Research Article pubs.acs.org/acscatalysis
Sorbitol Hydrogenolysis over Hybrid Cu/CaO-Al2O3 Catalysts: Tunable Activity and Selectivity with Solid Base Incorporation Xin Jin,† Jian Shen,§ Wenjuan Yan,†,‡ Meng Zhao,† Prem S. Thapa,∥ Bala Subramaniam,†,‡ and Raghunath V. Chaudhari*,†,‡ †
Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States Department of Chemical and Petroleum Engineering, University of Kansas, 1530 West 15th Street, Lawrence, Kansas 66045, United States § Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States ∥ Microscopy and Analytical Imaging Laboratory, University of Kansas, Haworth Hall, 1200 Sunnyside Avenue, Lawrence, Kansas 66045, United States
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‡
S Supporting Information *
ABSTRACT: We report for the first time the performance of hybridized Cu/CaO-Al2O3 catalysts for aqueous-phase hydrogenolysis of sorbitol to ethylene glycol (EG), 1,2-propanediol (1,2-PDO), and 1,2-butanediol (1,2-BDO) with linear alcohols as coproducts in a base-free liquid phase. These supported Cu catalysts with solid bases as promoters show significant activity for C−C cleavage and high selectivity (∼84%) to glycols and linear alcohols. The effects of Cu loading, catalyst pretreatment conditions, H2 pressure, and temperature on activity and selectivity of Cu/CaO-Al2O3 catalysts were investigated. The strong interaction between Cu and Ca2+ cations in the solid support is found to facilitate C−C and C−O cleavage of sorbitol, as evidenced from TEM, SEM, and TPR studies of the catalysts. Surface characterization and activity tests further suggest that CaxCuyAlzOp (Phase I) promotes dehydrogenation and isomerization reactions, whereas spinal CuAl2O4 (Phase II) species facilitates hydrogenation reactions. In addition, the overall activity and selectivity of the Cu catalysts may be easily tuned by the Cu/Ca2+ molar ratio and catalyst preparation conditions. Cu/CaO-Al2O3 catalysts also give higher overall yields of value-added glycols (63−82%) for facile conversion of various other sugar polyols such as glycerol (C3), erythritol (C4), xylitol (C5), and mannitol (C6) under similar reaction conditions. A surface reaction mechanism involving the formation of β-ketoses on multifunctional Cu−Ca2+ sites is proposed. KEYWORDS: sorbitol, hydrogenolysis, glycols, copper catalyst, solid base promoters
1. INTRODUCTION Hydrogenolysis of biomass-derived feedstocks (e.g., sugars and polyols) has been extensively studied in the past decade for developing sustainable technologies for fuels and chemicals.1,2 In this context, catalytic conversion of cellulosic and hemicellulosic biomass to fuel-grade compounds and value-added chemicals has received increasing attention. In a typical process scheme, cellulose and hemicellulose can be converted to sugarbased polyols via hydrolysis-hydrogenation route.3,4 The sugar polyols, including erythritol (C4), xylitol (C5), sorbitol (C6), and mannitol (C6), can be further converted to various useful products that are currently produced from fossil-based raw materials. In particular, sorbitol is commercially produced by catalytic hydrogenation of glucose and hence is a promising platform chemical. Hydrogenolysis or hydrodeoxygenation (HDO) of sorbitol can produce valuable chemicals such as ethylene glycol (EG), 1,2-propanediol (1,2-PDO), 1,2butanediol (1,2-BDO), and lower aliphatic alcohols.5−12 © 2015 American Chemical Society
These products are known commodities with several downstream applications in everyday products such as renewable plastics, paints, antifreeze, and pharmaceuticals. HDO of sorbitol has been studied previously using supported metal catalysts.7,13−16 However, one of the longstanding challenges is the development of cost-effective catalysts with high activity and selectivity. Sohounloue and co-workers reported Ru/SiO2 and Raney Ni catalysts for HDO of sorbitol and proposed a retro-aldol mechanism (C−C cleavage) on the basis of the observation of enhanced catalyst activity and the formation of 1,2-PDO and EG in a strong alkaline medium (pH > 12).13 Montassier et al.17 proposed that C−C bond cleavage by retro-Michael rather than retro-aldol mechanism was the main step on Ru/C catalysts because Received: June 24, 2015 Revised: September 22, 2015 Published: September 24, 2015 6545
DOI: 10.1021/acscatal.5b01324 ACS Catal. 2015, 5, 6545−6558
Research Article
ACS Catalysis
reaction mechanism involving Cu−Ca2+ interaction is proposed.
