G. L. thanks the Liaoning Natural Science Foundation (20180510050). BL14B and BL17B beamline of National Facility for Protein Science (NFPS), Shanghai Synchrotron Radiation Facility (SSRF) Shanghai, China for providing the beam time.

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NiO Composite: A Catalyst for One-Pot Cascade Conversion of

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