ZrO2 catalysts for hydrogenation of sec-butyl acetate: Structural evolution and catalytic performance

Molecular Catalysis xxx (xxxx) xxxx

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Boron-promoted Cu/ZrO2 catalysts for hydrogenation of sec-butyl acetate: Structural evolution and catalytic performance Xin Li, Haixing Wang, Peiyong Sun, Shenghong Zhang*, Zhilong Yao Beijing Key Laboratory of Enze Biomass Fine Chemicals, Beijing Institute of Petrochemical Technology, Beijing, 102617, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydrogenation sec-butyl acetate Cu catalysts Boron Stability

Cu/ZrO2 catalysts with or without boron modification were prepared, characterized, and evaluated in the hydrogenation of sec-butyl acetate (SBA) to 2-butanol. Cu/ZrO2 catalysts were active for the hydrogenation of SBA but suffered serious deactivation due to the leaching of Cu. Introduction of boron onto Cu/ZrO2 catalysts improved the dispersion of Cu species, the surface Cu+/Cu0 ratios, and the catalyst stability owing to the enhanced interaction between Cu species and support. However, it decreased the intrinsic activity of Cu towards the hydrogenation of SBA and complicated the reaction network by accelerating acid-catalyzed isomerization and transesterification processes as a result of the strengthened acidity by boron. A further kinetic analysis unravelled that the reaction orders for SBA and H2 are 0.58 and 0.83, respectively, and the apparent activation energy is 47 kJ/mol. Such understanding of the structural evolution of CuB/ZrO2 catalysts and the consequent catalytic performances with the B contents may provide insights useful for the design of Cu catalysts with desirable stability for hydrogenation of SBA and other esters.

Introduction

improving the selectivity of 2-butanol [10–12]. The doped catalysts are, however, still susceptible to deactivation [10], showing the declining SBA conversion with reaction time. The deactivation of Cu catalysts is generally related to the sintering of active Cu particles at the elevated temperatures (200–250 ℃) of hydrogenation reactions [13]. The nanoscale Cu particles tend to grow gradually via either Ostwalding ripening or coalescence of small particles [14], as the reaction temperature is comparable with the Hüttig (134 ℃) and Tamman (405 ℃) temperatures of Cu [13], at which atomic migration and crystallite migration can occur, respectively. To prevent the sintering of Cu particles, measures including strengthening Cu-support interaction by adding promoters [15–17], uniformizing Cu particles [18,19], and physical separation [20–23] by confining or encapsulating Cu particles, etc., have been developed for design and fabrication of stable Cu catalysts. For example, the hydrogenation performance of Cu/SiO2 catalysts is greatly promoted by metal oxide dopants such as La [15], Ce [24], Zn [25], Mg [26], and Mn [16] due to the enhanced interaction between Cu and the support, promoted dispersion of Cu species, and/or the optimal rebalance between Cu+ and Cu0 sites induced by the dopants. Besides, nonmetallic boron has also proven effective in dispersing Cu species and suppressing the growth of Cu particles, due to the stronger interaction of Cu with boron than with oxide supports such as alumina and silica [27–30]. The

sec-butyl acetate (SBA) is now commercially produced by direct addition of acetic acid to low valued C4 olefins [1], which contain mainly 1-butene and 2-butene after removal of valuable 1,3-butadiene and isobutene [2]. Catalytic hydrogenation of SBA to 2-butanol and ethanol provides a feasible route to valorize the C4 olefins to useful 2butanol [3], as the latter is an important chemical widely used as emulsifiers, plasticizers, and the starting material to produce versatile methyl ethyl ketone (MEK) via dehydrogenation [4,5]. However, the route is largely limited by the SBA hydrogenation process due to the poor performances and/or short lifespans of the employed catalysts [3]. Developing efficient catalysts for the hydrogenation of SBA is, thus, of great importance for the upgrading of these C4 olefins to high-value products. Cu-based catalysts have been intensively used in the hydrogenation of esters because of their excellent performances in selectively hydrogenating C]O bonds over dissociating CeC bonds [6–8]. Cu/Al2O3 catalysts are active for the hydrogenation of SBA but result in a poor selectivity of 2-butanol, as the medium acidity of alumina facilitates the dehydration of target 2-butanol [9]. Doping Cu/Al2O3 catalysts with alkaline promoters such as Mg, Ca, and Zn reduces significantly the surface acidic sites and the consequent acid-catalyzed products,

⁎

Corresponding author. E-mail address: [email protected] (S. Zhang).

https://doi.org/10.1016/j.mcat.2019.110698 Received 28 August 2019; Received in revised form 29 September 2019; Accepted 24 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Xin Li, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110698

