Chinese Journal of Catalysis 34 (13) 66 74 催化学报 13 年第 34 卷第 11 期 www.chxb.cn available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Article (Dedicated to Professor Yi Chen on the occasion of his th birthday) Efficient catalytic hydrogenolysis of glycerol using formic acid as hydrogen source Jing Yuan, Shushuang Li, Lei Yu, Yongmei Liu, Yong Cao * Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 433, China A R T I C L E I N F O Article history: Received 19 May 13 Accepted 11 July 13 Published November 13 Keywords: Copper Zirconium dioxide Hydrogenolysis Glycerol Formic acid A B S T R A C T We describe a sustainable, cost effective, and highly efficient H2 free protocol for catalytic hydrogenolysis of glycerol to 1,2 propanediol (1,2 PDO) using formic acid (FA) as the H2 source. The process is catalyzed by an earth abundant and robust Cu based metal oxide catalyst, in which the high performance of the Cu catalyst for the in situ generation of H2 gas in the system by highly selective decomposition of FA in an aqueous medium is essential. The activity test results showed that a synergy effect of well dispersed Cu and amphoteric ZrO2 is essential for FA decomposition as well as for glycerol conversion to 1,2 PDO. The Cu content of the Cu/ZrO2 catalyst prepared by the oxalate gel method has a significant role in the FA mediated glycerol conversion to 1,2 PDO, and a Cu content of wt% on ZrO2 was identified as the optimum Cu content. Moreover, the creation and maintenance of high component dispersion is essential for high glycerol hydrogenolysis activity of the Cu/ZrO2 system. Because selective hydrogenolysis with minimum use of external fossil fuel H2 is a critical issue in the realization of biorefinery concepts, the procedure described here is expected to be of broad applicability in biomass use. 13, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Diminishing petroleum reserves and growing concerns about global climate change necessitate the development of fuel and chemical production pathways based on renewable resources such as biomass materials [1,2]. In this respect, glycerol, obtained as a high volume by product in the biodiesel process, has emerged as a promising abundant feedstock for the renewable chemicals industry [3 5]. Among various transformations already reported for glycerol, hydrogenolysis to propanediols (PDOs) is of particular interest because of the large numbers of applications of 1,2 and 1,3 PDO (Scheme 1). 1,2 PDO is a major commodity chemical that is currently obtained through the selective hydrolysis of propylene oxide [6 8]. 1,3 PDO has traditionally been considered a specialty chemical [9,1]; however, it is fast emerging as a significant building block for the manufacture of polyesters such as poly(trimethylene terephthalate). Glycerol hydrogenolysis has mainly been performed using molecular hydrogen (H2) [4,11 14]. However, high H2 pressures are generally required to reach acceptable conversion and selectivity. As well as this practical inconvenience, the use of H2 has some other important drawbacks. First, most of the currently available H2 gas is still produced from fossil fuels, which makes the process dependent on fossil carbon. In addition, it ignites easily and has high diffusivity, and therefore presents considerable hazards when working at high pressures. One interesting alternative that could minimize external fossil fuel H2 consumption is the use of formic acid (FA), one of the major products formed during biomass processing, as the * Corresponding author. Tel: +86 21 55665287; Fax: +86 21 65643774; E mail: yongcao@fudan.edu.cn This work was supported by the National Natural Science Foundation of China (2127344). DOI: 1.116/S1872 67(12)656 1 http://www.sciencedirect.com/science/journal/187267 Chin. J. Catal., Vol. 34, No. 11, November 13
Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 67 HO +H 2,-H 2 O HO Cat. HO Glycerol 1,2-PDO 1,3-PDO Scheme 1. Reaction for conversion of glycerol to PDOs. H2 source. FA has recently attracted considerable interest in the area of green and sustainable chemistry because of its potential as a H2 carrier and as a means of using CO2 [3,15]. FA is liquid at room temperature, so it has the added technical advantage that its use as a replacement for H2 would not be limited by storage and delivery problems [15,16]. Compared with the impressive progress being made in glycerol hydrogenolysis with H2, very few reports dealing with the direct hydrogenolysis of glycerol using FA as a H2 source are available. In related previous studies, bimetallic Ni Cu supported on Al2O3 was used as an efficient catalyst for direct hydrogenolysis of glycerol with FA [17]. However, this catalyst has a low inherent efficiency, and the system requires the use of a heavily loaded catalyst and prolonged reaction times at high temperatures (up to 24 h at 2 C) to improve the hydrogenolysis kinetics. From both practical and economic points of view, a more efficient catalytic transformation that can be carried out at lower reaction temperatures using stoichiometric amounts of FA and glycerol is preferable. As part of our ongoing interest in the development of new catalytic methods that enable controlled and selective transformations of highly functionalized bio derived feedstocks, we recently discovered an efficient heterogeneous catalytic system for the direct conversion of bio derived aqueous levulinic acid/fa streams to γ valerolactone, catalyzed by ZrO2 supported high dispersed Au nanoparticles (NPs), in which high performance of the supported Au NPs for in situ generation of H2 gas via selective FA decomposition under mild conditions is essential [18]. This Au/ZrO2 FA protocol is also a suitable method for selective hydrogenolysis of glycerol to 1,2 PDO in high yields. However, the high cost and limited availability of Au have necessitated the development of earth abundant base metal alternatives for these applications. Simple Cu based catalysts are promising candidates because they have high activities toward FA decomposition in the vapor phase [19 22]. Moreover, it is also well established that Cu catalysts are very selective for catalytic hydrogenolysis of glycerol under a H2 atmosphere [23,24]. In this paper, we report that a simple Cu Zr nanocomposite featuring high component dispersion can be used as an efficient catalyst in the selective conversion of glycerol to 1,2 PDO using bio derived FA as a convenient H2 source. 