Research Article pubs.acs.org/journal/ascecg
Role of Surface Cooperative Effect in Copper Catalysts toward Highly Selective Synthesis of Valeric Biofuels Wei Li,† Yizhen Li,*,‡ Guoli Fan,† Lan Yang,† and Feng Li*,†,§ †
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State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, No. 15, Beisanhuan East Road, Beijing 100029, China ‡ Sinopec Group, Beijing Yanshan Petrochemical High-Tech Co., Ltd., No. 8, Yandong Road, Yanshan, Fangshan, Beijing 102500, China § Beijing Advanced Innovation Center for Soft Matter Science and Engineering, No. 15, Beisanhuan East Road, Beijing 100029, China S Supporting Information *
ABSTRACT: Currently, the catalytic conversion of biomass-derived compounds into biofuels is of great significance in terms of environmental protection and sustainable development. Among them, valeric esters derived from γ-valerolactone (GVL) are regarded as one of the most promising alternatives to fossil fuels. Herein, the highly efficient one-pot transformation of GVL to produce a series of valeric esters was successfully achieved over novel ZrO2-incorporated ZnAl2O4-compositesupported Cu-based catalysts. An extensive investigation gave clear evidence that the incorporation of ZrO2 into composites could lead to the enhanced metal dispersion and surface acidity. Especially, the catalyst with a Zr/Zn mass ratio of 0.2 exhibited the best selectivity of 99% in the transformation of GVL into pentyl valerate to date, together with a comparable conversion of 91% with respect to the Cu-based catalyst previously reported. The superior catalytic performance was attributable to the surface cooperation effect between highly dispersed active copper species and abundant surface acid sites. Especially, different surface types of acidic sites on catalysts could induce the reaction to efficiently proceed in different paths. The present work provides a valuable approach for precious metal substitution research in future largescale biorefineries. KEYWORDS: Valeric esters, Biofuels, Copper catalysts, Porous spinel, Acid sites
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INTRODUCTION With the increasing concern about environment and energy issues arising from growing greenhouse gas emission and the shortage of fossil resources, the search for a green renewable energy source has received substantial attention over the last two decades.1,2 Among them, biofuel resource has been regarded as one of the most promising alternatives to fossil fuels in the future. Represented by bioethanol, the first generation biofuels are mainly produced by anaerobic fermentation of edible biomass.3 The low energy density and high water solubility, however, limit their large-scale application.3 In this regard, many efforts are therefore devoted to the exploitation of catalytic conversion of next generation renewable and sustainable biomass resources.4−6 For example, the most abundant lignocellulosic biomass composed of cellulose, hemicellulose, and lignin, which is widely present in the plants cell walls, can be easily converted into several valuable platform molecules (e.g., levulinic acid and γ-valerolactone (GVL)) through a biorefinery process.7−12 At present, GVL has been identified as intermediate precursor for important chemicals and fuel additives.13−15 Especially, the production of valeric esters through the direct conversion of GVL is attracting great interest, because valeric © 2017 American Chemical Society