glycerol was the major product during sorbitol conversion. However, Wang et al.18 later observed that C−C cleavage follows a retro-aldol mechanism by using various diols as model compounds with NaOH as a base promoter.19 Clark and coworkers reported that HDO of sorbitol over kieselguhr supported Ni catalysts with CaO as a base promoter gave EG, 1,2-PDO and glycerol yields of 16%, 17%, and 40%, respectively, at 215 °C and 14 MPa hydrogen pressure (PH2).20 Recently, Zhao and co-workers investigated Ru/carbon nanofiber (CNF) and graphite felt composite catalysts for HDO of sorbitol at 220 °C and 7 MPa hydrogen, reporting 68% conversion and an overall glycol (EG and 1,2-PDO) selectivity of approximately 53%.5,6 It was found that the activity of Ru catalysts was enhanced when Ca(OH)2, instead of NaOH, was used as an alkali promoter. We recently reported that bimetallic RuRe/C showed improved performances during sorbitol HDO over monometallic Ru/C catalysts when MgO was added as a promoter, with selectivity to glycols in the range of 45−53% at 230 °C.15 In another study, Banu et al.7 investigated sorbitol conversion over bimetallic NiPt/NaY catalysts with CaO as a basic promoter at 220 °C and 6 MPa hydrogen, and they reported a combined selectivity to EG and 1,2-PDO of 76%. Ye et al.11 reported that the activity of Ni catalysts for HDO of sorbitol was enhanced by almost 3-fold at 230 °C and 7 MPa H2 with trace Ce4+ addition to the Ni/Al2O3+CaO system. However, side reactions including methanation and water gas shift reaction (WGS) were also significant (methane and CO2 selectivity = 10−22%). It is clear from the previous work that directing product selectivity toward liquid oxygenates is a major challenge during HDO of sorbitol over Pt-, Ru-, and Ni-based catalysts.21,22 Cu is an inexpensive candidate that has been investigated for HDO of glycerol.23 However, studies reveal that Cu catalysts are better for C−O cleavage with poor C−C cleavage activity.24 In general, multifunctional catalysts that promote dehydrogenation, isomerization, and hydrogenation are needed to favor the formation of carbonyl intermediates and their isomers,14,21 which may undergo C−C cleavage via retro-aldolization. Due to the monofunctional nature of conventional Cu catalysts,25 liquid alkalis [e.g., aqueous solutions of NaOH, KOH, Ca(OH)2] are often needed to promote the formation of CO species.9,25−27 This, however, results in the formation of excess amounts of undesired carboxylic salts, which are difficult to further convert to glycols (via C−O cleavage).25,27−29 The only previous work on Cu-catalyzed HDO of sorbitol reported poor activity and selectivity, even with liquid base promoters.24 In this work, we report for the first time a hybrid Cu/CaOAl2O3 material for selective HDO of sorbitol to glycols without using a liquid base. The proposed Cu catalysts immobilized on Al2O3 support containing either CaO, MgO, or ZnO as solid base (CaO-Al2O3, MgO-Al2O3 and ZnO-Al2O3) were prepared via a concurrent-precipitation method (CP)30,31 and tested for HDO reactions. The Cu catalysts were characterized by N2adsorption, temperature-programmed reduction (TPR), X-ray diffraction (XRD), UV−vis spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The interactions of Cu atoms with adjacent Ca2+ and Al3+ ions in the solid supports, the key to enhancing the activity of C−C and C−O cleavage, were investigated. Concentration−time profiles are presented and discussed using sorbitol as the model polyol. On the basis of the reactivity and product distribution observed on the hybrid Cu/CaO-Al2O3 catalysts, a general