2. Experimental 2.1. Catalyst preparation The catalysts were prepared using the conditions and methods listed in Table 1. 2.1.1. Oxalate gel (OG) coprecipitation (CP) method [25]. Briefly, an alcoholic solution of % excess oxalic acid was injected rapidly into an alcoholic solution of zirconium (cerium) nitrate (.1 mol/l) containing the desired amount of copper (iron, cobalt, nickel) nitrate at room temperature under vigorous stirring. After stirring for 2 h, the resultant gel like precipitates were separated by filtration, followed by air drying at 11 C overnight. The as obtained materials were then calcined in a muffle oven at 5 C for 4 h. 2.1.2. Na2CO3 CP method Typically, a mixed aqueous solution of copper nitrate and zinc nitrate (each.1 mol/l) and a solution of Na2CO3 (.1 mol/l) were added slowly and simultaneously to 15 ml of deionized water at C under vigorous stirring. The ph was kept constant at 6.5 7.. After stirring for 4 h, the precipitates were separated by filtration, followed by drying at 11 C overnight. The as obtained material was then calcined in a muffle oven at 5 C for 4 h. 2.1.3. Incipient wetness impregnation (Imp) method In a typical procedure, a certain amount of ZrO2 support or SiO2 support was soaked in an aqueous solution of copper nitrate of appropriate concentration. The mixture was stirred for 4 h at room temperature and then stirred at 85 C until the mixture was dry. The samples were dried in an oven at 11 C overnight. 2.1.4. Urea deposition precipitation (urea DP) method Typically, a mixed aqueous solution of copper nitrate (.1 mol/l) and urea (1 mol/l), and ZrO2 (or P25) were added to ml of deionized water at 9 C under vigorous stirring for 4 h. The solids were separated by filtration, followed by drying in an oven at 11 C overnight. The as obtained materials were Table 1 FA conversions and CO contents for different catalysts. Catalyst Preparation FA conversion a CO concentration b method (%) (ppm) Cu/ZnO Al2O3 c Na2CO3 CP 7 5 3%Cu/ZnO Al2O3 Na2CO3 CP 9 11 3%Cu/ZnO Na2CO3 CP 15 47 3%Fe/ZrO2 OG CP 1 45 3%Co/ZrO2 OG CP 1 57 3%Ni/ZrO2 OG CP 2 55 3%Cu/SiO2 Imp 1 5 3%Cu/CeO2 OG CP 32 3%Cu/TiO2 Urea DP 5 3 3%Cu/ZrO2 Urea DP 1 4 3%Cu/ZrO2 Imp 1 52 3%Cu/ZrO2 OG CP 5 12 Reaction conditions: FA 18 mmol, H2O 4 ml, nfa/nmetal = 24, 1 C, 5 h. aanalyzed by a HPLC (HP 1, Agilent, USA) equipped with a refractive index detector (RID) and a Platisil ODS C18 column. bmeasured by using a GC analysis system equipped with a methanizer and a flame ionization detector (detection limit ~1. ppmv). csupplied by Research Institute of Nanjing Chemical Industrial Group.
68 Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 then calcined at 5 C for 4 h. Prior to reaction, all samples were prereduced with 5% H2/Ar at 3 C for 2 h. 2.2. Characterization X ray diffraction (XRD) analysis of the catalysts was carried out on a Bruker D8 Advance (Germany) X ray diffractometer using Ni filtered Cu Kα radiation, with a scanning angle (2θ) of, a scanning speed of 2 /min, and a voltage and current of 4 kv and ma, respectively. Hydrogen temperature programmed reduction (H2 TPR) experiments were carried out using a laboratory made apparatus, as described elsewhere [26]. The specific surface areas of the prepared catalysts were determined by adsorption desorption of N2 at 196 o C using a Micromeritics TriStar 3 system. Sample degassing was carried out at 3 C for 4 h prior to acquiring the adsorption isotherm. X ray photoelectron spectroscopy (XPS) analysis was performed with a Perkin Elmer PHI 5C system equipped with a hemispherical electron energy analyzer using Mg Κα (hν = 1253.6 ev) radiation operated at 15 kv and ma. The energy scale was internally calibrated by setting the C 1s peak at 284.6 ev. The specific surface area of metallic Cu was measured based on the adsorption and decomposition of N2O on the surface of metallic Cu as follows: 2Cu(s) + N2O N2 + Cu2O(s). The pulse titration technique was employed. Pure N2 was used as the carrier gas, and a thermal conductivity detector (TCD) was used to detect the amount of N2O consumed [25,27]. The specific area of metallic Cu was calculated from the total N2O consumption, with 1.46 1 19 Cu atoms per m 2. The Cu dispersion was calculated based on the specific surface. The solution after reaction was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP AES) using a Thermo Electron IRIS Intrepid II XSP spectrometer. 2.3. Decomposition of FA and hydrogenolysis of glycerol For decomposition of FA, a mixture of FA (18 mmol), water (4 ml), and the prereduced supported metal catalyst (metal 4.2 mol%) was charged into a 5 ml Hastelloy C high pressure Parr reactor and stirred at a rate of r/min. The mixture of the substrate and the catalyst was heated to 1 C for less than 15 min. After the given reaction time, the concentration of residual FA was analyzed using a high performance liquid chromatography (HPLC; HP 1, Agilent, USA) system consisting of a Platisil ODS C18 column and a refractive index detector. H2SO4 (.5 mmol/l) was used as the mobile phase at a flow rate of 1 ml/min. The temperatures of the column and the detector were both 4 C. The gaseous products were analyzed using a GC equipped with a TDX 1 column and a TCD. The carrier gas was He. A standard gas sample consisting of 33.3% H2, 33.3% CO, and 33.3% CO2 was used to calibrate the GC. The composition of gaseous products was determined based on the standard gas. Very low CO concentrations can be reliably measured using a GC analysis system equipped with a methanizer and a flame ionization detector (FID, detection limit ~1. ppmv). For hydrogenolysis of glycerol, a mixture of glycerol (5 mmol), FA (a certain amount, according to the experimental requirements), the prereduced supported metal catalyst (metal 1 mol%), 2 methoxyethyl ether (2.5 mmol, internal standard), and water (1 ml) was charged into a 25 ml Hastelloy C high pressure Parr reactor and stirred at a rate of r/min under a.5 MPa N2 atmosphere for a given reaction time. The mixture of substrate and catalyst was heated to the desired temperature in less than 15 min. The liquid products were analyzed using a Shimadzu GC 17A gas chromatograph equipped with a capillary column HP FFAP (3 m.25 mm) and an FID. The identification of the products was performed using GC mass spectrometry. 