esters as a new generation of cellulose-based transportation biofuels are well-suited as gasoline additives and a diesel component depending on different alcoholic residues.16 Recently, some pioneering studies on the transformations of levulinic acid to valeric acid and valeric ester or the upgrading of GVL to pentyl valerate (PV), with different supported Pt, Ru, or Pd catalysts, were reported.3,17−20 In most cases, however, some drawbacks including the low selectivity to target valeric esters and the deactivation of catalysts still exist. Due to the advantages of both low cost and adequate reserves on Earth, designing and developing efficient non-noble-metal-based catalysts becomes imperative.21−23 For instance, Ravasio and co-workers22 reported a supported Cu/SiO2 catalyst with a copper loading of 16% that yielded a 92% selectivity to pentyl valerate at about 91% GVL conversion, and found that the reduction of copper species could enhance surface acidity of catalysts and thus catalytic performance. In spite of the above pioneering works, supported non-noble-metal-based catalysts are rarely reported in the production of valeric esters so far, Received: October 29, 2016 Revised: January 12, 2017 Published: January 20, 2017 2282
DOI: 10.1021/acssuschemeng.6b02618 ACS Sustainable Chem. Eng. 2017, 5, 2282−2291
Research Article
ACS Sustainable Chemistry & Engineering
x support (1.0 g) was added into the solution in an ultrasonic bath for 30 min. Subsequently, urea (4-fold amount versus Cu2+) was introduced into the system and aged at 70 °C for 4 h. The resultant precursor was washed with deionized water and ethanol as above, and dried at 60 °C overnight. The resultant powder was calcined in static air at 600 °C for 4 h to obtain C-Cu/ZZA-x. Finally, C-Cu/ZZA-x was reduced in 10% v/v H2/Ar atmosphere at 300 °C for 1 h at a ramping rate of 2 °C min−1 to obtain Cu/ZZA-x catalyst. The obtained catalyst was sealed in a sample tube filled with N2 before characterization and reactions. For comparison, Cu/ZnO, Cu/Al2O3, and Cu/ZrO2 were also prepared using the single-metal oxides as supports by similar methods. The loading amount of Cu was the same as that of Cu/ZZA-x. Also, the ZZA-0.2-supported Cu catalysts with different Cu loading are also prepared by similar methods. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku UItimaIII diffractometer with Cu Kα radiation (40 kV, 40 mA, λ = 0.15418 nm). The elemental analysis was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-AES). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were carried out on a JEOL 2010 instrument. Low-temperature N2 adsorption− desorption isotherms were obtained using a Micromeritics ASAP 2020 instrument. The total specific surface area was calculated by the multipoint Brunauer−Emmett−Teller (BET) method. The pore size distribution was calculated by the nonlocal density functional theory (NL-DFT) model for micropores and by the Barret−Jovner−Halenda (BJH) method for mesopores. X-ray photoelectron spectra (XPS) were recorded on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer using monochromatic Al Kα X-ray as the excitation source (1486.6 eV). X-ray induced Auger spectra (XAES) were recorded on a PHI Quantera SXM instrument. Hydrogen temperature-programmed reduction (H2-TPR) and ammonia temperature-programmed desorption (NH3-TPD) experiments were performed using a Thermo Fisher TPDRO-1100 chemical adsorption instrument with a thermal conductivity detector (TCD). For H2-TPR, 30 mg of the calcined sample was placed in a quartz tube reactor and degassed at 200 °C for 2 h under Ar flow (30 mL min−1). Then, a TPR measurement was conducted in a stream of 10% v/v H2/ Ar (30 mL min−1) with a heating rate of 5 °C min−1 up to 800 °C. For NH3-TPD, 200 mg of the calcined sample was placed in the quartz tube reactor and reduced in 10% v/v H2/Ar atmosphere at 300 °C for 1 h at a ramping rate of 2 °C min−1. After it cooled to room temperature, the sample was degassed by heating under a He environment at 450 °C for 1 h followed by treatment with a flowing ammonia gas mixture (5% NH3 in He) for 1.5 h at 100 °C. Chemisorbed NH3 was desorbed by heating from the saturation temperature up to 900 °C at a rate of 10 °C