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. 2.1.1. Supported Cu on Metal Oxides. Cu-based catalysts on various solid base supports were prepared using the CP method according to the following procedure (Figure S1). Required amounts of Cu(NO3)2· 2.5H2O (purum, ≥ 98%, Sigma), Ca(NO3)2·4H2O (≥99.0%, Sigma), and Al(NO3)3·9H2O (purum, ≥ 98.0%, Sigma) were mixed with deionized water (DI water), denoted as solution A. The total concentration of Cu2+, Ca2+, and Al3+ was about 0.2 kmol/m3. Predetermined amounts of NaOH (reagent grade, ≥ 98% pellets, anhydrous, Sigma) and Na2CO3·10H2O (purum, ≥ 99.0%, Sigma) were then mixed with DI water (solution B). The concentrations of NaOH and Na2CO3 were 0.25 kmol/m3 and 0.8 kmol/m3, respectively. In another 500 mL beaker, 50 mL of deionized water was introduced under vigorous stirring (>800 rpm) at 60 °C (solution C). In the next step, solutions A and B were added dropwise simultaneously (concurrently, as shown in Figure S1) to beaker C (approximately 2 drops every 3 s). A blue slurry was then formed. The pH of the solution in the beaker C was kept >10 throughout the preparation process to ensure the precipitation of metal cations. The resulting slurry was stirred at 60 °C for 16 h. Then, the mixture was filtered and the solids were washed with 2000 mL of DI water at 90 °C to remove surface Na+ ions. The solid cake obtained was then dried overnight in a vacuum oven at 100 °C. The dried solid catalyst sample was charged to a porcelain bowl, which was then transferred to a calcination furnace (Barnstead/Thermolyne 48000) under air flow (flow rate: 5 cm3/min/g catalyst). The furnace was then heated at a rate of 1 °C/min to 400 °C, for 5 h of dwelling (heating) time. Then, the contents in the furnace were cooled naturally by flowing air (Figure S2). As-prepared calcined Cu catalysts were denoted as CuO/CaO-Al2O3-1 (Cu/Ca2+: 5.4 molar ratio), CuO/CaOAl2O3-2 (Cu/Ca2+: 3.5), CuO/CaO-Al2O3-3 (Cu/Ca2+: 5.4, ramping rate used = 5 °C/min, instead of 1 °C/min). The catalysts were then activated by a procedure shown in Figure S2. The reduced (i.e., activated) Cu catalysts were denoted as Cu/CaO-Al2O3-1 (Cu loading = 43 wt %, 1 °C/min), Cu/ CaO-Al2O3-2 (Cu loading = 28 wt %, 1 °C/min), Cu/CaOAl2O3-3 (Cu loading = 43 wt%, ramping rate = 5 °C/min instead of 1 °C/min). The Cu content in the catalysts was determined by inductively coupled plasma (ICP-AES) analysis.32 The same procedure was followed for the preparation of Cu/ ZnO-Al2O3, and Cu/MgO-Al2O3catalysts. Zn(NO3)2·6H2O (reagent grade, 98%, Sigma) and Mg(NO3)2·6H2O (purum, ≥ 99.0%, Fluka) were used for preparation of solution A to replace Ca(NO3)2·3H2O. These catalyst samples were denoted as CuO/MgO-Al2O3, CuO/ZnO-Al2O3 for calcined catalysts, whereas Cu/MgO-Al2O3, Cu/ZnO-Al2O3 represent the corresponding reduced Cu catalysts. 2.1.2. Cu Catalyst on H-ZSM5. Cu/H-ZSM5 samples were prepared via impregnation method.33 A typical synthetic procedure followed in this approach is as follows: About 5 g of ZSM5 in ammonium form (NH4-ZSM5) was pretreated under calcination conditions under flow of air to remove NH3, the conditions of which were identical to those in Figure S2. The ammonium ion was removed during calcination such that ZSM5 is now in the form of H-ZSM5. Predetermined amounts of Cu(NO3)2·2.5H2O (purum, ≥ 98%, Sigma) were added to 6546
DOI: 10.1021/acscatal.5b01324 ACS Catal. 2015, 5, 6545−6558
Research Article
ACS Catalysis Table 1. HDO of Sorbitol over Different Hybrid, Base-Promoted Cu Catalysts selectivity (%) no.
