CaO-Al2O3 Catalysts

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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 ACS Catal., Just Accepted Manuscript • Publication Date (Web): 24 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015

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Sorbitol Hydrogenolysis over Hybrid Cu/CaO-Al2O3 Catalysts: Tunable Activity and Selectivity with Solid Base Incorporation Xin Jin,1 Jian Shen,3 Wenjuan Yan,1,2 Meng Zhao,1 Prem S. Thapa,4 Bala Subramaniam,1,2 Raghunath V. Chaudhari1,2* 1

Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa

Drive, Lawrence, Kansas 66047, USA 2

Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W 15th

St., Lawrence, Kansas 66045, USA 3

Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence,

Kansas 66045, USA 4

Microscopy and Analytical Imaging Laboratory, Haworth Hall, 1200 Sunnyside Ave,

University of Kansas, Lawrence, Kansas 66045, USA

*To whom correspondence should be addressed. E-mail: [email protected]

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Abstract We report for the first time hybridized Cu/CaO-Al2O3 catalysts for aqueous phase hydrogenolysis of sorbitol to ethylene glycol (EG), 1,2-propanediol (1,2-PDO) and 1,2butanediol (1,2-BDO) with linear alcohols as co-products 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 while 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 multi-functional Cu-Ca2+ sites is proposed. Key words: sorbitol, hydrogenolysis, glycols, copper catalyst, solid base promoters


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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 sugar-based 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), 1,2-butanediol (1,2-BDO), and lower aliphatic alcohols.5-12 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), based on observation of enhanced catalyst activity and the formation of 1,2PDO 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 based on the fact that glycerol was the major product during sorbitol 3 ACS Paragon Plus Environment

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conversion. But Wang et al.18 later observed that C-C cleavage follows retro-aldol mechanism by using various diols as model compounds with NaOH as a base promoter.19 Clark and co-workers 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 oC and 7 MPa hydrogen, reporting 68% conversion and overall glycols (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 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 threefold 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


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poor C-C cleavage activity.24 In general, multi-functional catalysts that promote dehydrogenation, isomerization, hydrogenation are needed to favor the formation of carbonyl intermediates and their isomers,14, 21 which may undergo C-C cleavage via retroaldolization. Due to the mono-functional 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 C=O 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/CaO-Al2O3 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 (CaOAl2O3, 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 N2-

adsorption, 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. Based on the reactivity and product distribution observed on the hybrid Cu/CaOAl2O3 catalysts, a general reaction mechanism involved Cu-Ca2+ interaction is proposed.


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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 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 were introduced under vigorous stirring (>800 rpm) at 60 °C (solution C). In the next step, solutions A and B were added drop wise simultaneously (concurrently, as shown in Figure S1) to beaker C (approximately 2 drops every 3 seconds). 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


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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). Calcined Cu catalysts as prepared were denoted as CuO/CaOAl2O3-1 (Cu/Ca2+: 5.4 molar ratio), CuO/CaO-Al2O3-2 (Cu/Ca2+: 3.5), CuO/CaO-Al2O3-3 (Cu/Ca2+: 5.4, ramping rate used = 5 oC/min, instead of 1 oC/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 = 43w%, 1 oC/min), Cu/CaOAl2O3-2 (Cu loading = 28w%, 1 oC/min), Cu/CaO-Al2O3-3 (Cu loading = 43 w%, 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/MgOAl2O3catalysts. 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/ZnOAl2O3 for calcined catalysts, while 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) were 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 HZSM5. Predetermined amounts of Cu(NO3)2.2.5H2O (purum, ≥ 98%, Sigma) were added 7 ACS Paragon Plus Environment

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to 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 oC. 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


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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 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. 10 µL of catalyst sample were 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, USA) with a silicon drift EDX detector (Oxford Instruments, X-Max, UK) was used to measure the surface morphology, elemental composition and distribution of metals. The SEM data were obtained at an acceleration voltage of 15kV, spot size 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, UK). Element mapping was carried out via element dispersion x-ray 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 9 ACS Paragon Plus Environment

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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 (CuKα 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-Al2O310 ACS Paragon Plus Environment

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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 towards glycerol, glycols and linear alcohols was found to increase from 68% to 84% in 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. 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 towards glycols (1,2-PDO 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 (< 6% in selectivity), while selectivity to C2-3 products was lower on Cu/ZnO-Al2O3 catalyst. The difference in the product distribution suggests that more C-O bond breakage of sorbitol molecules occurs over Cu/ZnO-Al2O3 compared to Cu/MgO-Al2O3 catalyst. This product distribution observed with Cu/ZnO11 ACS Paragon Plus Environment

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Al2O3 catalyst is consistent with that reported by Blanc and co-workers during HDO of sorbitol,24 where CuO/ZnO catalysts were found to favor the formation of C4-6 tetrols, triols and diols (about 56% selectivity) rather than C-C bond cleavage to smaller molecules (due to C-C bond cleavage) observed when ZnO was used as a support and promoter. Given that base promoters (Ca2+, Mg2+ and Zn2+) facilitate C-C and C-O cleavage, we also evaluated the performance of acidic promoters during sorbitol conversion. Cu/HZSM5 catalyst (Entry#7) showed completely different product distribution compared to the other three catalysts. Under similar conditions, the major products observed were anhydroglucitol and isosorbide (instead of glycols observed on the base promoted catalysts) with approximately ~ 30% and 46% selectivity (both products confirmed by GC-MS and HPLC), respectively, at a sorbitol conversion of 85%. These results suggest that dehydration (DHD), rather than retro-aldolization and HDO, is favored on an acidic support. The results over different Cu catalysts show that the acidity/basicity of solid supports significantly affects the activity of Cu catalysts and reaction pathways during HDO of sorbitol.34 Given our interest in glycols and alcohols as products, we compared the catalytic performances of the proposed Cu/CaO-Al2O3-1 catalyst with previously reported metal catalysts (see Figure 1). Ru/C and bimetallic RuRe/C catalysts, which were previously investigated by us and other research groups, show relatively high catalytic activity compared to Ni and Cu based catalysts. However, the selectivity to glycols was only in the range of 45 - 53% at 200 - 230 oC.15 In comparison, NiPt/NaY catalyst displays improved selectivity to glycols (S ~ 82%).7,