min−1. N2O titration experiments were carried out using a Micromeritics ChemiSorb 2920 instrument to determine the active copper surface area and dispersion of Cu0 particles. First, the calcined sample underwent a H2-TPR process in 10% H2/Ar mixture from 50 to 300 °C, and the temperature was held until there was no more H2 consumption. After it cooled to room temperature, the sample was oxidized in a stream of 10% v/v N2O/N2 (40 mL min−1) at 70 °C for 1h, followed by Ar purging and cooling down to room temperature. Finally, H2-TPR was carried out again with 10% v/v H2/Ar to 300 °C. Cu dispersion was calculated by dividing the amount of surface copper sites by the total number of supported Cu atoms, and copper surface area was calculated by assuming a spherical shape of the copper metal particles and a surface concentration of 1.47 × 1019 copper atoms per m2. In situ infrared spectroscopy of pyridine adsorbed (Py-IR) on samples was recorded on a Thermo Nicolet 380 spectrometer. The calcined sample (30 mg) was pressed into a self-supporting wafer and placed in an evacuable IR cell with CaF2 windows, and then reduced in situ under the same conditions as that for preparing the catalyst. The wafer was evacuated at 200 °C under argon flow and cooled down to room temperature before pyridine was introduced and balanced for 1
mainly due to the relatively low catalytic performance, as well as the irreversible deactivation of catalysts originating from the metal leaching under harsh reaction conditions. In this sense, developing new highly efficient and stable non-noble-metalbased catalysts still remains challenging toward the production of valeric esters. From the other perspective, thanks to the characteristics of variable microstructures and compositions, high chemical and thermal stability, and good mechanical resistance, spinel-type zinc aluminate (ZnAl2O4) is widely used as catalyst support in heterogeneous catalytic processes.24,25 Although a number of preparation methods for spinel materials have been set up,26−28 the requirement for high-temperature treatments in most cases makes the preparation processes energy-intensive and uneconomical. Moreover, porous ZnAl2O4 materials are rarely reported in the literature.29,30 Recently, we successfully synthesized micro/mesoporous ZnAl2O4 spinels by a homogeneous coprecipitation route,31 and the spinels were used as Pdbased catalyst supports for a highly efficient Suzuki−Miyaura coupling reaction. Inspired by pervious works, herein we reported the synthesis of ZrO2-incorporated ZnAl2O4 composites (ZrO2−ZnAl2O4) with high specific surface areas through a facile coprecipitation route followed by a solvothermal treatment. Further, one-pot catalytic transformations of GVL to produce valeric esters were evaluated over ZrO2−ZnAl2O4-supported copper-based catalysts. A series of systematic characterizations and catalytic tests revealed the effects of the incorporation of ZrO2 on the electronic states of active metal species, surface acidity, and catalytic performance of supported copper catalysts. It was found that surface cooperation between highly dispersed Cu0/ Cu+ species and Brønsted/Lewis acid sites in the present supported Cu-based catalysts contributed to high catalytic efficiency in the one-pot transformation of GVL into valeric esters. Especially, the catalyst with a Zr/Zn mass ratio of 0.2 exhibited the best pentyl valerate selectivity of 99%, together with a comparable conversion with respect to the Cu-based catalyst previously reported. Moreover, the Cu/ZrO2−ZnAl2O4 catalyst exhibited good versatility in the production of a wide range of valeric esters (i.e., methyl, ethyl, propyl, butyl, and pentyl valerate). Until now, such highly efficient supported non-precious-metal catalysts for the one-step conversion of GVL to produce valeric esters have not been reported.