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area (SBET) was estimated from nitrogen adsorption data in the relative pressure range from 0.05 to 0.20 using the Brunauer-Emmett-Teller (BET) method. Catalyst compositions were determined by an X-ray fluorescence (XRF) spectrometer (S4 Explorer, Bruker). Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max2500 diffractometer (Cu Kα, λ =1.5418 Å, 40 kV, 40 mA) at a scanning rate of 4°/min. Carbonaceous species deposited on the used catalyst were quantified by thermogravimetric-differential thermal analysis (TGDTA) on an SDT-650 simultaneous thermal analyzer (TA Instrument) from room temperature to 800 ℃ in air. Temperature-programmed reduction of the calcined catalysts in H2 (H2-TPR) was performed on a DAS-7000 analyzer (Hunan Huasi). Samples were diluted with silica and placed into a quartz tube (I.D. = 6 mm), followed by heating to 800 ℃ in a 10% H2/Ar flow (30 mL/min) at a constant rate of 10 ℃/min. The H2 concentration in the effluent stream was continuously monitored by a thermal conductivity detector (TCD) after removing water by 5A zeolites. The dispersion of surface metallic Cu sites was roughly measured using dissociative N2O chemisorption followed by H2-TPR. The calcined catalyst (100 mg) was first heated in a 10% H2/Ar flow (30 mL/min) to 300 ℃ and further kept isothermal there for 30 min. The reduced sample was cooled down to 40 ℃ and then exposed to a 10% N2O/Ar flow (30 mL/min) for 30 min to oxidize surface Cu species to Cu2O. Finally, the oxidized sample was purged by Ar (40 mL/min) to remove weakly adsorbed N2O until a constant TCD signal was recorded, followed by a second TPR run as described above. The Cu dispersion (D) was calculated as follows: D = 2n2/n1 ×100% [21], where n1 and n2 are the moles of the consumed H2 in the first and second TPR processes, respectively. In addition, the specific surface area of metallic Cu (SCu, m2/g) was also calculated according to the following equation [48]: SCu = 6.4955 × 10−2 × C × D, in which C and D represent the mass content of Cu (%) and Cu0+ dispersion (%), respectively. Temperature-programmed desorption of NH3 (NH3-TPD) was taken on a DAS-7200 TPD analyzer (Hunan Huasi). Typically, catalysts (300 mg) were first treated at 500 ℃ for 1 h in a He flow (40 mL/min) to remove the water and other molecules adsorbed on surface, then cooled to 100 ℃ in the He flow before exposure to NH3 (30 mL/min) for 30 min at the same temperature. Afterward, the NH3-saturated sample was purged by He (40 mL/min) at 100 ℃ until a constant baseline was recorded by the TCD connected to the reactor and finally heated to 800 ℃ at a rate of 10 ℃/min, with the desorbed NH3 continuously monitored by the TCD. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (XAES) were recorded on an Axis Ultra photoelectron spectrometer (Kratos, Shimadzu) equipped with a monochromatic Al Kα Xray (hν =1486.7 eV) source. After reduction by 10% H2/Ar (30 mL/ min) at 300 ℃ for 1 h, the sample was immediately sealed in a centrifuge tube full of the 10% H2/Ar mixture and transferred to the XPS sample holder. XPS analysis was then taken at an emission current of 15 mA under pressure of approximate 5 × 10−9 Torr, and the binding energies were calibrated according to the reference adventitious C 1s peak at 284.8 eV.

aggregation of Cu particles on CaO-Al2O3 mixed oxides was substantially suppressed by the boron dopant in the hydrogenation of SBA for more than 1000 h [3]. The slight deactivation of Cu/B/CaO-Al2O3 catalysts after the long-term reaction was, instead, found to be caused by the partial transformation of surface CaO to calcium acetate and the degradation in the mechanical strength of catalyst extrudates in the presence of steam [3]. Different from CaO-modified alumina, zirconia with high mechanical and (hydro)thermal stability is a promising carrier to load Cu particles [31]. Zirconia supported or fabricated Cu catalysts find many applications in heterogeneous catalysis, such as methanol synthesis [32] and steam reforming [33], ethanol dehydrogenation [34], and hydrogenation of CO2 [35] and oxalates [36]. In the case of CO2 hydrogenation to methanol, the rich oxygen vacancies of zirconia, compared with alumina, promote CO2 adsorption and the consequent methanol formation with hydrogen supplied by spillover from Cu sites [37]. Recent investigations have further revealed that Cu-ZrO2 interfacial sites are crucial for the conversion of formate species as reaction intermediates in the CO2 hydrogenation [38–42]. Cu species incorporated into amorphous zirconia exhibited, indeed, higher activity toward CO2 hydrogenation than those supported on monoclinic or tetragonal ZrO2 [41]. In spite of many efforts dedicated to probing active sites of Cu/ZrO2 catalysts, few studies have concerned the stability of Cu/ZrO2 catalysts in the hydrogenation reactions [43–46]. Herein, we examine the performances of Cu/ZrO2 catalysts in the hydrogenation of SBA to 2-butanol, as well as the promotion effect of boron on the catalyst stability. The resulting catalysts are characterized by comprehensive techniques to depict the structural evolution of surface Cu species with the B contents, which is further related to the intrinsic activity of Cu catalysts, the catalyst stability, and the reaction network. In addition, the kinetics of hydrogenation of SBA over CuB/ZrO2 catalysts is briefly analyzed to understand the reaction mechanism. Experimental Catalyst preparation Monoclinic zirconia (m-ZrO2) was prepared through the hydrothermal method described by Li et al. [47] Zirconium nitrate pentahydrate (Zr(NO3)4·5H2O, Aladdin Biochemical) was first dissolved in water to produce a solution of 0.4 mol/L, followed by addition of Urea (CO(NH2)2, Sinopharm Chemical) at a urea/Zr4+ molar ratio of 10. The above mixture was then sealed in a Teflon-lined stainless-steel autoclave and treated at 160 ℃ for 20 h under autogenous pressure. The resulting precipitate was separated by filtration, washed thoroughly with water, dried at 110 ℃ overnight, and finally heated to 400 ℃ in a flow of air (50 mL/min) and kept there for 4 h. Cu/ZrO2 catalysts were prepared by incipient wetness impregnation of the support with a solution of copper nitrate trihydrate (Cu (NO3)2·3H2O, Aladdin Biochemical) in ethanol. The mixture was sonicated for 30 min and dried at 60 ℃ before calcination at 500 ℃ for 4 h in air. As for the boron-doped Cu/ZrO2 catalysts, the dried Cu/ZrO2 precursors were further impregnated, with multiple impregnation cycles in case of high B loadings if required, with an ethanol solution of ammonium pentaborate tetrahydrate (NH4B5O8·4H2O, Aladdin Biochemical), dried, and calcined similarly. The resulting solid was ground to a fine powder and reduced in a flow of 20% H2/N2 mixture (30 mL/min) at 300 ℃ for 2 h before use. The finally obtained catalyst is denoted as CuxBy/ZrO2 for simplicity, where x and y represent the mass loadings (%) of Cu and B referred to zirconia in a recipe, respectively.