35

NiCe/Al2O3 catalysts exhibit relatively


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higher selectivity towards alcoholic products compared to Ru catalysts but tend to also form gaseous products (methane and CO2).11 CuO/ZnO catalyst displays poor activity for sorbitol conversion compared with Ru and Ni catalysts, with low selectivity to glycols and other lower products.24 Our new hybrid catalyst, Cu/CaO-Al2O3-1, shows improved activity compared to CuO/ZnO catalyst. More importantly, the overall yield of glycols and alcohols is higher compared to other metal catalysts under similar reaction conditions (T = 230 °C) although the activity is lower.6, 9, 27 Cu/CaO-Al2O3-1 catalyst thus shows good potential for HDO of sorbitol. 3.2 Effect Cu/Ca2+ ratio on activity and selectivity of Cu/CaO-Al2O3 catalysts Cu/CaO-Al2O3 catalysts with various Cu/Ca2+ ratios were prepared and tested in sorbitol conversion, the results of which are shown in Figure 2. Interestingly, we observed that CaO-Al2O3 support alone with no Cu (Cu/Ca2+ = 0) showed very low conversion of sorbitol but high selectivity for C3 products (with LA as the major product, S > 76%) in 6 h. Catalyst with Cu/Ca2+ ratio of 3.5, showed a TOF value of 0.24 h-1, and 81% selectivity to C3 products (1,2-PDO, glycerol, LA, PrOHs). In comparison, for the catalyst with a Cu/Ca2+ ratio of 5.4 (TOF = 1.2 h-1), the activity of Cu catalyst was increased by almost four fold, although the selectivity of C3 products (1,2-PDO, LA, glycerol, PrOHs) decreased to 64%. Clearly, the Cu content in the support has a strong influence on the activity and reaction pathways in sorbitol conversion. Therefore, textural and structural characterization studies, employing BET, TPR, UV-Vis, XRD, TEM and SEM tools, were carried out to gain further insights into the surface properties of Cu catalysts and understand the metal-support interaction.


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3.3 Catalyst characterization 3.3.1 BET BET adsorption results are presented in Figure 3 and Table 2 for Cu/CaO-Al2O3 catalysts with different Cu/Ca2+ ratios. Specifically, adsorption/desorption isotherms, surface area, pore size and pore volume of CaO-Al2O3 supported Cu catalysts with Cu/Ca2+ atomic ratio of 3.5, 4.0, 5.4 and 6.0 were measured. It is found that as Cu/Ca2+ atomic ratio increases from 3.5 to 6.0, the total surface area of Cu catalysts is enhanced from 39.4 to 81.9 m2/g. Nitrogen isotherms of these Cu catalysts confirm that these materials have macro-pores and the mean pore size is also larger as Cu/Ca2+ atomic ratios are higher. As seen from Table 2, the average pore size of the sample with Cu/Ca2+ ratio of 3.5 is approximately 7.69 nm while this value almost doubles for sample with a Cu/Ca2+ value of 6.0 (13.32 nm). The total pore volume follows the same trend for all four Cu catalyst samples. BET data of the four Cu catalysts clearly show that as more Cu was incorporated into the CaO-Al2O3 support, larger pores were formed during the precipitation process. This observation implies that Cu species favor the formation of CaO-Al2O3 framework in aqueous solution and appear to tune pore growth leading to particular pore size and volume. At this stage we do not have conclusive evidence to show that increasing pore size and volume could enhance catalytic activity for sorbitol conversion. But later it is shown that changing Cu/Ca2+ ratios will tune the interaction between Cu and Ca2+ species in the support, which might be the key for good HDO activity. This trend is different from previous experimental findings wherein higher Cu content usually inhibited pore


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formation and reduced the total pore volume in samples prepared from MgO-Al2O3, ZnO-Al2O3 and ZnO.30, 31, 33, 36 3.3.2 TPR The Cu/MgO-Al2O3, Cu/ZnO-Al2O3, Cu/CaO-Al2O3-1 (Cu/Ca2+ = 5.4) and Cu/CaO-Al2O3-2

(Cu/Ca2+ = 3.5) catalysts were further characterized by TPR. The TPR temperature

ramping program was set at conditions similar to the reductive atmosphere employed during catalyst activation in the tube furnace (Figure S3). Thus, the TPR profiles should reflect the actual metal-support interaction during catalyst activation. As shown in Figure 4, the hydrogen reduction peak of Cu/MgO-Al2O3 catalyst occurs at 180 °C, which is the normal reduction temperature for CuII to Cu0 species.37 Cu/ZnO-Al2O3 sample exhibits reduction behavior similar to Cu/MgO-Al2O3 catalyst. TPR profile for Cu/CaO-Al2O3-1 sample shows a peak at a different temperature. The reduction peak of CuII species is shifted from 180 °C to 225 - 230 °C. Further, the comparison between CuO/CaO-Al2O3-1 (Cu/Ca2+ = 5.4) and CuO/CaO-Al2O3-2 (Cu/Ca2+ = 3.5) catalysts suggests that the reduction peak of CuII species is progressively shifted to higher temperature (~ 250 °C) when Cu/Ca2+ ratio is lower, implying that Cu and Ca2+ interaction is stronger with an increase in basic sites. The shifts in reduction peak positions indicate that the interaction between Cu species and CaO-Al2O3 support is stronger compared with the other two metal oxides. Considering the fact that Cu/CaO-Al2O3-1 and Cu/CaO-Al2O3-2 catalysts display higher propensity for C-C cleavage than Cu/MgO-Al2O3, Cu/ZnO-Al2O3 catalysts, metalsupport interaction might be the key to activate metal sites during sorbitol conversion.


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3.3.3 UV-Vis Spectra In order to further understand the metal-support interaction, three selected Cu/CaO-Al2O3-1,

Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 catalysts were also characterized using

UV-Vis as shown in Figure 5. It is found that both Cu/CaO-Al2O3-1 (solid line) and Cu/CaO-Al2O3-2 (dash line) catalysts exhibit strong characteristic adsorption at 290 nm, although the absorbance magnitude is slightly different for the two catalysts. This peak is slightly blue shifted compared to that obtained with 300 nm on CuO nanoparticles.38 In contrast, the absorption associated with Cu/CaO-Al2O3-3 catalyst (dot-dash line) is significantly milder. Besides, the characteristic absorption peak for Cu particles at 570 nm39 was not observed on the three samples, indicating negligible presence of Cu particles. 3.3.4 XRD X-ray diffraction patterns for Cu/CaO-Al2O3-1, Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 catalysts are shown in Figure 6.