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EXPERIMENTAL SECTION
Materials. Zn(NO3)2·6H2O, Al(NO3)3·9H2O, Zr(NO3)4·5H2O, Cu(NO3)2·3H2O, methanol, ethanol, hexamethylene tetramine (HMT), and urea were purchased from Beijing Chemical Reagent Company. All of these chemicals were used as received. Synthesis of ZrO2−ZnAl2O4 Composites. ZrO2-incorporated ZnAl2O4 supports were prepared by a one-pot solvothermal route. Typically, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, and Zr(NO3)4·5H2O with a [Zn2+]/[Al3+] molar ratio of 1:2 and a [Zr4+]/[Zn2+] molar ratio of x (x = 0, 0.1, 0.2, and 0.3) were dissolved in 80 mL of a mixture solvent of deionized water and methanol (v/v = 1:1) under vigorous stirring. Then, HMT was added into the solution with twice the molar concentration of Zn2+. The resulting solution was transferred into a 100 mL Teflon-lined autoclave and heated at 180 °C for 12 h. The final product was filtered, washed with deionized water and ethanol for several times, and then dried at 60 °C overnight. The obtained sample was denoted as ZZA-x. Synthesis of Cu/ZrO2−ZnAl2O4 Catalysts. Cu/ZrO2−ZnAl2O4 catalysts were prepared by a urea-assisted precipitation method. First, a certain amount of Cu(NO3)2·3H2O was dissolved in 100 mL of deionized water under vigorous stirring, and then as-synthesized ZZA2283
DOI: 10.1021/acssuschemeng.6b02618 ACS Sustainable Chem. Eng. 2017, 5, 2282−2291
Research Article
ACS Sustainable Chemistry & Engineering h. Finally, the sample was heated stepwise under vacuum to 100 °C, and the Py-IR spectra were recorded. Catalytic Reaction. In a typical run, the batch autoclave reactor was loaded with catalyst (0.4 g), GVL (99%, J&K Scientific Ltd., 4 g), and solvent (pentanol), and then sealed and flushed with pure H2 10 times. After being placed in a heating jacket and preheated to the reaction temperature (250 °C), the autoclave was charged with H2 to 1.0 MPa and with stirring at a speed of 1000 rpm. After the reaction, the reactants were cooled rapidly in an ice bath, and the reaction mixture was filtered and quantitatively analyzed by gas chromatograph (Agilent 7890B) equipped with a DB-WAX capillary column (30.0 m × 250 μm × 0.25 μm) and a flame ionization detector using dioxane as internal standard substance. The products also were identified by GC− MS (Shimadzu QP 2010 with a DB-5 column). The spent catalyst was recovered from the reaction solution by centrifugation and washed several times with deionized water and ethanol before the next cyclic test. The turnover frequency (TOF) values were evaluated on the basis of the moles of GVL converted per mole of surface copper per unit of time after 1 h reaction (below 30% conversion).
(220) and (311) planes for the ZnAl2O4 phase in the cases of Zr-containing samples. In addition to the diffractions of ZrO2− ZnAl2O4 composites, several characteristic diffractions for the face-centered cubic (fcc) metallic copper phase (JCPDS 040836) can be observed at about 2θ of 43.2°, 50.4°, and 74.1°, respectively. Note that, with the Zr/Zn molar ratio increasing from 0 to 0.2, the average crystallite size of metallic copper particles estimated by the Scherrer formula according to the (111) plane decreases gradually from about 8.9 to 6.1 nm. Moreover, the change in the copper dispersion determined by N2O titration is in good agreement with that in the calculated particle size (Table 1 and Figure 1b). Such behavior is reasonably related to the incorporation of ZrO2, which can prevent the growth and agglomeration of Cu particles due to the barrier effect of surrounding ZrO2, as well as the enhanced interactions between metal and supports. Similarly, such a phenomenon also was found in other supported copper-based catalysts on Zr-containing supports previously reported.32−38 The microstructures of reduced Cu/ZZA-x samples were also examined by HRTEM images (Figure 2). Apart from some
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RESULTS AND DISCUSSION Structural Characterization. Figure 1a shows the XRD patterns of Cu/ZZA-x catalysts with different Zr content. All
Figure 2. HRTEM images of Cu/ZZA-x samples. (a) x = 0, (b) x = 0.1, (c, e) x = 0.2, (d) x = 0.3.
small dark Cu nanoparticles (NPs), several aggregated Cu clusters can also be found in the Zr-free sample. Obviously, the dispersion of Cu NPs is improved with the introduction of zirconium, as evidenced by the above XRD and N2O titration results. Further, a HRTEM image of a representative Cu/ZZA0.2 sample depicts the lattice fringes with two different interplanar spacings of about 0.245 and 0.208 nm indexed to exposed (311) and (111) planes of spinel phase and metallic Cu phase. However, no obvious lattice fringes corresponding to the ZrO2 phase are found. This is because of the relatively low crystallinity of the ZrO2 phase with the broad diffractions in XRD patterns, as well as uniform dispersion of ZrO2 on the
Figure 1. (a) XRD patterns of Cu/ZZA-x samples. (b) Mean size of Cu particles calculated by Scherrer formula (upper) and Cu dispersion degree determined by N2O titration (down).