Hydrogenation of sec-butyl acetate Hydrogenation of SBA was carried out in a 50-mL high-pressure vessel made of stainless steel. In a typical run, 0.2 g catalyst, 5 mL SBA, and 20 mL tetrahydrofuran (THF) were placed into the vessel, followed by successive purges with 0.5 MPa N2 for three times to remove O2 and 0.5 MPa H2 for another three times to remove N2. The reactor was then charged with 2 MPa H2 and heated to 240 ℃ in 30 min with vigorous stirring. After reaction for 4 h, the mixture was cooled quickly to room temperature in a water bath, sampled, and filtered by a syringe filter (5 μm, Durapore™ PVFD membrane) for analysis. The products were carefully identified by gas chromatography-mass spectrometry (Shimadzu GCMS-QP2010 Plus, seen in Fig. S1) and then quantified by

Catalyst characterization Nitrogen adsorption-desorption isotherms were measured at -196 ℃ using an Autosorb-iQ analyzer (Quantachrome). The specific surface 2

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a GC (Shimadzu GC-2014) equipped with a capillary column (Supercowax™ 10, 30 m × 0.53 mm ×1 μm) and a flame ionization detector (FID). The conversion (X) of SBA and selectivities (S) of products including 2-butanol, ethanol, MEK, n-butanol, n-butyl acetate, and C4 hydrocarbons were calculated as follows:

Table 1 Physiochemical properties of CuxBy/ZrO2 catalysts with different Cu (2.5–20%) and B (0–2%) loadings. Catalysts

X (SBA) = ∑ni/(nSBA + ∑ni) × 100% Cu2.5/ZrO2 Cu5/ZrO2 Cu7.5/ZrO2 Cu10/ZrO2 Cu12.5/ZrO2 Cu15/ZrO2 Cu20/ZrO2 Cu20B0.5/ZrO2 Cu20B1/ZrO2 Cu20B1.5/ZrO2 Cu20B2/ZrO2

Si = ni/∑ni × 100% where nSBA and ni represent the moles of SBA and an individual C4 product in the reaction mixture. The selectivities of C2 products, which were composed of ethanol (> 98%), ethyl acetate (⁓1%), and trivial acetic acid, were ignored for simplification in the following discussion. All data in this work were carefully checked (and repeated if needed) to make a carbon balance within 100 ± 5%. Hydrogenation of SBA was also performed in a continuous flow reactor (Φ10 mm × 600 mm) to investigate the reaction kinetics. CuB/ ZrO2 catalysts (2 g) with pellet sizes of ca. 0.38-0.83 mm were diluted by inert glass beads (O.D. = 0.5 mm) and loaded into the reactor between two glass bead zones. The catalysts were first reduced by 20% H2/N2 at 300 ℃ for 2 h prior to the introduction of reactants at the desired temperatures. The SBA conversions were strictly controlled below 10%, with the temperature varied from 200 to 250 ℃, H2 pressure from 2 to 4 MPa, weight hourly space velocity (WHSV) of SBA from 1 to 5 h−1, and the H2/SBA molar ratios from 5 to 20. The rate equation was given by a power law as r = kpα SBApβ H2, and the orders to SBA (α) and H2 (β) were determined using the logarithmic plots of rates to the partial pressures of SBA and H2, respectively.