All catalyst samples show characteristic peaks of

CuAl2O4, CaO and Al2O3 species although their positions and intensities are different. Specifically, Cu/CaO-Al2O3-1 displays strong peaks for CaO at 30°, 43°, 56° and 68°,40-43 while both Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 exhibit lower intensity at these positions. Interestingly, we also found that diffraction peaks of CuAl2O4 species in Cu/CaO-Al2O3-1 are shifted to lower diffraction angles, from 23° and 30° 44-46 to 20° and 27°, respectively. This indicates possible interaction of spinel CuAl2O4 with other species within the catalyst structure. For Al2O3 diffraction, we found that the two characteristic peaks are slightly moved to higher angles (from 36° and 39°

47-49

to 37° and 40°,

respectively) for Cu/CaO-Al2O3-1 catalyst compared with the other two samples. 16 ACS Paragon Plus Environment

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3.3.5 TEM The surface morphology of Cu/CaO-Al2O3-1, Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 was investigated using TEM characterization, as shown in Figures 7, S5 and S6. Unlike the conventional Cu/MgO-Al2O3 and Cu/ZnO-Al2O3 catalysts, which exhibit Cu particles of 50-200 nm in size,30,

31, 36, 50

Cu/CaO-Al2O3-1 sample displays a highly crystallized

structure, as shown in Figures 7 (a) and (b). The Cu/CaO-Al2O3-1 sample does not show any Cu particles with detectable sizes. In other words, Cu element is well dispersed and mixed with CaO-Al2O3 support. This observation is consistent with UV-Vis spectra shown in Figure 5. EDX analysis of bulk composition of CuO/CaO-Al2O3-1 [Figure 7 (c)] also confirms that Cu, Ca and Al elements are well distributed in the catalyst sample. Similarly, highly crystalline structures were observed on Cu/CaO-Al2O3-2 (Figure S5) sample. Besides, Cu/CaO-Al2O3-3 (Figure S6) exhibits ordered structured lattices, but several bulky phases appear to exist, indicating that fast ramping rate (5 oC/min) during synthesis may result in a phase separation. 3.3.6 SEM SEM characterization of Cu/CaO-Al2O3-1, Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 catalysts confirms the phase distribution pattern observed earlier by XRD. In particular, Figure S7 shows SEM images of the distribution of Cu, Ca, Al and O elements on Cu/CaO-Al2O3-1 catalyst surface. Elemental analysis of the selected catalyst particle shows that all four elements are well dispersed.


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As already shown in TEM images (Figure 7), Cu, Ca, Al and O in Cu/CaO-Al2O3-1 form hybrid structures and hence the size of Cu particles is not detectable. The hybrid composition is however not known from the elemental analysis. Phase analysis was therefore conducted in order to reveal the chemical composition of Cu catalysts. Figure 8 shows that CuAlO is the major phase, while Ca is concentrated as CaCuAlO being dominant on the surface of Cu/CaO-Al2O3-1. The molar ratio of the two phases is about 18/1 as evaluated using a data processing software counted for 693 thousand points in the region shown Figure S12. Therefore, it is clear that the highly crystalline structure mainly represents CuAlO and CaCuAlO phases. The existence of CuAlO and CaCuAlO phases also confirm a strong interaction between Cu and metal oxide supports, consistent with TPR results and TEM images. It is known that CuAl2O4 spinel structure is the major active phase for HDO reactions.5153

Based on the EDX analysis for CuAlO phase, the molar ratio of Cu, Al and O is

roughly 0.6/1.1/2.5 (or 1.8/3.3/7.5 for another spot on the same sample), which is consistent with the composition of spinel CuAl2O4 species or its dimer (dicopper aluminum oxide, Cu2Al4O7) structure. The EDX analysis of another phase CaCuAlO phase is consistent with a structure such as Ca5Cu3Al6O17. SEM images of Cu/CaO-Al2O3-2 sample are shown in Figures S8 and S9. It is found that Ca, Al and O are well distributed throughout the sample, while Cu species displays only a slight phase separation. Compared with Cu/CaO-Al2O3-1 sample (Figures 8 and S7, Cu loading = 43 w%), Cu content in this sample is much lower (Cu loading = 28 w%). The difference in SEM images clearly indicates that the active phases in the two samples are distinct from each other. Phase diagram of Cu/CaO-Al2O3-2 sample was thus carried out. 18 ACS Paragon Plus Environment

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As shown in Figure S9, we counted for more than 708 thousand points for the region shown and found that major phases for Cu/CaO-Al2O3-2 sample are CuO/Cu and CaCuAlO. In sharp contrast, CaCuAlO is the dominant phase (98.1%) in this sample compared with Cu/CaO-Al2O3-1 (CaCuAlO ≈ 6%). CuO/Cu is < 2% in this sample. Figures S10 and S11 presents the distribution of Cu, Ca, Al and O elements on Cu/CaOAl2O3-3 catalyst surface. Elemental analysis on a selected catalyst particle shows that all four elements are not well dispersed. It shows a trend of elemental segregation on the catalyst surface. It is important to point out that the bulk composition of Cu/CaO-Al2O3-3 catalyst is almost identical with Cu/CaO-Al2O3-1 catalyst. The pretreatment conditions are however different from each other. As mentioned in the experimental section, Cu/CaO-Al2O3-1 sample underwent a 1 oC/min ramping rate during calcination (under flowing air), while Cu/CaO-Al2O3-3 experienced a 5

o

C/min ramping rate. The

uniformity and crystallinity of Cu/CaO-Al2O3-3 are not as good as the Cu/CaO-Al2O3-1 sample. Figure S11 shows that Al2O3 is a dominant phase, which means that phase segregation led to the formation of mixed metal oxides rather than hybridized ones. Therefore, it is highly possible that fast heating rate results in phase separation. SEM images of used Cu/CaO-Al2O3-1 catalysts after three recycles are shown in Figure 9. The surface morphologies have changed from layered structures (Figure S7) to layered and spherical mixtures (Figure 9). We find that both Ca and Al are still well distributed after reactions, while Cu species tend to agglomerated during recycles. Particularly, EDX mapping in Figure 9 shows that Cu tend to phase out from Cu/CaO-Al2O3-1 materials and form spherical particles. This observation is also supported by STEM images shown in