samples display a series of characteristic diffractions corresponding to the (220), (311), (400), (511), and (440) planes of the cubic ZnAl2O4 spinel phase (JCPDS 05-0669). In addition, two broad diffractions at 31.3° and 35.1°, which are assigned to the (111) and (020) planes of the monoclinic ZrO2 phase (JCPDS 01-0750), are partially overlapped with the
Table 1. Textual and Structural Data of Different Supported Copper Samples contenta (wt %)
acidity (mmol/g) SBETb 2
Vtotalc 3
d
sample
Cu
Zr
(m /g)
(cm /g)
Vmicro (cm3/g)
D111e (nm)
Cudisf (%)
C(NH3)g
C(PyL)h
C(PyB)h
Cu+/(Cu0 + Cu+) ratioi
Cu/ZZA-0 Cu/ZZA-0.1 Cu/ZZA-0.2 Cu/ZZA-0.3
9.81 9.78 9.80 9.79
4.01 7.92 11.21
172 185 191 180
0.2129 0.2424 0.2443 0.2254
0.0225 0.0149 0.0151 0.0125
8.9 7.2 6.1 6.3
10.4 14.8 18.3 17.3
0.603 0.710 0.722 0.731
0.079 0.103 0.159 0.215
0.012 0.035 0.058 0.088
0.51 0.54 0.56 0.62
a
Determined by ICP-AES. bBET specific surface area. cSingle-point total pore volume calculated form nitrogen adsorption at P/P0 = 0.994. Micropore volume calculated by the NL-DFT method. eThe mean size of Cu particles calculated by Scherrer formula. fCopper dispersion degree determined by N2O titration. gCalculated by NH3-TPD. hCalculated by Py-IR analysis after evacuation under 100 °C. iDetermined by Cu LMM XAES analysis.
d
2284
DOI: 10.1021/acssuschemeng.6b02618 ACS Sustainable Chem. Eng. 2017, 5, 2282−2291
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compared with other Zr-containing samples. In this case, highly dispersed cupric species both on the surface and in bulk can interact strongly with the support, which makes cupric species much more difficult to reduce, leading to the weakened reducibility and thus elevated reduction temperatures.41,42 From another perspective, noticeably, the reduction peaks for the C-Cu/ZZA-0.2 sample with the lower Cu loading of 6.53 wt % obviously shift to lower temperatures in comparison to that of C-Cu/ZZA-0.2 with the Cu loading of 9.80 wt % (Figure S1), whereas those for C-Cu/ZZA-0.2 with the higher Cu loading of 13.1 wt % present an opposite trend. This result can be interpreted as the gradually reduced dispersion character of cupric species both on the surface and in the bulk phase with increasing Cu loading, thus leading to the weakened reducibility. On the basis of the above results, this clearly demonstrates that the incorporation of ZrO2 into composite supports has a positive effect on the dispersion of metal species and the metal−support interactions. Commonly, basic pyridine can be used as a probe molecule to detect surface acidity of samples and distinguish the types of acid sites.43 In this sense, in situ IR spectra of adsorbed pyridine (Py-IR) over Cu/ZrO2−ZnAl2O4 samples were analyzed. As shown in Figure 5a, several absorption bands appearing at 1380−1700 cm−1 are associated with pyridine adsorption on surface Lewis acid (LA) sites and Brønsted acid (BA) sites, as well as physically adsorbed (Ph) and H-bonding (HB) adsorbed pyridine.44−48 Also, the amounts of acid sites based on pyridine adsorption (mmolPy/g) were calculated according to the method reported by Emeis et al.49 The result clearly shows that the introduction of Zr into composite supports brings out the increased amounts of surface LA (Py-L) and BA (Py-B) sites (Table 1). As for the present Cu/ZrO2−ZnAl2O4 samples, LA sites mainly originate from surface coordinatively unsaturated Zr4+ cations, whereas BA sites may be correlated with surface Zr−OH species.50−53 To further gain insights into the surface acidity of samples, NH3-TPD was measured. In