3. Results and discussion We commenced our study by investigating the selective decomposition of FA in an aqueous medium over various Cu based solid catalysts. As early as the 19s, the decomposition of FA was widely investigated as a probe reaction for characterizing the surface properties of metals and metal oxides. Among these reports, two different pathways, namely dehydrogenation (HCO H2 + CO2, ΔG = 48.4 kj/mol) and dehydration (HCO H2O + CO, ΔG = 28.5 kj/mol), were identified [16]. From the stand point of catalysis, what is of particular interest is that the selectivities for the two possible pathways can be controlled by the choice of catalyst. In the specific use of FA as a H2 source for chemical synthesis, the key point is selective decomposition of liquid phase FA to H2 and CO2, while avoiding undesirable formation of CO. In general, studies of heterogeneous FA decomposition have mostly concentrated on gas phase catalysis. For the liquid phase decomposition of FA by heterogeneous catalysts, a number of supported as well as unsupported noble metals, including Pd, Pt, Au, and their corresponding bimetallic alloys such as Pd Au, have been reported to be effective [28 3]. The development of new non noble catalytic systems that give fast, selective decomposition of FA under mild aqueous conditions is therefore still of great interest. In a first set of experiments, the decomposition of a diluted aqueous solution of FA (.43 mol/l) for 5 h at 1 C was studied in a batch autoclave reactor. The results in terms of the direct production of H2 and CO2 over various Cu based catalysts are compiled in Table 1. The H2/CO2 molar ratios (n) obtained from most catalysts under these conditions were around 1:1, with very low CO contents, indicating that the selective decomposition of FA can take place under these reaction conditions. However, most catalysts were not very active in the direct dehydrogenation of FA (conversion level < 3%) under our experimental conditions. Conventional 3%Cu/ZnO, 3%Cu/ZnO Al2O3, and industrial Cu/ZnO Al2O3 methanol synthesis catalysts gave either low activities or high CO contents. It is interesting to note that the Cu/ZrO2 based catalyst prepared using the OG CP method gave good FA conversion, with a consistently low CO content (< ppm); this is of particular interest for the direct use of FA as a convenient source of
Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 69 FA conversion (%) 4 (6) (2) (7) 1 2 3 4 5 6 Time (h) (3) (4) (1) (5) Fig. 1. Time profiles of FA decomposition over Cu/ZrO2 catalysts with different Cu loadings. (1) 1%Cu/ZrO2; (2) %Cu/ZrO2; (3) 3%Cu/ZrO2; (4) 4%Cu/ZrO2; (5) 5%Cu/ZrO2; (6) %Cu/ZrO2 (FA 18 mmol, H2O ml); (7) %Cu/ZrO2 (FA 9 mmol, H2O 4 ml). Reaction conditions: FA 18 mmol, H2O 4 ml, nfa/nmetal = 24, 1 C. Table 2 Physiochemical properties of various Cu/ZrO2 catalysts. Catalyst SBET SCu a (m 2 /gcu) H2. Moreover, a clear advantage of the 3%Cu/ZrO2 OG catalyst over other first row Group VIII metals (base metals) with the same metal loading was also observed when aqueous FA was subjected to decomposition using identical OG derived Fe/ZrO2, Ni/ZrO2, or Co/ZrO2 under otherwise identical conditions. To further improve the FA conversion while minimizing undesirable CO formation, Cu/ZrO2 catalysts with different Cu loadings were prepared, and their performance in FA decomposition was measured. As indicated in Fig. 1, the catalytic performance was strongly dependent on the Cu contents. Among the catalysts examined at this stage of the study, the Cu/ZrO2 OG catalyst containing wt% Cu exhibited by far the best catalytic performance, and it was found that % FA conversion was achieved within 3 h at 1 C. This result is significant, especially as, similar to all these experiments, only a very low level of CO (< ppm) was detected in the gas phase products. The reaction parameters were further optimized for FA decomposition by varying the feed concentration, which is very important for the product distribution in the glycerol hydrogenolysis reaction; this will be discussed further. It was found that even more efficient FA decomposition was achieved by using a FA solution of higher concentration. To clarify the origin of their high FA decomposition activities, the (1 5 wt%)cu/zro2 OG catalysts were characterized in detail. Typical data such as BET specific surface areas and Cu metal surface areas (determined from chemical N2O titration) are summarized in Table 2. It can be seen that there is only a weak relationship between the specific surface area and the performance of the Cu/ZrO2 OG samples, indicating that the external texture is not the key factor governing the catalytic performance of the samples prepared using the OG CP method. However, there is a strong positive correlation between Cu dispersion and FA decomposition activity (not shown). The fact that a specific composition of %Cu/ZrO2 OG can maximize exposure of the fraction of catalytically active species at the catalyst surface is crucial for achieving high activity in FA decomposition, in line with the many literature reports documenting the structure