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Figure 10. Clearly, Cu species tend to form bulky phases after reactions [see overlayered element mapping in Figure 10 (b)]. 3.4 Structure-activity correlation The conversion of sorbitol on the three Cu catalysts, Cu/CaO-Al2O3-1, Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 was investigated in a batch reactor. As shown in Table 3, Cu/CaOAl2O3-1 catalyst displayed almost 100% conversion of sorbitol in 6 h, while Cu/CaO-Al2O3-2

and Cu/CaO-Al2O3-3 only showed < 20% conversion under the same reaction

conditions. The selectivities towards 1,2-PDO, LA+PAD and EG are significantly different on the three catalysts. It is found that the combined selectivity to 1,2-PDO, glycerol and EG was about 73% on Cu/CaO-Al2O3-1 catalyst, but this value was only 20 - 32% on other two catalysts. Further, whereas the selectivity of LA+PAD was < 2% on the active Cu/CaO-Al2O3-1 catalyst, the selectivity values on the other two catalysts were as high as 64 - 75%. It is believed that C3-C3 cleavage occurs after dehydrogenation (DH) of sorbitol leading to the formation glyceraldehyde (GLA) and dihydroxyacetone (DHA) as C3 intermediates, which undergo rearrangement to form LA (in alkali medium or supercritical water54) or HDO reactions to generate 1,2-PDO,35 as shown in Scheme 1. It can be inferred that whereas Cu/CaO-Al2O3-1 is active for both DH and HDO reactions, only DH occurs on Cu/CaO-Al2O3-2 and Cu/CaO-Al2O3-3 catalysts. Furthermore, the selectivity towards C3 products (1,2-PDO, LA and glycerol) is 61 - 98%, on all the three catalysts implying that C3-C3 cleavage is dominant. Further, as shown earlier, XRD and SEM characterizations reveal that Cu/CaO-Al2O3-2 sample has only one phase (CaCuAlO), while Cu/CaO-Al2O3-1 displays both CaCuAlO and CuAlO


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phases. It is therefore hypothesized that CaCuAlO is the active site for the DH and C3-C3 cleavage reactions of sorbitol and CuAlO (spinel CuAl2O4) species are active for hydrogenation reactions. Hydrogenation reactions consume C3 intermediates species formed from DH and retroaldolization of sorbitol on Cu catalysts, which in turn helps overcome the equilibrium limitation associated with DH reactions due to product accumulation. Therefore, the Cu/CaO-Al2O3-1 catalyst shows much higher sorbitol conversion. In contrast, Cu/CaOAl2O3-2 and Cu/CaO-Al2O3-3 catalysts have almost no active sites for hydrogenation reactions, causing the sorbitol conversion on these catalysts to be hindered by equilibrium limitations. The higher activities for C-C and C-O cleavage reactions observed with Cu/CaO-Al2O3-1 compared to other catalysts suggests that the activity and selectivity of Cu/CaO-Al2O3 catalyst in general are dictated by the relative distributions of DH (CaCuAlO phase) and hydrogenation (spinel CuAl2O4 phase) sites. 3.5 Reaction profiles Since Cu/CaO-Al2O3-1 catalyst shows superior performances compared to other catalysts, detailed concentration-time profiles were obtained to better discern possible reaction pathways. Figures 11 (a and b) show typical temporal product distribution [including 1,2-PDO, EG, glycerol, butylene glycol (1,2-butanediol, 1,2-BDO)] profiles during sorbitol conversion. The concentrations of 1,2-PDO, EG and glycerol increase almost linearly with reaction time, while that of LA+PAD (pyruvaldehyde) first increase and then decrease at longer reaction times, implying that LA and PAD under further 21 ACS Paragon Plus Environment

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hydrogenated to linear alcohols and glycols as products. The concentration of linear alcohols (Figure 11) increased sharply with reaction time, as a result of consecutive HDO of 1,2-PDO and EG to propanols (PrOH), EtOH and MeOH. A linear dependence of sorbitol conversion with time is observed until almost 100% conversion. As inferred from Figures 11 (a) and (b), the HDO rates show a weak dependence on sorbitol concentration (230 oC). While lower sorbitol conversion and product formation rates are observed at lower reaction temperature (210 oC), the product distribution is similar at the two temperatures. Pressure effect on the product distribution during sorbitol conversion is unclear from previous literature reports.13,

35

As shown in Figure 12, more LA is formed, with

relatively low selectivity compared to 1,2-PDO at low H2 pressure (2.8 MPa). With increasing hydrogen pressures, the combined selectivity of glycerol and LA decreased by about 22% while the initial selectivity of 1,2-PDO as well as other alcohols increased by 20%. The difference in product selectivity further confirms the possible reaction network shown in Scheme 1. 3.6 Plausible reaction pathways and mechanism The product distribution pattern observed in sorbitol conversion with Cu/CaO-Al2O3 catalysts shows that a series of consecutive and parallel reactions occur. In particular, CC and C-O cleavage, HDO, DH and DHD reactions are expected to occur on the catalyst surface and a possible reaction pathway is proposed in Scheme 2. Generally, sorbitol is believed to undergo DH to form different unsaturated aldoses and ketoses, such as aldohexose, β-ketohexose and γ-ketohexose.16 The formation of aldohexose (primary C6


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aldehyde) initially will generate C2 and C4 (Route I) products, while β-ketohexose will lead to C3-C3 cleavage (Route III). γ-Ketohexose formed initially results in C1-C5 breakage (Route II) and C5 may further convert to β-ketopentose (which can be isomerized to aldopentose) to generate C2 and C3 products (Route IV). The likelihood of the formation of β-ketohexose is high under low hydrogen pressure on Cu catalysts, because combined selectivity of C3 products is much higher than other products, as shown in Figure 12. However, it should also be noted that selectivity of C2 products (mainly EG) is higher than C4 (mainly 1,2-BDO). This finding suggests that C1-C5 cleavage also occurs over Cu catalysts, although C1 is formed in minor quantities compared to C3-C3 cleavage products. This also implies that DH over Cu catalysts tends to generate β-ketohexose rather than γ-ketohexose and aldohexose. In other words, at higher H2 pressures, aldehyde-like intermediate compounds are likely to be formed (still minor compared with β-ketohexose) while at lower hydrogen pressures β-ketone like intermediate is favored which leads to 1,2-PDO and EG formation. Sun & Liu’s work implies that in xylitol conversion, the formation of aldopentose (primary C5 aldehyde) was favored rather than ketopentose because C1 and C4 products were found to be in trace amounts.9, 27 In fact, only γ-ketopentose leads to the formation of C1 and C4 products. From the mechanism shown (Scheme 2), it is obvious that βketopentose will also facilitate C2-C3 cleavage. In order to further understand the possible position where the C=O bond is formed after DH reactions, xylitol (C5 polyol), erythritol (C4 polyol) and glycerol (C3 polyol) were also studied as substrates. Results on catalytic HDO of mannitol, xylitol, erythritol and glycerol on Cu/CaO-Al2O31 catalyst are summarized in Table 4. We observed that mannitol conversion led to > 65% 23 ACS Paragon Plus Environment