Cu contenta

SBET

SCub

Db

dCuc(nm)

(wt %)

(m2/g)

(m2/g)

(%)

XRD

N2O

2.0 4.6 6.3 8.3 10.2 12.3 16.5 16.1 15.6 15.4 15.1

69.7 74.7 83.2 82.5 80.0 73.5 71.5 63.0 61.6 53.9 42.8

4.1 8.5 8.4 7.9 8.2 8.2 8.9 9.1 10.2 10.4 9.5

31.3 26.8 19.4 13.5 11.2 9.1 7.4 8.7 10.0 10.4 9.7

– 8.9 12.7 16.7 21.8 24.6 30.0 26.3 24.0 21.4 12.1

3.2 3.7 5.2 7.4 8.9 11.0 13.4 11.5 9.9 9.6 10.3

a

Mass content determined by XRF. The surface area (SCu) and dispersion (D) of Cu0 measured by N2O chemisorption. c The average Cu crystallite size (dCu) estimated by the Scherrer equation and N2O chemisorption. b

report by Li et al. [47] Loading 2.5% Cu on ZrO2 led to an identical XRD pattern to that of the support, suggesting that Cu species were highly dispersed on the surface of Cu2.5/ZrO2 catalyst. However, additional small diffraction peaks at 2θ of 43.3° and 50.5°, which correspond to Cu metal (JCPDS 04-0836), are visible for the catalyst with a Cu loading of 5%. The two peaks become stronger as the Cu loading increases further from 5% to 20%, indicating the formation of large Cu particles on the catalysts with high Cu loadings. The average Cu crystallite size estimated by the Scherrer equation increased consequently from 8.9 to 30.0 nm (Table 1). These XRD results are consistent with the textural properties of m-ZrO2 catalysts discussed below. The specific surface area of metallic, the dispersion of Cu, and the average Cu particle size determined from N2O titration are listed in Table 1. The Cu0 surface area increased initially with the Cu contents but fluctuated slightly around 8.4 m2/g with Cu loadings at 5% or above; While the Cu dispersion declined continuously from 31.3% to 7.4% as the Cu loading increased from 2.5% to 20%. Accordingly, the average Cu particle size calculated from the Cu dispersion increased from 3.2 nm to 13.4 nm, which were surprisingly smaller than those estimated from XRD data. It should be pointed out that the results from N2O titration are more reliable since all surface Cu0 species are theoretically detectable by the method. The XRD technique, by contrast, is generally able to provide information about well-crystallized phases with particles sizes above 4 nm but fails to detect crystallites smaller than it. The apparent discrepancy between the values determined by XRD technique and those by N2O titration suggests a broad particle size distribution of Cu species, probably with the coexistence of highly dispersed Cu species and Cu crystallites on Cu/ZrO2 catalysts. Such a proposition is verified by TPR profiles of the calcined Cu/ ZrO2 catalysts shown in Fig. 2. The main peak at 120 ℃ (α peak), along with a shoulder at ca. 137 ℃ (β peak), appears in the TPR profile of Cu2.5/ZrO2, corresponding to the reduction of Cu2+ ions incorporated into zirconia and dispersed CuO clusters weakly bound on the support surface, respectively [49]. The lower temperature of α peak reflects better reducibility of the Cu2+ ions incorporated in an octahedral environment, which has been attributed to the promotion from zirconia with abundant surface oxygen vacancies [50]. As the Cu loading increases, an additional peak at 189 ℃ (γ peak) is observable for Cu5/ ZrO2 catalyst and assigned to the reduction of CuO crystallites. The catalysts with Cu loadings at 10% or above exhibit the similar H2-TPR feature to Cu5/ZrO2, showing β and γ peaks at 138 ℃ and ca. 192 ℃, respectively. It is interesting to find that the β peak changes slightly from 138 ℃ to 140 ℃ as the Cu loading increases from 2.5% to 20%, which is closely related to the nature of highly dispersed CuO species

Results and discussion Structures of Cu/ZrO2 catalysts Fig. 1 shows the XRD patterns of the reduced Cu/ZrO2 catalysts with different Cu loadings. The dominant diffraction peaks at 2θ of 24.1°, 28.1°, 31.3°, 34.2°, and 50.1° are the characteristic feature of m-ZrO2 (JCPDS 37–1484) crystal phase. There was no diffraction peak identified for tetragonal ZrO2 (t-ZrO2), implying the absence of t-ZrO2 structure in the as-prepared zirconia, in accordance with the previous

Fig. 1. XRD patterns of Cu/ZrO2 catalysts with different Cu loadings (0–20 wt %). 3

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Fig. 3. XRD patterns of Cu20By/ZrO2 catalysts with different B loadings (0–2 wt %).

Fig. 2. H2-TPR profiles of Cu/ZrO2 catalysts with different Cu loadings (2.5–20 wt%).

interacting weakly with zirconia. By contrast, the γ peak broadens and shifts to higher temperature from 189 ℃ to 201 ℃ because of the increasing crystallinity of CuO. The disappearance of α peak with an increase in the Cu loadings, together with the appearance of γ peak, suggests the structural evolution of Cu species from the incorporated Cu2+ ions and highly dispersed CuO to the coexistence of CuO clusters and crystallites on the calcined Cu/ZrO2 catalysts. A further quantitative analysis by deconvolution of these TPR peaks (summarized in Table S1) identified the contribution of different CuO species. The percentage of Cu2+ ions incorporated into zirconia decreased from 61% to zero with increasing the Cu loading from 2.5% to 15%; While the content of dispersed CuO species oscillated roughly around 36% at low Cu loadings and then declined as the Cu loading increased above 10%. By contrast, the content of crystalline CuO increased monotonously up to 80% with the Cu loadings. Similar results have been reported by Águila et al. [51] that an increase in Cu loadings favoured the formation of large CuO particles with the concentration of highly dispersed Cu species remaining almost constant. After reduction by H2, the coexistence of dispersed Cu species and Cu crystallites seemed to be preserved, in line with the structures of Cu/ ZrO2 catalysts identified by high-resolution transmission electron microscopy (HRTEM) in the work of Pakharukova et al. [52] The H2-TPR results infer a bimodal particle size distribution of metallic Cu on the reduced Cu/ZrO2 catalysts, explaining well the disagreement between the XRD and N2O titration data discussed above.