catalysts
time (h)
Xa (%)
1,2-PDO
1 2 3 4 5 6 7
Cu/CaO-Al2O3-1
3 6 6 12 6 12 6
57.1 98.1 54.1 80.6 55.9 94.9 85.7
39.0 46.1 29.1 31.6 38.3 35.9
Cu/MgO-Al2O3 Cu/ZnO-Al2O3 Cu/H-ZSM5
LA
glycerol
16.6 14.3 1.8 11.8 19.0 4.9 11.5 5.7 11.0 2.2 1.1 6.3 anhydroglucitol: ∼ 30%
EG
C4−6b
othersc
14.3 15.4 12.8 10.2 12.2 11.7
6.4 6.5 3.2 5.5 14.4 29.0 isosorbide: 46%
6.9 14.5 7.7 12.9 3.8 10.5
Conversion at 9.8 kg/m3, Cu/Mg2+, Zn2+, Ca2+ molar ratio: 5.4, sorbitol: 0.18 kmol/m3, T: 230 °C, PH2: 7.6 MPa; bC4−6 tetrols, triols, diols, etc.; c Mainly MeOH and EtOH, trace 1-propanol (1-PrOH), 2-PrOH, methane and carbon dioxide, etc. a
H-ZSM5 in a 50 mL flask (Chemglass) followed by addition of a small amount (5−10 mL) of DI water. The resulting slurry was stirred at ∼500 rpm for at least 4 h at room temperature in order to obtain well-dispersed solid−liquid suspension. The solvent (water) in the slurry was then removed using rotary evaporator at 55 °C. Next, the sample was dried at 120 °C in a vacuum oven overnight to further remove the remaining water. Calcination (ramping rate = 1 °C/min) and activation processes were identical to the procedures described in Figures S2 and S3. This sample was denoted as Cu/H-ZSM5. 2.2. Catalyst Activity Tests. Performance evaluation of Cu catalysts for HDO of sorbitol, mannitol, xylitol, erythritol, and glycerol was carried out in a 300 mL stirred Parr reactor with H2 addition from external sources to maintain a constant hydrogen partial pressure. Details of the experimental setup and procedure are described in a previous publication.15 2.3. Analytical Methods. Gas- and liquid-phase products were analyzed using GC (a RT-APLOT column) and HPLC (a Rezex ROA-Organic Acid H+ column). An example of a typical HPLC chromatogram of the product mixture is shown in Figure S4. 2.4. Catalyst Characterization. Brunauer−Emmett−Teller (BET) Measurement. N2 adsorption studies were carried out using a NOVA 2200e Instrument. The measurement procedure is similar to that described previously.32 Temperature-Programmed Reduction (TPR). TPR study was carried out using an Autochem 2910 Instrument. Details are presented in Supporting Information. UV−vis Spectroscopy. Surface absorbance under UV−vis spectroscopy was carried out using Shimadzu UV-3600 UV− vis-NIR Spectrophotometer. The samples were dispersed in hexane solution, and the solvent was dried on a quartz plate before characterization. Transmission Electron Microscopy (TEM). Sample preparation and detailed procedures were similar to that described previously.28 Samples were prepared by suspending the solid catalyst in ethanol and agitating in an ultrasonic bath. Ten microliters of catalyst sample was placed onto a copper mesh grid with lacey carbon film (from Ted Pella Inc.). The wet grid was allowed to air-dry for several minutes prior to being examined under TEM.15 Scanning Electron Microscopy (SEM). A Versa 3D dual beam Scanning Electron Microscope/Focused Ion Beam (FEI, Hillsboro, OR, U.S.A.) with a silicon drift EDX detector (Oxford Instruments, X-Max, U.K.) was used to measure the surface morphology, elemental composition, and distribution of metals. The SEM data were obtained at an acceleration voltage of 15 kV, spot size of 3.0, and the images were collected with an
ET (Everhart Thornley) detector. The elemental mapping and energy spectra were acquired with Aztec tools (Oxford Instruments, U.K.). Element mapping was carried out via element dispersion Xray analysis during SEM characterization. The X-ray maps were converted into phase maps. The phase maps show the constituent elements of the phase and how the phases are distributed over the sample (in different colors). The name of each phase is derived from its mapping of main elements (e.g., CuCaAlO indicates Cu, Ca, Al, and O are major elements in CuCaAlO phase). The composition of each element in each phase can also be quantified. Each phase detected by phase mapping was further confirmed by X-ray diffraction analysis. X-ray Diffraction (XRD). This measurement was performed on a Bruker D8 powder diffractometer with a copper target (Cu Kα radiation) operating at 40 kV and a current of 40 mA to analyze the crystal structures of materials.