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C3 selectivity (Entry#1), while the combined selectivity to EG and EtOH (C2 products) was only 15%. The conversion of xylitol showed mainly C2 and C3 products (Entry#2), possibly following retro-aldolization mechanism.18 It is obvious that the theoretical selectivities of C3 and C2 should be 60% and 40%, respectively. Here, it is found that the total selectivity of 1,2-PDO, LA, PAD and glycerol is approximately 56.7% over Cu/CaO-Al2O3-1 catalyst and combined selectivity toward EG and EtOH is about 31%. The ratio of C3/C2 is thus almost equal to the theoretical value (60/40). The limited selectivity of MeOH and 1,2-BDO implies that xylitol conversion follows primarily C2C3 cleavage. These results agree well with Sun & Liu’s observations on Ru/C catalyst.9 HDO of meso-erythritol showed that the dominant product is 1,2-BDO (S ≈ 39%). This suggests that, as the carbon number of the polyol decreases from 6 (sorbitol, mannitol) to 5 (xylitol) to 4 (erythritol), the tendency of C-C cleavage on the Cu catalysts becomes less significant, while the propensity toward C-O cleavage is enhanced. Therefore, it is proposed that erythritol follows sequential dehydration (DHD) and HDO reactions to form 1,2-BDO (Route V in Scheme 3). C1-3 products were also detected in HPLC, including glycerol, LA, 1,2-PDO, EG and MeOH. The ratio of C3/C1 was found to be almost 3/1. Besides, as shown in Table 4, the combined selectivity of C1+C3 products is much higher than C2. This observation clearly indicates that C1-C3 bond breakage (Route VI in Scheme 2) rather than C2-C2 cleavage (Route VII) prevails over Cu/CaO-Al2O3 catalyst. The observed facts support our hypothesis that the formation of β-ketose (e.g. βketohexose, ketopentose and ketoerythrose) is favored on Cu/CaO-Al2O3 catalysts. The argument on whether 1,2,3-butanetriol (1,2,3-BTO, Route V) or 1,2,4-butanetriol (1,2,4-BTO, Route VIII) is the precursor for 1,2-BDO can be answered by the detected 24 ACS Paragon Plus Environment

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species in GC-MS. Trace 1,2,4-BTO (S < 1.8%) was found at small sorbitol conversion values (X < 10%), which indicates that the generation of 1,2-BDO is instantaneous on Cu/CaO-Al2O3-1 catalyst. Moreover, this observation confirms that active basic species tend to attack terminal carbon and promote DHD at primary and secondary carbons. However, the reason that sequential DHD and hydrogenation occur to form 1,2-BDO is still not clear. Here a representative mechanism is proposed for the formation of 1,2-BDO. Unlike sorbitol and xylitol, the β-ketose intermediate from erythritol might undergo either retro-aldolization (Route VI) or secondary DHD (Route V in Scheme 3) because the stable conjugated structure might also be favored. A detailed description of the reaction mechanism over Cu/CaO-Al2O3-1 catalyst is presented in the next section. In sharp contrast to the results reported on Ru catalysts,9,

15

the observed product

distribution during conversions of sorbitol, mannitol, xylitol and erythritol over Cu/CaOAl2O3-1 catalyst implies that the potential of C-C breakage of C3 molecules is restrained on this catalyst. These observations are further confirmed by our study of glycerol HDO over the Cu/CaO-Al2O3-1 catalyst that shows a conversion of 67.5% with 1,2-PDO (S = 81%) as a major product in 10 h under 2.8 MPa H2 pressure. Interestingly, compared with the HDO of sorbitol (C6), xylitol (C5) and erythritol (C4), the C-C cleavage of glycerol (C3) molecules is significantly restrained as inferred from the low yields of C1-2 products. Previous studies found that the addition of liquid base (NaOH and CaO) to the reaction mixture increased glycerol conversion to both EG (S ≈ 20%) and 1,2-PDO (S ≈ 27%) because the retro aldolization and HDO reactions are enhanced by adsorbed hydroxyls even under much milder conditions.5 Again, such a difference can be explained by alternate representative pathways such as the formation of DHA, which is a type of β-


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ketose in Scheme 4 (Route IX) over Cu/CaO-Al2O3 catalyst rather than GLA, which can lead to C-C cleavage in glycerol conversion. Once DHA is formed, it undergoes further DHD to generate a conjugated unsaturated alcohol, which hydrogenates to form 1,2-PDO. As indicated above, product distribution from HDO of sorbitol, xylitol and erythritol suggests that C-C cleavage is likely to be initiated by β-ketone and thus displays a Cx-C3 (x = 1, 2, 3 for erythritol, xylitol and sorbitol, respectively) like cleavage in the presence of divalent Ca2+ species. Therefore, a general C-C cleavage mechanism over the Cu/CaOAl2O3 catalysts is proposed here for the first time. Because DH of α-alcohols occurs easily on Cu catalysts whereas DH of secondary and tertiary alcohols demands extremely harsh conditions,55 sorbitol most likely undergoes DH reaction to form aldehexose catalyzed by Cu sites, as shown in Scheme 5. As supported by TPR (reduction peak position) profiles and SEM (phase diagram) characterization, Ca2+ cations neighboring Cu species have a strong influence on Cu sites and tend to isomerize the aldohexose intermediates forming β-ketohexose (or γ-ketohexose). This is on the basis of retro-aldol mechanism by which the formation of β-ketohexose results in C3-C3 cleavage. In the case of xylitol, this mechanism also applies and similar equilibrium structures (aldopentose, βketopentose and γ-ketopentose) are formed. It is also clear that aldopentose and βketopentose both result in C2-C3 scission, although in different ways. There have been several debates on the role of cations during C-C and C-O cleavage. It was found that Cr3+ could isomerize aldohexose (glucose) molecules via bonding with α and β carbon chains to form glucose-fructose equilibrium structures, which easily undergo DHD in the presence of an acidic promoter (pH < 5 in the solution containing Cr3+).56 Later reports also found similar effects over Lewis acid, Sn-containing zeolites 26 ACS Paragon Plus Environment