Fig. 4. H2-TPR profiles of Cu20By/ZrO2 catalysts with different B loadings (0–2 wt%).

ZrO2 integrated the β and γ reduction peaks and shifted further the integrated peak to higher temperature with increasing the B loading from zero to 2%. Considering the decreasing crystallinity of CuO with the B contents (Fig. S2), the shift of H2-TPR peaks towards high temperature is unambiguously indicative of the enhanced interaction between CuO species and support in CuB/ZrO2 catalysts. Similar results [29] have also been reported for boron modified Cu/SiO2 catalysts and interpreted by the incorporation of small B3+ species into CuO structures, which restrains consequently the nucleation of Cu species during calcination process and the following reduction by H2. Another evidence for the enhanced interaction between Cu and support in CuB/ZrO2 catalysts is the electron transfer from metallic Cu to boron, as revealed by Cu 2p XPS and Cu LMM XAES shown in Fig. 5. Two intensive peaks at binding energies (BE) of 932.5 and 952.3 eV are observable for both Cu/ZrO2 and CuB/ZrO2 catalysts, assigned to Cu 2p3/2 and Cu 2p1/2 peaks of the reduced Cu0/Cu+ species [27], respectively. The absence of a shakeup satellite at ca. 943 eV indicates that all Cu2+ species has been thoroughly reduced to Cu0 or Cu+ species during the H2 treatment at 300 ℃。However, Cu0 and Cu+ species cannot be discriminated from each other by XPS spectra because of their similar BE values. The Cu LMM XAES, shown in Fig. 5(b), was thus employed to distinguish the two species. The broad and asymmetrical Auger kinetic energy peak was deconvoluted into two symmetrical peaks at around 569.5 and 572.6 eV, respectively, corresponding to Cu0 and Cu+ species in sequence [53]. According to the deconvolution

Structures of CuB/ZrO2 catalysts To clarify the role of boron on the structure of Cu/ZrO2, the Cu20/ ZrO2 catalyst with a relatively high Cu loading was selected and modified by boron and their XRD patterns are displayed in Fig. 3. The diffraction peak at 2θ of 43.3° corresponding to Cu metal attenuates obviously with increasing the B loading from zero to 2%, indicating the decreasing crystallinity of Cu particles with the B contents. Meanwhile, the Cu dispersion measured by N2O titration increased from 7.5% to 10.4% and then declines to 9.7% (Table 1). These data verified the positive effect of boron in improving the dispersion of Cu species, although the Cu dispersion tended to decline at higher B contents due to the lowered accessibility of metallic Cu species surrounded by excessive boron [28–30]. The promoted Cu dispersion of CuB/ZrO2 catalysts originated probably from the enhanced interaction between Cu species and zirconia by boron, which was strongly supported by the H2-TPR profiles of CuB/ZrO2 catalysts plotted in Fig. 4. Introduction of boron to Cu20/ 4

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Fig. 5. (a) Cu 2p XPS and (b) Cu LMM XAES of Cu/ZrO2 and CuB/ZrO2 catalysts.

results, the molar ratio of surface Cu+/Cu0 in the reduced catalysts increased significantly from 1.01 for Cu20/ZrO2 to 1.52 for Cu20B1/ ZrO2. The result was considered to be caused by the partial electron transfer from Cu species to boron with a relatively high affinity for electrons [27–30], serving as solid evidence for the enhanced interaction between Cu species and catalyst support by boron. Performances of Cu/ZrO2 catalysts Catalytic hydrogenation of SBA over Cu/ZrO2 catalysts was carried out in a batch reactor with the SBA conversions controlled generally below 20% to approach a kinetically controlled regime. Under such conditions, the SBA conversion rates (r) were expressed in terms of turnover frequencies (TOF) by normalizing activities to per surface metallic Cu sites determined by N2O chemisorption (mol-SBA/s/molCusur or s−1). The measured turnover rates and selectivities are summarized in Table 2. The rates of Cu/ZrO2 catalysts decreased gradually from 6.9 × 10-6 to 4.2 × 10-6 s−1 with increasing the Cu loading from 2.5% to 20%. Considering the structural transformation of Cu from highly dispersed Cu species to Cu crystallites with increasing the Cu loadings, small Cu particles proved more active for the SBA hydrogenation than the large ones. Catalytic hydrogenation of SBA over Cu/ZrO2 catalysts produced 2butanol, ethanol, MEK, C4 hydrocarbons and other acid-catalyzed byproducts such as ethyl acetate, acetic acid, n-butanol, and n-butyl acetate. As ethanol dominated always the C2 products with a selectivity higher than 98%, special attention was paid to the C4 products and their selectivities are listed in Table 2. Regardless of Cu loadings, 2-butanol and MEK were the main C4 products accounting for more than 95% selectivities. Specifically, the selectivity of 2-butanol declined monotonically from 90.0% to 83.8% with increasing the Cu loading from 2.5% to 20%; While a reversed trend from 7.6% to 11.3% was observed for the selectivity of MEK. MEK is the primary dehydrogenation product of 2-butanol over Cu catalysts [54], so its presence reflected a balance