3. RESULTS AND DISCUSSION Initial experiments on HDO of sorbitol were carried out in a stirred slurry reactor to compare the activity/selectivity profiles on different Cu catalysts. Selected Cu catalysts were characterized using XRD, BET, TEM, SEM, TPR, and UV− vis spectroscopy to gain insights into structure−performance relationship. Effect of reaction conditions and concentration− time profiles were also studied on selected catalysts in a batch slurry reactor. The results are discussed in terms of TOF, conversion, and selectivity. With selected catalysts, HDO of other substrates such as mannitol, xylitol, erythritol, and glycerol was also studied. 3.1. Catalyst Performance Evaluation. The performances of Cu/CaO-Al2O3-1, Cu/MgO-Al2O3, Cu/ZnO-Al2O3, and Cu/H-ZSM5 catalysts for HDO of sorbitol are compared in Table 1. Cu/CaO-Al2O3-1 catalyst outperforms the other three investigated catalysts in terms of conversion and product selectivity. Notably, it is found that Cu/CaO-Al2O3-1 catalyst (Entries 1 and 2) shows significantly higher conversion compared with Cu/MgO-Al2O3 (Entries 3 and 4) and Cu/ ZnO-Al2O3 (Entries 5 and 6) catalysts under similar reaction conditions. In particular, as shown in Entry 1, sorbitol conversion reached 57.1% with Cu/CaO-Al2O3-1 within only 3 h. The combined selectivity to 1,2-PDO, glycerol and EG was 68%. Furthermore, as the conversion reached completion (Entry 2), the total selectivity toward glycerol, glycols, and linear alcohols was found to increase from 68% to 84% in the presence of Cu/CaO-Al2O3-1 catalyst. This is the first time that such high conversion and excellent selectivity are reported for both C−C and C−O bond cleavage during HDO of sorbitol. 6547
DOI: 10.1021/acscatal.5b01324 ACS Catal. 2015, 5, 6545−6558
Research Article
ACS Catalysis Moreover, the pH values of fresh substrate solution and final reaction products were approximately 7, indicating that our hybridized Cu/CaO-Al2O3 materials display base-catalyzed retro aldolization activity even in an alkali-free liquid reaction medium. Compared with Cu/CaO-Al2O3-1 catalyst, only about 54% sorbitol conversion was achieved on Cu/MgO-Al2O3 catalyst after 6 h (Entry 3), with lower selectivity toward glycols (1,2PDO and EG). With prolonged reaction time (12 h, Entry 4), combined selectivity of C3 products (1,2-PDO, LA and glycerol) was found to decrease, while that of ethanol (EtOH) and methanol (MeOH) increased, suggesting that further conversion of C3 products occurred possibly via C−C and C−O cleavage and HDO reaction to form C1−2 products. Compared to Cu/MgO-Al2O3 catalyst, we observed that Cu/ ZnO-Al2O3 catalyst displayed slightly higher conversion (56%) after 6 h (Entry 5) under identical reaction conditions. It is worth noting that after 12 h (Entry 6), 95% conversion was achieved with more C4−6 polyols (including tetrols, triols, diols, about 29% in selectivity, identified by GC-MS) in comparison with Cu/MgO-Al2O3 catalyst (