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and depicted the same mechanism for the formation of hydroxymethylfurfural during glucose and fructose conversion.57, 58 In contrast, Yan and coworkers found that the LA (C3) yield from glucose (C6) was facilitated by Ca(OH)2 to a greater extent than NaOH (at the same initial OH- concentration) during hydrothermal conversion (270-400 oC).54 These observations confirm that Ca2+ is the key to C3-C3 scission over Cu catalysts during HDO of sorbitol and xylitol (Scheme 5). For xylitol, dominant C2-C3 cleavage occurs because of the formation of the β-ketose. Analogously, formation of C1 and C3 via asymmetrical C1-C3 cleavage can also occur via forming a β-ketose from a C4 polyol and is confirmed by our experimental results from the conversion of erythritol, although the potential of C-C scission is much lower than sorbitol and xylitol. The asymmetrical cleavage seems to be not favored for sorbitol (C1C5) and xylitol (C1-C4) conversions on Cu/CaO-Al2O3 catalyst. These unsaturated intermediates generated from DH and C-C cleavage are then hydrogenated to alcoholic products. 3.7 Recycle studies Recycle studies on sorbitol conversion using Cu/CaO-Al2O3-1 catalyst (refer Table 1 for reaction conditions) were further carried out. As shown in Table 5, an observable deactivation of Cu/CaO-Al2O3-1 catalyst was found. In particular, conversion of sorbitol at 230 oC decreases from 98% (Table 1) to 88% after 1st test, and then further to < 70% after three recycles (see Table 5). The selectivities towards 1,2-PDO, EtOH and MeOH remain almost unchanged during recycles. But selectivity to glycerol and EG decreases with increasing LA+PAD selectivity after three recycle tests. It is highly plausible that


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the decreased activity of Cu/CaO-Al2O3-1 catalyst is due to the phase separation of Cu species from Cu/CaO-Al2O3-1 materials (Figures 9 and 10), which results in relatively weaker interaction between Cu and Ca2+ cations in the support.

4. Conclusion Hydrogenolysis of sugar polyols such as sorbitol, mannitol, xylitol, erythritol and glycerol using the novel Cu-based tri-functional catalysts has been investigated. It is found that, in comparison with Mg2+ and Zn2+, Ca2+ displays significant promotional effects on the activity as well as the selectivity of Cu catalysts. Detailed surface characterization confirms the existence of binary phases, CaCuAlO and CuAlO in Cu catalysts, which are responsible for DH and HDO, respectively, during the conversion of polyols. Results with different polyols further show that Ca2+ also promotes the isomerization of aldoses, and that C3-Cx (x = 1, 2, 3) cleavage is significant. A possible reaction mechanism involving DH and isomerization is proposed for the first time for Cubased catalysts. The proposed Cu-based catalysts display good performances for catalytic upgrading of sugar polyols to value-added glycols and alcohols.

Associated content Supporting Information. This material is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional information on catalyst preparation, analytical conditions, TEM and SEM characterization.


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Acknowledgement Partial support from United States Department of Agriculture (USDA/NIFA Award 2011-10006-30362) and National Science Foundation (CHE-1543673) is gratefully acknowledged. We want to thank Dr. Victor Day for XRD analysis.

References 1. Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Angew. Chem., Int. Ed. 2004, 43, 1549−1551. 2. Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044−4098. 3. Ohara, M.; Takagaki, A.; Nishimura, S.; Ebitani, K. Appl. Catal., A 2010, 383, 149−155. 4. Tong, X. L.; Li, M. R.; Yan, N.; Ma, Y.; Dyson, P. J.; Li, Y. D. Catal. Today 2011, 175, 524−527. 5. Zhao, L.; Zhou, J. H.; Chen, H.; Zhang, M. G.; Sui, Z. J.; Zhou, X. G. Korean J. Chem. Eng. 2010, 27, 1412−1418. 6. Zhao, L.; Zhou, J. H.; Sui, Z. J.; Zhou, X. G. Chem. Eng. Sci. 2010, 65, 30−35. 7. Banu, M.; Sivasanker, S.; Sankaranarayanan, T. M.; Venuvanalingam, P. Catal. Commun. 2011, 12, 673−677. 8. Jin, X.; Subramaniam, B.; Chaudhari, R. V. Novel Materials for Catalysis and Fuels Processing; Bravo-Suarez, J. J., Kidder, K. K., Schwartz, S., Eds.; ACS Symposium Series 1132; American Chemical Society: Washington, DC, 2013; pp 273−285. 9. Sun, J. Y.; Liu, H. C. Green Chem. 2011, 13, 135−142. 10. Jin, X.; Subramaniam, B.; Chaudhari, R. V. Proceedings of the International Congress of Catalysis Meeting, Munich, Germany, July 1−6, 2012. 11. Ye, L. M.; Duan, X. P.; Lin, H. Q.; Yuan, Y. Z. Catal. Today 2012, 183, 65−71. 12. Jin, X.; Subramaniam, B.; Chaudhari, R. V. Presented at the Center for Environmentally Beneficial Catalysis Industry Advisory Board Meeting, Lawrence, KS, Oct. 3−4, 2011. 13. Sohounloue, D. K.; Montassier, C.; Barbier, J. React. Kinet. Catal. Lett. 1983, 22, 391−397.