Fig. 6. Catalytic performance of Cu7.5/ZrO2 catalysts in the five consecutive cycles of SBA hydrogenation reaction (0.3 g catalysts, 25 mL 10% SBA/THF, 240 ℃, 2 MPa H2, 4 h).

between 2-butanol dehydrogenation and MEK hydrogenation, even under the hydrogen-rich reaction conditions. Stability of Cu/ZrO2 catalysts for the SBA hydrogenation was tested in five consecutive reactions and the results are illustrated in Fig. 6. Despite the almost constant selectivities of 2-butanol and MEK, the SBA conversion deteriorated rapidly from 31.8% for the first run to 14.3% for the fifth run, indicating a 55% loss in catalyst activity after the five successive runs. Catalyst deactivation can be caused by sintering, leaching or blocking of active sites [55]. To identify the main reason for the deactivation, the used catalyst was characterized in detail by XRD, XRF, and TG-DTA techniques. The identical XRD pattern of the used catalyst to that of fresh one (Fig. S3) excluded the sintering of Cu particles during the reaction. In addition, carbonaceous species

Table 2 Catalytic performances of CuxBy/ZrO2 catalysts with different Cu (2.5–20 wt%) and B (0–2 wt%) loadings in the hydrogenation of SBA. Catalysts

Cu2.5/ZrO2 Cu5/ZrO2 Cu7.5/ZrO2 Cu10/ZrO2 Cu12.5/ZrO2 Cu15/ZrO2 Cu20/ZrO2 Cu20B0.5/ZrO2 Cu20B1/ZrO2 Cu20B1.5/ZrO2 Cu20B2/ZrO2

Conversion

Rate

Selectivity (%)

(%)

(10−6 s-1)

2-butanol

MEK

n-butanol

n-butyl acetate

C4 hydrocarbons

23.9 23.5 23.9 20.8 18.3 21.9 19.7 14.5 10.4 8.9 6.8

6.9 6.1 5.7 5.5 4.7 4.9 4.2 2.6 1.8 1.4 1.1

90.0 89.8 86.8 86.1 86.0 86.3 83.8 81.6 84.0 77.6 53.8

7.6 8.1 8.6 9.7 8.9 9.8 11.3 8.6 7.9 6.7 4.8

0.2 0.3 0.6 0.5 0.5 0.4 0.7 1.7 2.0 5.3 16.9

0.6 0.6 1.2 1.2 1.3 0.8 1.4 7.6 6.0 10.2 24.3

1.6 1.3 2.8 2.5 3.3 2.7 2.8 0.5 0.1 0.2 0.2

Conditions: 10% SBA/THF (V/V), T = 240 ℃, p(H2) = 2 MPa. 5

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promotion could be attributed to the stabilization effect of boron on structures of Cu/ZrO2 catalysts. Previous characterization results have demonstrated that the interaction between Cu species and supports was greatly strengthened after the introduction of boron to Cu/ZrO2 catalysts. As a result, both the leaching of Cu species and the growth of Cu particles were substantially inhibited by the boron dopant, yielding the observed promotion on the catalyst stability in the hydrogenation of SBA. Despite the positive effect of boron on the catalyst stability, the activity of Cu/ZrO2 for the hydrogenation of SBA was significantly lowered by the boron dopant, as listed in Table 2. The turnover rate decayed from 4.2×10−6 to 1.1 × 10−6 s-1 with increasing the B content from zero to 2%. Similar trend has also been observed in the hydrogenation of dimethyl oxalate on the boron modified Cu/SiO2 catalysts and interpreted in terms of the surface distribution of Cu0 and Cu+ sites [27]. Hydrogenation of esters over Cu catalysts has been unravelled to proceed via a synergistic mechanism involving both Cu0 sites to dissociate the adsorbed H2 and Cu+ sites to stabilize the activated acyl species [53,56,57]. The two sites compete against each other to exert a dominant effect on the catalyst activity, e.g., the rate of Cu/ SiO2 catalysts is proportional to the accessible Cu0 surface area in the hydrogenation of dimethyl oxalate when the Cu0 sites are insufficient [53]. A similar explanation may also apply to the case discussed here. Considering the aforementioned enrichment of surface Cu+ species by boron, it can be expected that increasing the B content would increase the surface Cu+/Cu0 ratios and aggravate the imbalance between Cu+ and Cu0 species. Under such conditions, adsorption and activation of acyl species over excessive Cu+ sites would have reduced significantly the accessibility of Cu sites, competed with and influenced the H2 dissociation over Cu0 sites, thus leading to insufficient H2 decomposition and the consequent lower reaction rates. Besides activity, reaction selectivity was also largely affected by the boron dopant, as shown in Table 2. The selectivities of 2-butanol and MEK decreased from 83.8% and 11.3% to 53.8% and 4.8%, respectively, with increasing the B content from zero to 2%. As the primary hydrogenation product of SBA, 2-butanol can be further converted to MEK via dehydrogenation catalyzed by metallic Cu sites. The decline in both selectivities, therefore, indicated a deteriorating (de)hydrogenation ability of CuB/ZrO2 catalysts with the B contents, in line with the improved Cu+/Cu0 ratios by boron (Fig. 5(b)). Different from 2-butanol and MEK, the selectivities of n-butanol and n-butyl acetate increased continuously from 0.7% and 1.4% to 16.9% and 24.3%, respectively. nbutanol is an isomer of 2-butanol and derives probably from 2-butanol via acid-catalyzed isomerization. The hypothesis was supported by the formation of a considerable amount of n-butanol in the additional experiment using 2-butanol as reactants under the identical reaction conditions (Fig. S8). Moreover, the formed n-butanol can be further converted to n-butyl acetate through transesterification with SBA. There is a consensus that both isomerization and transesterification processes depend closely on the strong acidity of catalysts. Therefore, the boost of n-butanol and n-butyl acetate in the hydrogenation of SBA was an indicator of the enhancive acidity of CuB/ZrO2 catalysts with the increasing B loadings, which was indeed verified by the NH3-TPD profiles shown in Fig. S9. An additional broad NH3 desorption peak centered at ca. 330 ℃ appears in the NH3-TPD profile of Cu20B0.5/ZrO2 catalyst, suggesting the presence of moderately strong acidic sites on the B-doped catalyst. Its further shift to higher temperature implies strengthened catalyst acidity with the B contents, in line with the rising selectivities of acid-catalyzed products such as n-butanol and n-butyl acetate.