activity relationships of supported Cu catalysts [25,27,31,32]. The XRD patterns of the calcined forms of the as synthesized catalysts showed only tetragonal ZrO2 and CuO crystallites (not shown). After prereduction under a 5% H2/Ar atmosphere at 3 C for 2 h, a strong peak at 43.3, ascribed to the Cu (111) lattice plane of metallic Cu, appeared. This, together with the fact that no diffraction features of CuO could be identified in the reduced form of the catalyst (Fig. 2), indicates that CuO was completely reduced to metallic Cu in the working sample. H2 TPR (Fig. 3) showed that %Cu/ZrO2 can be completely reduced at a lower temperature ( C), which indicates stronger Cu Zr interactions; this was considered to be the most important reason for FA decomposition over Cu/ZrO2 catalysts without leaching of Cu. Based on the above results, we next focused on the direct use of FA as the sole H2 source to facilitate the hydrogenolysis of glycerol in an aqueous medium. Figure 4 shows plots of glycerol conversion versus temperature in the presence of %Cu/ZrO2 OG for 5 h. With increasing temperature, the reaction rate increased, but the selectivity for 1,2 PDO decreased significantly as a result of formation of appreciable amounts of n propanol as a by product. As a result, C was found to be the optimum temperature for targeted 1,2 PDO formation. The molar ratio of FA to glycerol, which might be very important for the product distribution, was also investigated to maximize the 1,2 PDO yield. As shown in Fig. 5, the conversion of glycerol DCu dcu b (nm) (m 2 /g) (%) 1%Cu/ZrO2 OG 28 28. 4.3 11.2 %Cu/ZrO2 OG 4 47.5 7.4 12.3 3%Cu/ZrO2 OG 37 41.7 6.5 14.6 4%Cu/ZrO2 OG 63 36.7 5.7.8 5%Cu/ZrO2 OG 36 33.8 5.2 21.6 3%Cu/ZrO2 DP 31 4..6 25.3 3%Cu/ZrO2 Imp 3.3.5 79. adetermined by N2O titration. bestimated from XRD data using the Scherrer equation. Intensity t = tetragonal t Cu 3 4 5 7 2 /( o ) t t t (5) (4) (3) (2) Fig. 2. XRD patterns of reduced Cu/ZrO2 catalysts with different Cu loadings prepared by the OG CP method. (1) 1%Cu/ZrO2; (2) %Cu/ZrO2; (3) 3%Cu/ZrO2; (4) 4%Cu/ZrO2; (5) 5%Cu/ZrO2. (1)
7 Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 H2 consumption 5%CuO/ZrO 2 4%CuO/ZrO 2 3%CuO/ZrO 2 %CuO/ZrO 2 1%CuO/ZrO 2 CuO 5 15 25 3 35 Temperature ( o C) Fig. 3. TPR profiles of CuO/ZrO2 catalysts with different Cu loadings prepared by the OG CP method. Glycerol conversion (%) 4 1 19 21 2 23 Temperature ( o C) Fig. 4. Glycerol conversions and 1,2 PDO selectivities at different temperatures. Reaction conditions: glycerol 5 mmol, H2O 1 ml, FA 5 mmol,.5 MPa N2, nglycerol/nmetal = 1, 5 h. increased from 69% to 89% when the molar ratio of FA to glycerol changed from 1:2 to 3:1; this is consistent with the results in Fig. 1, which shows that FA decomposition is more favored at high FA concentrations. The selectivity for 1,2 PDO was quite low when nfa/nglycerol = 1:2 because the lack of H2 could favor the undesirable degradation of glycerol to other products. However, more 1,2 PDO was converted to n propanol by excess H2 (nfa/nglycerol > 1). The highest selectivity (96%) for 1,2 PDO was obtained with nfa/nglycerol = 1. Given the fact that the yield of the target product can be up to 94%, it appeared that complete use of FA was achieved in this particular case. To explore the reaction performed with %Cu/ZrO2 OG in detail, the product evolution for the direct hydrogenolysis of glycerol with FA at C was followed by continuous sampling using GC. As shown in Fig. 6, the conversion of glycerol reached 85% within 5 h. The reaction rate then leveled off and an extended time of 18 h was required to attain a conversion of 97%. Throughout the process, the 1,2 PDO selectivity remained above 95%, and an approximately 94% yield of 1,2 PDO was achieved after reaction for 18 h at C using FA. It is important to emphasize that this value was much better than that reported by Gandarias et al. [17] (conversion of 9% and 1,2 PDO selectivity of 82% after operating for 24 h at 2 C by 4 Selectivity for 1,2-PDO (%) Glycerol conversion (%) 95 9 85 75 7 65 1/2 1/1 2/1 3/1 n FA/n glycerol Fig. 5. Glycerol conversions and 1,2 PDO selectivities with different molar ratios of FA and glycerol. Reaction conditions: glycerol 5 mmol, H2O 1 ml,.5 MPa N2, nglycerol/nmetal = 1, C, 5 h. Glycerol conversion (%) 4 with FA with H 2 continuously introducing FA). As stability is critical for the efficient use of any catalyst, possible metal leaching during the reaction was studied. As can be seen from Fig. 7, the conversion of glycerol and selectivity for 1,2 PDO remained essentially the same after five cycles. Catalyst leaching was not observed by ICP AES analysis. This finding demonstrates the high stability of the %Cu/ZrO2 OG catalyst and rules out the possibility of catalyst deactivation during the reaction. To further understand the reaction pathways involved in the FA mediated reduction, we investigated a separate hydrogenolysis of an aqueous solution of glycerol (.5 mol/l) at C under a H2 atmosphere (.8 MPa). As shown in Fig. 6, the hydrogenolysis performance of Cu/ZrO2 using H2 closely resembled that observed with FA as the reductant. It is important to note that the pressure inside the autoclave reactor increased rapidly from.5 MPa to a maximum value of ca. 4 MPa in the first hour of reaction during the hydrogenolysis of glycerol over %Cu/ZrO2, an indication that significant FA decomposition, leading to H2/CO2 formation, occurred in the initial stage of the reaction. A set of control experiments indicated that the FA concentration was constant at C in the absence of catalysts. Taken together, these results demonstrated that the hydrogenolysis of glycerol with FA did not proceed through 95 9 85 75 7 65 Selectivity for 1,2-PDO (%) 2 4 6 8 1 12 14 16 18 Time (h) Fig. 6. Profiles of glycerol hydrogenolysis over time using FA and H2 as sources. Reaction conditions: glycerol 5 mmol, H2O 1 ml, FA 5 mmol (or.8 MPa H2; nh2 = nfa), nglycerol/nmetal = 1, C.
Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 71 Glycerol conversion (%) 4 Cu catalyzed transfer hydrogenolysis, as emphasized by Gandarias et al. [17], but rather through indirect hydrogenolysis of glycerol with H2 generated in situ by FA decomposition. More specifically, the fact that the Cu/ZrO2 catalyst can substantially facilitate the crucial FA dehydrogenation appeared to be the key factor for achieving high activity in glycerol hydrogenolysis. 4. Conclusions 1 2 3 4 5 6 Cycle Fig. 7. Recycling study of glycerol hydrogenolysis. Reaction conditions: glycerol 5 mmol, water 1 ml, FA 5 mmol, nglycerol/nmetal = 1, C, 5 h. We have demonstrated that a highly active, robust, non noble, Cu/ZrO2 based catalyst can be used to convert glycerol to 1,2 propanediol (1,2 PDO) in excellent yields using bio derived formic acid (FA) as a convenient H2 source. It was observed that there is an optimum FA/glycerol molar ratio (1:1) and an optimum temperature that maximize the yield of 1,2 PDO (94% yield after 18 h at C). This catalyst can be reused at least five times. The similar performance of the Cu/ZrO2 catalysts in glycerol hydrogenolysis to 1,2 PDO under a H2 atmosphere and using FA confirms that the present glycerol reaction with FA proceeds through a straightforward hydrogenolysis of glycerol by H2 generated in situ by FA decomposition. The present findings form the basis for cost competitive production of 1,2 PDO from glycerol and the development of new sustainable, 4 Selectivity for 1,2-PDO (%) affordable processes for the targeted conversion of bio derived feedstock, with minimum use of external fossil fuel based H2 sources. References [1] Huber G W, Iborra S, Corma A. Chem Rev, 6, 16: 444 [2] Corma A, Iborra S, Velty A. Chem Rev, 7, 17: 2411 [3] Martin A, Armbruster U, Gandarias I, Arias P L. Eur J Lipid Sci Technol, 13, 115: 9 [4] Brandner A, Lehnert K, Bienholz A, Lucas M, Claus P. Top Catal, 9, 52: 278 [5] Nakagawa Y, Tomishige K. Catal Sci Technol, 11, 1: 179 [6] Gandarias I, Arias P L, Requies J, Doukkali M E, Güemez M B. J Catal, 11, 282: 237 [7] Bolado S, Treviño R E, García Cubero M T, González Benito G. Catal Commun, 1, 12: 122 [8] Yuan Z L, Wu P, Gao J, Lu X Y, Hou Z Y, Zheng X M. Catal Lett, 9, 13: 261 [9] Qin L Z, Song M J, Chen C L. Green Chem, 1, 12: 1466 [1] Oh J, Dash S, Lee H. Green Chem, 11, 13: 4 [11] Miyazawa T, Kusunoki Y, Kunimori K, Tomishige K. J Catal, 6, 24: 213 [12] Maris E P, Davis R J. J Catal, 7, 249: 328 [13] Meher L C, Gopinath R, Naik S N, Dalai A K. Ind Eng Chem Res, 9, 48: 184 [14] Furikado I, Miyazawa T, Koso S, Shimao A, Kunimori K, Tomishige K. Green Chem, 7, 9: 582 [15] Jones S, Qu J, Tedsree K, Gong X Q, Tsang S C E. Angew Chem Int Ed, 12, 51: 11275 [16] Yi N, Saltsburg H, Flytzani Stephanopoulos M. ChemSusChem, 13, 6: 816 [17] Gandariasa I, Requies