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14. Zhao, L.; Zhou, J.; Yang, G.; Ji, Y.; Zhang, M.; Chen, H.; Zhou, X. Energ. Source A 2012, 34, 430−438. 15. Jin, X.; Subramaniam, B.; Chaudhari, R. V. Proceedings of the AIChE Annual Meeting, Minneapolis, MN, Sept. 17−21, 2011. 16. Zhou, Z.; Li, X.; Zeng, T.; Hong, W.; Cheng, Z.; Yuan, W. Chin. J. Chem. Eng. 2010, 18, 384−390. 17. Montassier, C.; Menezo, J. C.; Hoang, L. C.; Renaud, C.; Barbier, J. J. Mol. Catal. 1991, 70, 99−110. 18. Wang, K. Y.; Hawley, M. C.; Furney, T. D. Ind. Eng. Chem. Res. 1995, 34, 3766−3770. 19. Chopade, S. P.; Miller, D. J.; Jacson, J. E.; Werpy, T. A.; Frye, J. G.; Zacher, A. H. U.S. Patent 6291725B1, Sept. 18, 2001. 20. Clark, I. T. Ind. Eng. Chem. 1958, 50, 1125−1126. 21. Leo, I. M.; Granados, M. L.; Fierro, J. L. G.; Mariscal, R. Chin. J. Catal. 2014, 35, 614−621. 22. Chen, X. G.; Wang, X. C.; Yao, S. X.; Mu, X. D. Catal. Commun. 2013, 39, 86−89. 23. Yuan, Z. L.; Wang, L. N.; Wang, J. H.; Xia, S. X.; Chen, P.; Hou, Z. Y.; Zheng, X. M. Appl. Catal., B 2011, 101, 431−440. 24. Blanc, B.; Bourrel, A.; Gallezot, P.; Haas, T.; Taylor, P. Green Chem. 2000, 2, 89−91. 25. Huang, Z. W.; Chen, J.; Jia, Y. Q.; Liu, H. L.; Xia, C. G.; Liu, H. C. Appl. Catal., B 2014, 147, 377−386. 26. Auneau, F.; Berchu, M.; Aubert, G.; Pinel, C.; Besson, M.; Todaro, D.; Bernardi, M.; Ponsetti, T.; Di Felice, R. Catal. Today 2014, 234, 100−106. 27. Sun, J. Y.; Liu, H. C. Catal. Today 2014, 234, 75−82. 28. Jin, X.; Roy, D.; Thapa, P. S.; Subramaniam, B.; Chaudhari, R. V. ACS Sustain. Chem. Eng. 2013, 1, 1453−1462. 29. Jin, X.; Dang, L.; Lohrman, J.; Subramaniam, B.; Ren, S.; Chaudhari, R. V. ACS Nano 2013, 7, 1309−1316. 30. Xia, S. X.; Yuan, Z. L.; Wang, L. N.; Chen, P.; Hou, Z. Y. Appl. Catal., A 2011, 403, 173−182. 31. Xia, S. X.; Nie, R. F.; Lu, X. Y.; Wang, L. N.; Chen, P.; Hou, Z. Y. J. Catal. 2012, 296, 1−11. 30 ACS Paragon Plus Environment

Page 30 of 55

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32. Yan, W. J.; Ramanathan, A.; Ghanta, M.; Subramaniam, B. Catal. Sci. Technol. 2014, 4, 4433−4439. 33. Huang, Z. W.; Cui, F.; Kang, H. X.; Chen, J.; Zhang, X. Z.; Xia, C. G. Chem. Mater. 2008, 20, 5090−5099. 34. Haynes, W. M. CRC Handbook of Chemistry and Physics: Boca Raton, 2010; pp 4-55−4-74. 35. Banu, M.; Venuvanalingam, P.; Shanmugam, R.; Viswanathan, B.; Sivasanker, S. Top. Catal. 2012, 55, 897−907. 36. Bienholz, A.; Schwab, F.; Claus, P. Bienholz, A.; Schwab, F.; Claus, P. Green Chem. 2010, 12, 290−295. 37. Bravo-Suarez, J. J.; Subramaniam, B.; Chaudhari, R. V. Appl. Catal., A 2013, 455, 234−246. 38. Zhu, J. W.; Li, D.; Chen, H. Q.; Yang, X. J.; Lu, L.; Wang, X. Mater. Lett. 2004, 58, 3324−3327. 39. Wu, S. H.; Chen, D. H. J. Colloid. Interf. Sci. 2004, 273, 165−169. 40. Yilmaz, G. J. Mol. Struct. 2012, 1019, 37−42. 41. Kerstan, M.; Wuller, M.; Russel, C. Mater. Res. Bull. 2011, 46, 2456−2463. 42. Pickup, D. M.; Wetherall, K. M.; Knowles, J. C.; Smith, M. E.; Newport, R. J. J. Mater. SciMater. Med.. 2008, 19, 1661−1668. 43. Schmid, S.; Lobo, C.; Withers, R. L.; Thompson, J. G. J. Solid State Chem. 1996, 127, 82−86. 44. Ilinich, O.; Ruettinger, W.; Liu, X. S.; Farrauto, R. J. Catal. 2007, 247, 112−118. 45. Ghose, J.; Rao, E. N. Studies on the formation of a solid solution compound in Li2O impregnated CuAl2O4 catalyst. Mater. Res. Bull. 1983, 18, 73−78. 46. Valenzuela, M. A.; Aguilar, G.; Bosch, P.; Armendariz, H.; Salas, P.; Montoya, A. Catal. Lett. 1992, 15, 179−188. 47. Baylet, A.; Marecot, P.; Duprez, D.; Castellazzi, P.; Groppi, G.; Forzatti, P. Phys. Chem. Chem. Phys. 2011, 13, 4607−4613. 48. He, M.; Luo, M. F.; Fang, P. J. Rare Earth 2006, 24, 188−192. 49. Krieger, T. A.; Tsybulya, S. V.; Tsyrulnikov, P. G. React. Kinet. Catal. Lett. 2002, 75, 141−146.


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50. Xia, S. X.; Zheng, L. P.; Ning, W. S.; Wang, L. N.; Chen, P.; Hou, Z. Y. J. Mater. Chem., A 2013, 1, 11548−11552. 51. Vila, F.; Granados, M. L.; Ojeda, M.; Fierro, J. L. G.; Mariscal, R. Catal. Today 2012, 187, 122−128. 52. Wolosiak-Hnat, A.; Milchert, E.; Lewandowski, G.; Grzmil, B. Pol. J. Chem. Technol. 2011, 13, 71−76. 53. Kwak, B. K.; Park, D. S.; Yun, Y. S.; Yi, J. Catal. Commun. 2012, 24, 90−95. 54. Yan, X. Y.; Jin, F. M.; Tohji, K.; Kishita, A.; Enomoto, H. AIChE J. 2010, 56, 2727−2733. 55. Nagaraja, B. M.; Padmasri, A. H.; Seetharamulu, P.; Reddy, K. H. P.; Raju, B. D.; Rao, K. S. R. J. Mol. Catal., A 2007, 278, 29−37. 56. Peng, L. C.; Lin, L.; Zhang, J. H.; Zhuang, J. P.; Zhang, B. X.; Gong, Y. Molecules 2010, 15, 5258−5272. 57. Roman-Leshkov, Y.; Moliner, M.; Labinger, J. A.; Davis, M. E. Angew. Chem., Int. Ed. 2010, 49, 8954−8957. 58. Davis, L.; Glaser, L. Biochem. Biophys. Res. Commun. 1971, 43, 1429−1435.