Fig. 7. Catalytic performance of Cu20B1/ZrO2 catalysts in the five consecutive cycles of SBA hydrogenation reaction (1.0 g catalysts, 25 mL 10% SBA/THF, 240 ℃, 2 MPa H2, 4 h).

deposited on the used catalyst, which was quantified by TG-DTA technique (Fig. S4), was as low as 3.4%, suggesting the less contribution of site blocking to the observed deactivation. The Cu content of Cu7.5/ ZrO2 catalyst determined by XRF, however, dropped sharply from 6.3% to 3.2% after the consecutive runs, giving a Cu leaching of 49% (Table 2). Cu10/ZrO2 catalysts also showed a comparable Cu leaching of 52% under identical reaction conditions, consistent with the serious leaching of Cu reported previously in the hydrogenation of levulinic acid over Cu/ZrO2 catalysts [43]. However, the leached Cu species in the solution were inactive for the hydrogenation of SBA, which was confirmed by the stable SBA conversions with the prolonged reaction time after removing Cu/ZrO2 catalysts by filtration (Fig. S5). Taken together, the leaching of Cu caused probably by the weak interaction between Cu species and zirconia should be mainly responsible for the measured loss of activity. Promotion of boron on performances of Cu/ZrO2 catalysts The durability of Cu/ZrO2 catalyst for the hydrogenation of SBA was remarkably improved by the boron dopant, as shown in Fig. 7. Both SBA conversion and 2-butanol selectivity changed slightly during the five consecutive reactions, remaining approximately 11.3% and 83%, respectively. In addition, XRD examination of the used Cu20B1/ZrO2 catalyst confirmed its phase structure identical to that of the fresh one (Fig. S6), and the TG-DTA curves of the used catalyst (Fig. S7) showed a similar amount of carbonaceous deposits to its counterpart without boron modification (Fig. S4). Most importantly, the leaching of Cu was greatly suppressed by boron, giving a loss of only 4.3% in the Cu content of Cu20B1/ZrO2 catalysts after the five successive cycles of reaction (Table 3), much less than that of Cu7.5/ZrO2 catalysts (49%). The promoting effect of boron on the stability of Cu catalysts can be seen more clearly in comparing the Cu leaching of Cu10B1/ZrO2 (3.9%) and Cu10/ZrO2 (52%) catalysts with the same Cu loading (Table 3). The Table 3 Changes in the Cu contents of CuxBy/ZrO2 catalysts after five consecutive reactions. Catalysts

Cu content (wt %)

Cu leaching

Fresh cat.

Used cat.

(%)

6.3 8.3 7.7 15.6

3.2 4.0 7.4 14.9

49 52 3.9 4.3

Reaction network and kinetics Cu7.5/ZrO2 Cu10/ZrO2 Cu10B1/ZrO2 Cu20B1/ZrO2

The product distribution as a function of reaction temperature, H2 pressure, catalyst dosage, and reaction time are illustrated in Fig. S10. Generally, 2-butanol dominated overwhelmingly in the products but 6

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Fig. 8. Proposed reaction network for hydrogenation of sec-butyl acetate over CuB/ZrO2 catalysts.