J, Arias P L, Armbruster U, Martin A. J Catal, 12, 29: 79 [18] Du X L, He L, Zhao S, Liu Y M, Cao Y, He H Y, Fan K N. Angew Chem Int Ed, 11, 5: 7815 [19] Tedsree K, Li T, Jones S, Chan C W A, Yu K M K, Bagot P A J, Marquis E A, Smith G D W, Tsang S C E. Nature Nanotech, 11, 6: 32 [] Ojeda M, Iglesia E. Angew Chem Int Ed, 9, 48: 4 [21] Solymosi F, Koós Á, Liliom N, Ugrai I. J Catal, 11, 279: 213 [22] Gazsi A, Bansagi T, Solymosi F. J Phys Chem C, 11, 115: 15459 [23] Huang Z W, Cui F, Kang H X, Chen J, Zhang X Z, Xia C G. Chem Mater, 8, : 59 [24] Liang C H, Ma Z Q, Ding L, Qiu J S. Catal Lett, 9, 13: 169 Chin. J. Catal., 13, 34: 66 74 Graphical Abstract doi: 1.116/S1872 67(12)656 1 Efficient catalytic hydrogenolysis of glycerol using formic acid as hydrogen source Jing Yuan, Shushuang Li, Lei Yu, Yongmei Liu, Yong Cao * Fudan University Biorenewable formic acid as a convenient H2 source and cheap and earth abundant Cu based catalysts can be used for the facile conversion of glycerol to 1,2 propanediol (PDO). The present findings form the basis of cost competitive production of 1,2 PDO from glycerol and the development of new sustainable, affordable processes for the targeted conversion of bio derived feedstock, with minimum use of external fossil fuel based H2 sources. HCO Glycerol HO Cu/ZrO 2 CO 2 1,2-PDO HCO Cu/ZrO 2 H2 + CO 2 Cu/ZrO H 2 2 HO
72 Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 [25] Wang L C, Liu Q, Chen M, Liu Y M, Cao Y, He H Y, Fan K N. J Phys Chem C, 7, 11: 16549 [26] Zhang X R, Wang L C, Yao C Z, Cao Y, Dai W L, He H Y, Fan K N. Catal Lett, 5, 12: 183 [27] Yao C Z, Wang L C, Liu Y M, Wu G S, Cao Y, Dai W L, He H Y, Fan K N. Appl Catal A, 6, 297: 151 [28] Bi Q Y, Du X L, Liu Y M, Cao Y, He H Y, Fan K N. J Am Chem Soc, 12, 134: 8926 [29] Zhou X C, Huang Y J, Xing W, Liu C P, Liao J H, Lu T H. Chem Commun, 8: 354 [3] Huang Y J, Zhou X C, Yin M, Liu C P, Xing W. Chem Mater, 1, 22: 5122 [31] Xia S X, Yuan Z L, Wang L N, Chen P, Hou Z Y. Appl Catal A, 11, 43: 173 [32] Xia S X, Nie R F, Lu X Y, Wang L N, Chen P, Hou Z Y. J Catal, 12, 296: 1 甲酸为氢源的甘油催化选择氢解 * 袁静, 李舒爽, 于磊, 刘永梅, 曹勇复旦大学化学系, 上海市分子催化和功能材料重点实验室, 上海 433 摘要 : 以甲酸作为氢源, 采用铜基复合金属氧化物催化剂, 催化氢解甘油制备 1,2- 丙二醇, 其中液相甲酸的高选择性分解是实现甘油氢解的必要和关键步骤. 活性测试表明, 高分散的铜和 ZrO 2 载体间的协同作用对甲酸分解和甘油到 1,2- 丙二醇的转化至关重要, %Cu/ZrO 2 催化剂的活性最佳. 由于避免使用相对昂贵的化石燃料氢, 因而该催化体系在生物质的高值利用方面具有潜在的应用前景. 关键词 : 铜 ; 二氧化锆 ; 氢解 ; 甘油 ; 甲酸 收稿日期 : 13-5-19. 接受日期 : 13-7-11. 出版日期 : 13-11-. * 通讯联系人. 电话 : (21)55665287; 传真 : (21)65643774; 电子信箱 : yongcao@fudan.edu.cn 基金来源 : 国家自然科学基金 (2127344). 本文的英文电子版由 Elsevier 出版社在 ScienceDirect 上出版 (http://www.sciencedirect.com/science/journal/187267). 1. 前言随着石油资源日益减少和全球气候变暖, 开发基于可再生资源 ( 如生物质材料 ) 的燃料和化学品生产过程具有重要意义 [1,2]. 目前, 生物柴油工业副产的大量的甘油已成为化学工业中非常有前景的基础化工原料 [3 5]. 由甘油氢解可制得 1,2- 丙二醇和 1,3- 丙二醇等用途广泛的高值化学品 [6 8] ( 图式 1). 