NH 3-TPD profiles (Figure 5b), there are mainly two desorption peaks below 200 °C, corresponding to desorption of NH3 from the weak acid sites. Moreover, the desorption peaks at approximately 200−450 °C are associated with desorption of NH3 from surface moderate strength acid sites, while the peaks above 450 °C are assigned to desorption of NH3 from surface strong acid sites. On the basis of the integrated areas of desorption peaks, it is clearly found that the incorporation of ZrO2 into composite supports can lead to the increment of the amount of total surface acid sites (Table 1), very consistent with the Py-IR results. XPS measurements were conducted to determine surface electronic structures of copper species on Cu/ZZA-x samples. In all cases, clearly, a Cu 2p 3/2 peak appears at about 932.7 eV (Figure 6a), and no satellite peak between 940 and 945 eV can be found. It is an indicator of the reduction of Cu2+ species to Cu0 and/or Cu+ species.54,55 Further, a broad and asymmetric peak can be obviously seen in Cu LMM XAES of samples (inset in Figure 6a). Deconvolution of the spectra clearly discriminates between Cu0 and Cu+ species on the basis of different kinetic energy (about 916.5 eV for Cu+ and 918.7 eV for Cu0).56,57It is noted that the incorporation of Zr results in an increased surface Cu+/(Cu0+Cu+) ratio (Figure 6b), probably due to the existence of a Cu−O−Zr-like structure at the metal−support interface. Such a change in the proportion of surface Cu+ species is also supported by the above TPR results. The TPR results indicate that the incorporation of ZrO2
surface of ZnAl2O4 spinel. The nitrogen adsorption−desorption measurements of as-synthesized Cu/ZZA-x samples show combined isotherms of types I and IV (Figure 3), reflecting
Figure 3. Low-temperature N2 adsorption−desorption isotherms (a) and pore size distributions (b) of Cu/ZZA-x samples.
the presence of micro/mesoporous structure. Meanwhile, the mesopore volume increased gradually with the increasing Zr/ Zn molar ratio from 0 to 0.2 (Table 1). Noticeably, supported Cu/ZZA-x samples possess high surface areas (>170 m2/g), which would be beneficial to heterogeneous catalytic processes. H2-TPR analysis was performed to investigate the redox behaviors of calcined C-Cu/ZZA-x samples. As displayed in Figure 4, TPR profiles of all samples show a broad and
Figure 4. H2-TPR profiles of C-Cu/ZZA-x samples.
asymmetric reduction peak in the temperature range of 170− 270 °C, indicative of the existence of different surface chemical states of copper species in samples. For C−Cu/ZZA-0 sample, two reduction peaks around 224 and 245 °C can be assigned to surface Cu species with relatively high dispersion and CuO in bulk, respectively. For C-Cu/ZZA-0.1 and C-Cu/ZZA-0.2, the reduction of surface Cu species shifts to lower temperatures of about 208 and 201 °C, respectively, strongly reflecting the enhanced dispersion of surface Cu species in the case of Zrcontaining samples.39,40 In addition, the reduction of bulk CuO also simultaneously shifts to lower temperatures, which can be assigned to the enhancement of the dispersion of CuO in bulk, probably due to the increase in the specific surface area (Table 1). With the further increase in the Zr content, however, the CCu/ZZA-0.3 sample shows an inverse reduction behavior 2285
DOI: 10.1021/acssuschemeng.6b02618 ACS Sustainable Chem. Eng. 2017, 5, 2282−2291
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ACS Sustainable Chemistry & Engineering
Figure 5. (a) Py-IR spectra and (b) NH3-TPD profiles of Cu/ZZA-x samples.