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Figures, Tables and Schemes

Figure 1. Comparison of Cu/CaO-Al2O3 catalysts with previous work for sorbitol conversion (Conditions: T: 200 - 230 oC, PH2: 4 - 8 MPa)


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Figure 2. Effect of Cu/Ca2+ ratio on catalyst activity and product distribution (Reaction time: product distribution obtained after 6 h, TOF values calculated with < 23% conversion level, other conditions same as Table 1)


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Figure 3. Adsorption/desorption isotherms of Cu/CaO-Al2O3 catalysts with different Cu/Ca2+ atomic ratios


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Figure 4. TPR profiles of CuO/MgO-Al2O3, CuO/ZnO-Al2O3, CuO/CaO-Al2O3-1 and CuO/CaOAl2O3-2 catalysts


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Figure 5. UV-Vis spectra for selected Cu/CaO-Al2O3 catalysts


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Figure 6. XRD spectra for selected Cu/CaO-Al2O3 catalysts


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Figure 7. TEM data of (a - b) Cu/CaO-Al2O3-1 catalyst and (c) its EDX mapping (white bars indicate 5 nm)


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Figure 8. Phase diagram of Cu/CaO-Al2O3-1 sample (obtained from SEM characterization, refer Experimental for analysis details and Figure S12 for element analysis in each phase): (a) layered phase image of a selected sample region; phase mapping of (b) CuAlO and (c) CuCaAlO phases in the sample region (white bars indicate 5 µm)


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(a)

(b) Figure 9. SEM images and EDX mapping of used Cu/CaO-Al2O3-1 sample [after 3rd recycle, (a) and (b) are selected two different regions. White bars indicate 500 nm]


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Figure 10. STEM images of used Cu/CaO-Al2O3-1 sample after 3rd recycle, (a) STEM image (white bar indicates 50 nm), (b) element mapping of Cu, Ca and Al species


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(a)


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(b)


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(c) Figure 11. Concentration-time profiles of sorbitol conversion on Cu/CaO-Al2O3-1 catalyst (a) T: 230 oC, PH2: 7.6 MPa; (b) T: 230 oC, PH2: 4.9 MPa; (c) T: 210 oC, PH2: 4.9 MPa


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Figure 12. Effect of hydrogen pressure on product selectivity at 230 oC (conversion = 17 - 23%)


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Table 1. HDO of sorbitol over different hybrid, base-promoted Cu catalysts

#

Catalysts

1 2

Cu/CaO-Al2O3-1

3 4

Cu/MgO-Al2O3

5 6 7

Cu/ZnO-Al2O3 Cu/H-ZSM5d

Time (h)

Xa (%)

3

Selectivity (%) 1,2-PDO

LA

glycerol

EG

C4-6b

Othersc

57.1

39.0

16.6

14.3

14.3

6.4

6.9

6

98.1

46.1

1.8

11.8

15.4

6.5

14.5

6

54.1

29.1

19.0

4.9

12.8

3.2

7.7

12

80.6

31.6

11.5

5.7

10.2

5.5

12.9

6

55.9

38.3

11.0

2.2

12.2

14.4

3.8

12

94.9

35.9

1.1

6.3

11.7

29.0

10.5

6 3

85.7 2+

2+

Anhydroglucitol: ~30%

Isosorbide: 46%

2+

a. Conversion at 9.8 kg/m , Cu/Mg , Zn , Ca molar ratio: 5.4, sorbitol: 0.18 kmol/m3, T: 230 o C, PH2: 7.6 MPa; b. C4-6 tetrols, triols, diols, etc; c. Mainly MeOH and EtOH, trace 1-propanol (1-PrOH), 2-PrOH, methane and carbon dioxide, etc.

Table 2. Physical properties for various Cu/CaO-Al2O3 catalysts Cu/Ca2+

Surface area (m3/g)

Pore size (nm)

Pore volume (cm3/g)

Powder size* (mesh)

3.5

39.4

7.7

0.09

200

4.0

42.9

9.3

0.08

200

5.4

47.6

10.9

0.13

200

6.0 81.9 13.3 0.19 200 *Particle sizes of as prepared catalysts were in the range of 10 - 16 mesh, which was crumbled into powder forms before activity tests.


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Table 3. Product distribution on different Cu catalysts in sorbitol conversion Selectivity (%)

#

Catalysts

X (%)

1,2-PDO

LA+PAD

glycerol

EG

Others

1

Cu/CaO-Al2O3-1

98.1

46.1

1.8

11.8

15.4

21

2

Cu/CaO-Al2O3-2

17.9

12.9

64.3

4.3

9.5

-

3

Cu/CaO-Al2O3-3

12.1

8.4

75.2

15.1

1.1

-

Catalyst charge: 9.8 kg/m3, sorbitol: 0.18 kmol/m3, T: 230 oC, PH2: 7 MPa, reaction time: 6 h. PAD: pyruvaldehyde.

Table 4. HDO of xylitol, erythritol and glycerol on Cu/CaO-Al2O3-1 catalyst Selectivity (%) #

Substrate

X (%) 1,2-BDO

1,2-PDO

LA+PAD

glycerol

EG

EtOH

MeOH

1

Mannitol

100

trace

51.5

4.1

9.5

11.4

3.6

6.6

2

Xylitol

98.9

trace

44.4

6.7

5.6

26.0

5.6

1.5

3

Erythritol

85.4

38.6

15.7

3.9

1.0

10.9

trace

7.7

4

Glycerol

67.5

-

81.1

5.7

-

8.1

-

3.8

Cu/CaO-Al2O3-1 catalyst: 9.8 kg/m3, sorbitol: 0.18 kmol/m3, T: 230 oC, PH2: 2.8 MPa, reaction time: 10 h.


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Table 5. Recycle studies of Cu/CaO-Al2O3-1 catalyst Recycle #

X (%)

1

Selectivity (%) 1,2-BDO

1,2-PDO

LA+PAD glycerol

EG

EtOH

MeOH

88.1

5.6

48.1

3.1

11.5

18.1

3.2

5.1

2

74.4

9.2

44.5

5.7

6.6

16.0

5.7

2.5

3

68.9

4.5

46.1

9.9

3.0

10.9

3.1

8.7

Sorbitol: 0.18 kmol/m3, reaction conditions same as Table 1


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Scheme 1. Formation of 1,2-PDO, glycerol and LA from C3 intermediates


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Scheme 2. Possible reaction pathways of sorbitol on Cu/CaO-Al2O3 catalyst


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Scheme 3. Possible reaction pathways of erythritol on Cu/CaO-Al2O3 catalyst


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Scheme 4. Possible reaction pathways of glycerol on Cu/CaO-Al2O3 catalyst


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Scheme 5. Reaction mechanism of bio-derived polyols in Cu/CaO-Al2O3 catalysts


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ACS Catalysis

Abstract Figure


CaO-Al2O3 Catalysts

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