1,2- 丙二醇是一类重要的大宗化学品, 目前主要通过基于化石原料的环氧丙烷水合过程制得 ; 而 1,3- 丙二醇则一度被视为一种精细化学品 [9,1], 近来作为聚对苯二甲酸丙二醇酯 (PTT) 等重要聚酯的单体而备受关注. 目前甘油氢解过程主要采用价格相对昂贵的分子氢作原料 [4,11 14], 为了得到较高的活性和选择性, H 2 压力需达 4 6 MPa. 由于目前工业上氢气仍主要从化石资源获得, 同时高压氢气的使用存在储运不便以及易燃爆等安全方面的问题, 因此从经济 环保及实用的角度来看, 有必要开发完全不依赖传统化石资源氢气的甘油氢解催化新过程. 甲酸是重要的化学中间体和可再生能源载体, 其低温催化选择分解是最近发现的一类在微型氢燃料电池及化学品合成领域具有重要应用前景的供氢反应体系 [3,15]. 甲酸在常温下为液体, 将它作为氢源便于储存和运输 [15,16]. 因而相对于高压氢气, 它在环保及碳排放 等方面具有很大优势. 最近发现双金属 Ni-Cu/Al 2 O 3 体系可催化甲酸氢解甘油 [17], 但该催化体系无论活性还是选择性均有待提高 ( 在 o C 反应 24 h, 1,2- 丙二醇收率不到 75%). 因此, 开发甲酸氢解甘油的低温高效催化剂显得尤为重要. 本课题组研究发现, ZrO 2 负载的纳米 Au 可实现水相生物基乙酰丙酸和等量甲酸直接转化为 γ- 戊内酯, 且高分散负载 Au 催化作用下的甲酸选择分解制氢是实现该过程的必要和关键步骤 [18]. 尽管该体系也可高效催化甘油氢解制 1,2- 丙二醇反应, 但 Au 资源的高度稀缺很大程度上限制了该过程的广泛使用. 因此, 有必要研究非贵金属催化剂来实现甲酸氢解甘油. 众所周知, Cu 基催化剂不仅在气相甲酸分解反应中 [19 22], 而且在以 H 2 为氢源的甘油选择氢解反应中表现出良好的活性和选择性 [23,24]. 因此本文将 Cu/ZrO 2 用于甲酸氢解甘油转化为 1,2- 丙二醇的反应. 2. 实验部分 2.1. 催化剂的制备 [25] 2.1.1. 草酸凝胶共沉淀法室温下, 将过量 % 的草酸的乙醇溶液在搅拌下快速加入到含有所需量的硝酸铜 ( 或硝酸铁 硝酸钴 硝酸镍 ) 和硝酸锆 ( 或硝酸铈 ) 的乙醇溶液中. 搅拌 2 h 后, 得
Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 73 到凝胶状沉淀物, 经离心 11 o C 干燥过夜后, 在马弗炉中 5 o C 焙烧 4 h 得到待还原材料. 2.1.2. 碳酸钠共沉淀法将硝酸铜 硝酸锌和碳酸钠的水溶液 ( 均为.1 mol/l) 在 o C 混合均匀, 溶液 ph 值维持在 6.5 7., 剧烈搅拌 4 h 后得到沉淀物, 经过滤 11 o C 干燥过夜后, 在马弗炉中 5 o C 焙烧 4 h 得到待还原材料. 2.1.3. 浸渍方法取一定量的 ZrO 2 或 SiO 2 载体浸没在一定浓度的硝酸铜溶液中, 于室温下搅拌 4 h, 然后在 85 o C 下继续搅拌至蒸干, 再在 11 o C 干燥过夜. 2.1.4. 尿素沉积 - 沉淀法将 ZrO 2 或 TiO 2 (P25) 浸没于硝酸铜 (.1, mol/l) 和尿素 (1 mol/l) 的混合溶液中, 在 9 o C 剧烈搅拌 4 h. 所得固体经离心 11 o C 干燥过夜后, 在马弗炉中 5 o C 焙烧 4 h 得到待还原材料. 所有样品反应前在 5%H 2 /Ar 气氛下于 3 o C 还原 2 h. 2.2. 表征采用德国 Bruker 公司的 D8 Advance 型 X 射线衍射仪 (XRD) 进行样品的物相分析, 使用 Cu K α 辐射为射线源 (λ =.154 nm), 石墨单色器, 管压 4 kv, 管流 4 ma, 扫描范围 2θ = o o, 扫描速率 2 o /min. H 2 程序升温还原 (H 2 -TPR) 在自制装置上进行, 采用上海分析仪器厂的 12G 型气相色谱仪检测 [26]. 采用 Micromeritics 公司的 Tristar 3 型自动物理吸附仪在 196 C 下测定样品的 N 2 吸附 - 脱附等温线, 由 BET 方程计算样品比表面积, 用 BJH 等效圆柱模型计算孔分布. 预处理条件为 3 C, 1 2 Torr, 4 h. XPS 测试 : 在 Perkin Elmer 公司的 PHI 5C ESCA 型 X 射线光电子能谱 (XPS) 仪上进行, 使用 Mg K α 射线 (hv = 1253.6 ev) 为激发源, 半球型能量分析器 (EA11/), 高压 14. kv, 功率 25 W, 通能 93.9 ev, X 射线与样品夹角 θ = 54 o, 测量时分析室压力为 1 9 Torr, 以碳的结合能 (C 1s = 284.6 ev) 为基准进行结合能的校正. 活性 Cu 表面积由 N 2 O 脉冲滴定测定 : 2Cu(s) + N 2 O N 2 + Cu 2 O(s). 载气为 N 2, TCD 检测器检测消耗的 N 2 O 的量 [25,27]. 通过计算消耗的 N 2 O 量得到催化剂样品的活性 Cu 表面积, 每 m 2 活性 Cu 表面对应的 Cu 原子数为 1.46 1 19. 根据 Cu 比表面积计算 Cu 分散度. 反应后溶液由 IRIS Intrepid 型等离子体发射光谱仪 (Thermo Elemental 公司 ) 测定. 2.3. 甲酸分解与甘油氢解对于甲酸分解, 将 18 mmol 甲酸 4 ml 水和一定量催化剂加至 5 ml 哈氏合金 Parr 高压反应釜中, 在 1 o C 搅拌 ( r/min) 反应一定时间后, 采用美国 Agilent 公司的 HP 1 型高效液相色谱仪 (HPLC) 分析甲酸含量. 分析条件 : Platisil ODS C18 分析柱 ; 示差检测器 ; 柱温和检测器温度均为 4 o C; 流动相.5 mol/min H 2 SO 4, 流速 1 ml/min. 采用气相色谱分析气相产物 (TDX-1 型分析柱 ;TCD 检测器 ;He 为载气 ;33.3% H 2-33.3% CO 2-33.3% CO 2 混合气为标准校正气 ). 当产物 CO 浓度过低时使用附带甲烷转化炉和 FID ( 检测限为 1. ppmv) 的气相色谱系统进行准确测定. 对于甘油氢解, 将 5 mmol 甘油 一定量甲酸 一定量催化剂 (2.5 mmol 2- 甲氧基乙基醚, 内标 ) 和 1 ml 水加至 25 ml 哈氏合金 Parr 高压反应釜中, 充入.5 MPa N 2, 在一定反应温度下搅拌 ( r/min) 一定时间后, 通过 GC-MS 进行产物定性并采用岛津 GC-17A 对产物进行定量分析. 