Figure 6. (a) Cu 2p XPS of Cu/ZZA-x samples. (b) Surface Cu+/(Cu0+Cu+) ratio (upper) and the contents of Cu and Zn (down) vs the Zr/Zn molar ratio. The inset in part a shows Cu LMM XAES of Cu/ZZA-x samples.
oxide supports could lead to the formation of Cu+ species in a similar way.58−61,68 Catalytic Performance of Supported Copper Catalysts. Commonly, it is considered that transformation of GVL with pentanol into PV probably goes through complicated multistep reactions including ring-opening of GVL (i),
plays an important role in the metal−support interaction, thus resulting in the enhanced metal dispersion with the increasing Zr content. Consequently, with the increasing introduction of ZrO2, more Cu+ species stabilized by the Cu−O−Zr-like structure can be formed after reduction. Previous studies also suggested that the strong interaction between Cu NPs and 2286
DOI: 10.1021/acssuschemeng.6b02618 ACS Sustainable Chem. Eng. 2017, 5, 2282−2291
Research Article
ACS Sustainable Chemistry & Engineering Table 2. Results of Catalytic Transformation of GVL with Alcohols over Different Samplesa entry
catalysts
alcohol
temp (°C)
conversion (%)
selectivity to valeric esters (%)
TOFb (h−1)
1 2 3 4 5 6 7 8 9 10 11
Cu/ZZA-0 Cu/ZZA-0.1 Cu/ZZA-0.2 Cu/ZZA-0.3 Cu/ZZA-0.2 Cu/ZZA-0.2 Cu/ZZA-0.2 Cu/ZZA-0.2 Cu/ZnO Cu/Al2O3 Cu/ZrO2
pentanol pentanol pentanol pentanol methanol ethanol propanol butanol pentanol pentanol pentanol
250 250 250 250 240 240 250 250 250 250 250
70 80 91 88 98 99 95 93 56 63 69
99 99 99 81c 99 99 98 99 95 98 98
67.1 70.0 76.4 71.9 82.3 83.1 79.7 78.0
a
Reaction conditions: GVL, 4 g; alcohol, 20 mL; catalyst, 0.4 g; H2, 1.0 MPa; reaction time, 10 h. bEvaluated after 1 h reaction. cPentyl-4pentoxypentanoate as main byproduct.
further confirming the advantage of the ZnAl2O4 component in the support. In addition, the stability of the Cu/ZZA-0.2 catalyst was studied through recycling tests in the batch reactor (Figure 7).
hydrogenation/esterification (ii), and dehydration/hydrogenation (iii) (eqs −3).22,23 Among those processes, a series of intermediates like pentenoic acid, 4-hydroxy-pentylvalerate, valeric acid, pentyl pentenoate, and pentyl-4-pentoxypentanoate may be formed. Table 2 shows the GVL conversion and selectivity to pentyl valerate (PV) over different catalysts at 250 °C and 1.0 MPa after reaction for 10 h. Noticeably, Cu/ZrO2−ZnAl2O4 catalysts are catalytically active for the transformation of GVL into PV. As the Zr/Zn ratio increases from 0 to 0.2, the GVL conversion is continuously improved while the PV selectivity remains above 99% (entries 1−3). Especially, the Cu/ZZA-0.2 catalyst exhibits the best selectivity of 99% to date, together with a comparable conversion of 91% (entry 3) with respect to the best Cu-based catalyst previously reported.22 Further incorporation of Zr leads to a decline in the PV selectivity, together with a slight drop of conversion (entry 4). In the present catalytic systems, pentyl-4-pentoxypentanoate is detected as the main byproduct. Among as-formed Cu catalysts, Cu/ZZA-0.2 delivers a highest turnover frequency (TOF) value of 76.4 h−1, indicative of an excellent catalytic activity. Further, the one-pot conversions of GVL with other alcohols (i.e., methanol, ethanol, propanol, and butanol) were also performed using Cu/ZZA-0.2 catalyst (entries 5−8). It is found that the yields of corresponding valeric esters are above 90% in all cases, indicating the broad versatility of the catalyst in the production of valeric biofuels. In comparison, much lower GVL conversions (