Ea value for hydrogenation of isopropyl acetate on CuZn/K-SiO2 (41 kJ/ mol) [60] and that of ethyl acetate on Cu/SiO2 (62 kJ/mol) [58]. Such similarity implies that the hydrogenation of SBA on Cu catalysts may involve a similar mechanism to that of ethyl acetate. That is, Cu+ sites are responsible for the rate-determining dissociative adsorption of SBA molecules and the following stabilization of isobutyl and acyl species, while Cu0 sites are crucial for the decomposition of H2 [53]. Nevertheless, the intrinsic performance of Cu/ZrO2 catalysts can be largely regulated by the boron dopant, as discussed above, owing to the enhanced interaction between Cu species and support.

tended to convert to MEK at high temperature and low H2 pressures; While the product distribution seemed to be independent of the catalyst dosage and reaction time. The observed product distribution under different reaction conditions, together with the responses of selectivity to CuB/ZrO2 catalysts with distinct structures and acidities, allows identification of different side reactions and a proposition of the reaction pathway. As illustrated in Fig. 8, hydrogenation of SBA over CuB/ ZrO2 catalysts follows a sequential and parallel reaction network. Primary hydrogenation of SBA produces 2-butanol and ethanol. The produced 2-butanol then undergoes dehydrogenation to MEK or dehydration to butenes (releasing water), which can be further hydrogenated to butanes. In parallel to the consecutive hydrogenation reactions, acidcatalyzed side reactions complicate the product distribution. n-butanol is presumed to derive from the isomerization of 2-butanol on acidic sites, and it can react with SBA via transesterification to form n-butyl acetate and 2-butanol. Note that the acid-catalyzed processes such as hydrolysis, dehydration, esterification, and transesterification compete always against the Cu-catalyzed (de)hydrogenation reactions and the balance between them depends closely on the intrinsic properties of the employed catalysts. The kinetics for the hydrogenation of SBA over CuB/ZrO2 catalysts was also investigated in a purge flow reactor within a regime without obvious mass diffusion limitations. The influence of each reactant was examined by changing its partial pressure at an approximately constant pressure of the other, where the space velocity of the reactants remained roughly the same. Dependences of the measured rates on the SBA and H2 partial pressures are plotted in Fig. 9. Both SBA and H2 pressures had positive effects on the rates, and the determined orders for SBA and H2 were 0.58 and 0.83 (Table 4), respectively, in the temperature range from 200 to 250 ℃. The data clarify that H2 has a relatively strong influence on the hydrogenation of SBA catalyzed by CuB/ZrO2, similar to the reported results for hydrogenation of ethyl acetate on Cu/SiO2 [58] and Cu/ZrO2 catalysts [59]. For example, the reaction order with respect to ethyl acetate was reported by Schittkowski et al. [59] to be roughly zero, much lower than that of H2 (0.2), for the hydrogenation of ethyl acetate on Cu/ZrO2 catalysts. Fig. 10 further presents the conversion rates of SBA as a function of temperature, with the estimated apparent activation energy (Ea) listed in Table 4. The Ea derived from the Arrhenius plot was 47 kJ/mol for the hydrogenation of SBA on CuB/ZrO2 catalysts, comparable with the

Conclusions Cu/ZrO2 catalysts with or without boron modification were prepared by impregnation, characterized in terms of structural evolution of surface Cu species, and evaluated in the hydrogenation of SBA to 2butanol. Cu/ZrO2 catalysts were active for the hydrogenation of SBA but suffered serious deactivation due to the leaching of Cu. Introduction of boron onto Cu/ZrO2 catalysts improved the Cu dispersion and enriched the surface Cu+ species due to the enhanced interaction between Cu species and catalyst supports and the electron transfer from Cu to boron. As a result, the leaching of Cu was greatly suppressed, yielding a durable CuB/ZrO2 catalyst for the hydrogenation of SBA. The intrinsic activity of Cu catalyst, however, was found to decline continuously with increasing the B content, and the reaction products were complicated by the strong acidity brought by boron. In addition, kinetical studies showed that the hydrogenation of SBA over CuB/ZrO2 catalysts proceeds with the reaction orders of 0.58 and 0.83, respectively, for SBA and H2 and an apparent activation energy of 47 kJ/mol. These results, especially the structural origin behind the promotion of boron on the hydrogenation performance of Cu/ZrO2 catalysts, should be beneficial to the design of stable Cu catalysts for hydrogenation of SBA and other esters. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant 21703012), the Scientific Research Project of Beijing Municipal Education Commission (KM 201910017010), and the Construction of Scientific Research Platform of BIPT (2018XK002).

Fig. 9. Dependences of the SBA conversion rates on partial pressures of (a) SBA and (b) hydrogen (Reaction conditions: (a) 230 ℃, p(H2) = 1.7–1.9 MPa, WHSV =4 h−1) and (b) 230 ℃, p(SBA) = 0.2 MPa, WHSV =4 h−1). 7

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Table 4 Comparison of the SBA hydrogenation with other reactions in kinetic parameters. Catalysts

Reactants

Temperature

Ea

Reaction orders

( C)

(kJ/mol)

Esters

H2

200-250 250-280 230-300 190-230 273-302

47 41 62 74 62

0.58 0.82 0.5 0 0.67

0.83 – 0.66 0.2 0.25

o

CuB/ZrO2 CuZn/K-SiO2 Cu/SiO2 Cu/ZrO2 CuZn/Al2O3

iso-butyl acetate iso-propyl acetate Ethyl acetate Ethyl acetate Butyl butyrate

Refs.

This work [60] [58] [59] [61]

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Fig. 10. Temperature dependence of the SBA conversion rates for Cu20B1/ZrO2 catalysts (200–250 ℃, 2 MPa, H2/SBA = 10/1 (mol/mol), WHSV = 4 h−1).

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