色谱条件为 : HP-FFAP (3 m.25 mm) 毛细管柱, FID 检测器. 3. 结果与讨论首先研究了不同 Cu 基催化剂上水相甲酸选择分解反应. 相关报道中, 甲酸主要以如下两种路径分解 [16] : HCO H 2 + CO 2, ΔG = 48.4 kj/mol HCO H 2 O + CO, ΔG = 28.5 kj/mol 特别令人感兴趣的是, 可通过选择不同的催化剂来实现上述反应路径的调控. 在以甲酸为氢源的应用中, 甲酸应选择分解为 H 2 和 CO 2, 同时最大限度避免 CO 的形成. 甲酸分解的相关研究主要集中于气相反应, 而液相甲酸分解所使用的多相催化剂多为负载贵金属催化剂, 如 Pd, Pt 和 Au, 以及非负载的双金属合金, 如 Pt-Au 等 [28 3]. 因此, 开发能快速实现甲酸在温和条件下选择性分解的非贵金属催化剂具有重要的现实意义. 表 1 比较了不同方法制备的 Cu 基催化剂上低浓度 (.43 mol/l) 甲酸溶液的分解活性. 发现 1 o C 反应 5 h 后得到的产物 H 2 /CO 2 摩尔比均为 1:1, 副产物 CO 浓度均低于 ppm, 表明各 Cu 基催化剂在该反应条件下均有一定程度的甲酸选择分解活性. 然而, 大部分催化剂的活性较低, 如 3Cu%/ZnO, 3%Cu/ ZnO-Al 2 O 3, 商业 Cu/ZnO-Al 2 O 3, Fe/ZrO 2, Ni/ZrO 2 和 Co/ZrO 2. 而采用草酸共沉淀法制备的 3% Cu/ZrO 2 催化剂在该反应中表现出较高的甲酸分解制氢活性, 甲酸转化率可达 5%. 并生
74 Jing Yuan et al. / Chinese Journal of Catalysis 34 (13) 66 74 成 CO ( 浓度仅为 12 ppm). 图 1 为采用草酸共沉淀法制备的不同 Cu 负载量的 Cu/ZrO 2 催化剂的甲酸分解行为. 可以看出, 负载量对甲酸分解活性影响较大, 当 Cu 含量为 wt% 时, 催化剂活性最高. Cu 于 1 o C 反应 3 h 后即可将甲酸完全转化为 H 2 与 CO 2. 另一方面, 各催化剂上 CO 浓度均较低 (< ppm). 图 1 还考察了甲酸浓度对甘油氢解反应性能的影响, 可以看出高浓度甲酸有利于其分解. 为进一步探讨 Cu/ZrO 2 催化剂对甲酸分解具有高活性的原因, 对 (1 5 wt%)cu/zro 2 -OG 进行了详细的表征. 催化剂的表面积和活性金属铜表面积等参数列于表 2. 可以看出, Cu/ZrO 2 比表面积和其活性之间相关性较弱, 表明织构性质不是影响甲酸分解活性的关键因素. 而金属铜分散度 ( 或铜比表面积 ) 和甲酸分解活性之间具有较强的正关联, 与文献结果一致 [25,27,31,32]. 因此, wt%cu/zro 2 -OG 对甲酸分解表现出的高活性可归因于其较大的金属铜表面积. 图 2 为还原后不同 Cu 含量的 Cu/ZrO 2 催化剂的 XRD 谱. 由图可见, 焙烧后样品出现四方相 ZrO 2 和 CuO 的晶相 ( 未给出 ), 而还原后各催化剂在 43.3 o 出现了一较强的衍射峰, 对应于金属 Cu(111) 晶面 ;CuO 物相峰的消失表明它已被完全还原. 图 3 为各 CuO/ZrO 2 的 H 2 -TPR 谱. 可以看出, Cu/ZrO 2 催化剂中的 Cu 物种可在较低的温度 ( o C) 下被还原, 说明铜和锆之间具有较强的相互作用. 这可最大限度地避免反应条件下 Cu 组分的流失. 图 4 为 wt%cu/zro 2 -OG 催化剂上甲酸为氢源的甘油水相氢解反应 5 h 的结果. 由图可见, 尽管反应温度的增加可提高反应活性, 但过高则使 1, 2- 丙二醇选择性下降. 事实上, 当反应温度高于 o C 时, 生成大量正丙醇等深度氢解产物. 与此同时, 甲酸与甘油的摩尔比对产物分布的调控也十分重要. 如图 5 所示, 随着甲酸与甘油投料摩尔比的增大, 甘油转化率逐渐增加, 而 1,2- 丙二醇选择性先增大后减小, 这与图 1 中高浓度甲酸有利于其转化相一致. 由于甘油和甲酸在催化剂表面上存在竞争吸附, 甲酸与甘油的摩尔比过小, 吸附在催化剂上的甲酸量少, 导致甘油转化不充分, 且易降解为其它产物, 甘 油转化率和 1,2- 丙二醇选择性均较低. 当甲酸与甘油的投料摩尔比为 1 时, 甘油转化率达到 85%; 继续增大甲酸与甘油摩尔比, 甘油转化率增加不大, 但 1,2- 丙二醇选择性却显著降低. 这是由于过多的甲酸易导致副反应加剧所致. 综合反应的选择性与甲酸利用效率, 确定最佳投料比为 1:1. 图 6 给出了使用气相色谱对上述反应历程的跟踪检测结果. 甘油转化率在反应 5 h 内可达到 85%; 继续延长反应时间, 甘油转化率缓慢增加至 18 h 达 97%. 在整个过程中, 1,2- 丙二醇选择性始终保持在 95%. 该结果要远 [17] 优于相关文献报道结果 (2 o C 连续进甲酸反应 24 h 甘油转化率 9%, 1,2-PDO 选择性 82%). 图 7 为催化剂稳定性的考察结果. 由图可见, 重复使用 5 次后甘油转化率和 1,2- 丙二醇选择性均无明显变化, 且 ICP-AES 检测结果表明无金属流失, 可见催化剂具有较高的稳定性. 为了进一步理解反应过程, 使用等当量的氢气 (.8 MPa) 为氢源研究了甘油氢解反应, 结果如图 6 所示, 分别以氢气和甲酸为氢源时甘油的氢解曲线相似. 另外, 需要指出的是, 在前 1 h, 反应釜内压迅速从.5 MPa 增加到 4 MPa, 表明甲酸在起始阶段迅速分解为 H 2 和 CO 2. 同时, 对照实验表明, 在 o C, 无催化剂条件下, 甲酸浓度保持不变. 由此可见, 以甲酸为氢源时, Cu/ZrO 2 催化的 [17] 甘油氢解反应并非按转移氢解路径进行, 而是通过甲酸选择分解制氢, 进而氢解甘油的连串路径进行的. 另外, 甲酸在 Cu/ZrO 2 催化剂上高选择性制氢是获得较高甘油氢解活性的关键因素. 4. 结论 以生物基甲酸为氢源, Cu/ZrO 2 为催化剂进行甘油氢解制 1,2- 丙二醇, 最优反应条件为 : o C, 甲酸与甘油摩尔比 1:1, 反应时间 18 h. 1,2- 丙二醇收率可达 94%. 初步的反应机理研究表明, 甘油氢解是通过甲酸选择分解制氢进而氢解甘油的连串路径进行的. 该结果为甘油制取 1,2- 丙二醇提供了一条有成本竞争优势的新范例, 并从原理上解决了当前生物炼制中普遍需要外源石化氢的问题.