Glycerol (GL) is one of the main platform molecules in bio-refinery, being currently produced as a co-product (ca. 250 million tons per year) of biodiesel manufacture from either vegetable or animal oils. One of the important routes for value-added chemicals from GL would be its aerobic oxidation in water [1]. However, GL oxidation could potentially produce a number of valuable products including dihydroxyacetone (DHA), glyceraldehyde (GLD), glyceric acid (GLA), tartronic acid (TTA) and lactic acid (LA) [2, 3, 4, 5, 6]. These many product candidates make it a great challenge to the catalysis community to identify or develop efficient catalyst that can offer a high selectivity for production of a specific product.
The catalytic GL oxidation reaction was often conducted in water and with the presence of NaOH, using an aqueous solution of GL as the reaction feed. Oxide-supported or oxide-immobilized Au nanoparticles (NPs) were identified as very efficient catalyst, producing a fairly high GLA selectivity (40%-70%) with TTA, LA, glycolic acid (GCA) and/or oxalic acid (OA) being the by-products [2, 4, 6, 7]. The presence of NaOH in water (OH-, in essence) has been believed to be indispensable for initiating the reaction by abstraction of a proton from one of the hydroxyls in the GL molecules [2, 8, 9]. GLD and/or DHA were considered to be the reactive intermediate(s) for the formation of GLA though a clear detection of GLD during the reaction was never reported over supported monometallic Au catalysts [2, 5]. It has been also observed that the feature of product selectivity appeared little affected by the size and morphology of the catalytic Au NPs, and nature of the supporting materials as well. A recent study of our laboratory disclosed that when conducted under base-free (neutral and even acidic) conditions or without the presence of NaOH in water the Au-catalyzed GL oxidation on oxide-supported Au catalyst would show a different feature; instead of producing GLA as the main product, the reaction in the base-free water produced a specific selectivity for DHA production [3]. These results are therefore indicative that the presence of NaOH in water can modify the direction (selectivity) of GL oxidation over the Au catalyst, which motivates us to investigate how the catalysis for GL oxidation in base-free water (absence of NaOH) of supported Au NPs would be affected by the surface acid-base property of their supporting materials.
In the present study, a series of Au/MgO-Al2O3 samples with varying surface acid-base property are prepared by changing the composition (molar Mg/Al ratio, denoted as x) of the supporting MgO-Al2O3 and then employed to catalyze the oxidation of GL in base-free water. Au NPs on the most acidic and least basic MgO-Al2O3 support exhibit the highest activity for GL activation and the highest selectivity for DHA production. Increasing the surface basicity or lowering the acidity of the MgO-Al2O3 support results in continuously improved selectivity for GLA but lowered selectivity for DHA, disclosing for the first time a counter relationship in the production of DHA and GLA during the oxidation. These results demonstrate that the surface acidity/basicity of the supporting material is a key to the selectivity control of their carrying Au NPs for aerobic GL oxidation in base-free water.
All the MgO-Al2O3 samples were prepared by co-precipitation according to Ref. [10]. A solution (A) containing a total of 100 mmol Mg(NO3)2 and Al(NO3)3 (Beijing Modern Eastern Fine Chemical Co., Ltd.) dissolved in 100 mL deionized water was prepared by varying the molar Mg/Al ratio between 0 and 5.0. For the preparation of Al2O3 (x = 0), only 100 mmol of Al(NO3)3 was used as the precursor. The solution A was added dropwise under vigorous stirring in ca. 1 h into a solution B, which contained 100 mL aqueous solution of NaOH (200 mmol) and Na2CO3 (40 mmol, Beijing Chemical Reagent Company). The as-formed suspensions were stirred and aged for 18 h at 65 ℃, followed by filtration, extensive washing with deionized water and then drying overnight at 110 ℃. Calcination in flowing air at 600 ℃ for 5 h of the dried samples produced the as-prepared MgO-Al2O3(x) samples, in which x denotes the actual molar ratio of Mg/Al.
The Au/MgO-Al2O3(x) catalysts with a nominal Au loading of 1 wt% were prepared by deposition-precipitation with urea [12]. Briefly, 1 mL aqueous HAuCl4 (10 mgAu/mL, Acros Organics Inc.), 1.22 g urea (urea/Au = 400 (molar), Beijing Modern Eastern Fine Chemical Co., Ltd.), 1.00 g MgO-Al2O3(x) and 50 mL deionized water were added into a three-neck flask kept in a water bath of 80 ℃. The flask was covered with an aluminum foil to keep off the room light. The suspensions were stirred vigorously for 6 h, followed by overnight aging at room temperature. The precipitate was filtered, washed extensively with deionized water, and then dried at 110 ℃ for 5 h. Unless otherwise specified, the dried samples were calcined in flowing air at 400 ℃ for 5 h to produce the Au/MgO-Al2O3(x) catalysts.
The actual composition by x of the supporting MgO-Al2O3(x) materials, as well as the actual Au loading of the Au/MgO-Al2O3(x) catalysts were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) on a Perkin Elmer Optima 3300 RL instrument. X-ray diffraction (XRD) patterns of Au/MgO-Al2O3(x) were recorded at a rate of 8°/min on a Bruker D8 Advance X-ray diffractometer with a Ni-filtered Cu Ka (λ = 0.15406 nm) radiation at 40 kV and 40 mA. The morphology and size of Au NPs were characterized by transmission electron microscopy (TEM) on a JEOL JEM-2010 operating at 120 kV. At least 200 particles were randomly measured to determine the mean diameter of Au NPs for every catalyst sample. The sample surface area, pore volume and average pore size were measured at -196 ℃ from the N2 adsorption-desorption isotherm on a Micromeritics ASAP 2010 C apparatus. NH3-and CO2-temperature programmed desorption (TPD) experiments were conducted to evaluate the acid-base property, which were done on a Cat-Lab (BEL JAPAN, INC.) equipped with an on-line QIC-200 quadrupole mass spectrometer (Inprocess Instruments, GAM 200) as the detector, as detailed previously [11]. In order to prevent a possible intervention of H2O desorption, the NH3 desorption profiles were measured as the change in the signal of m/z = 15, rather than m/z = 16 or 17. And, the CO2 desorption profiles were measured as the change in the signal for molecular CO2 at m/z = 44.
The GL oxidation was conducted in a 50-mL stainless steel autoclave with 20 mL aqueous GL solution (0.1 mol/L) and 160 mg catalyst (molar GL/Au = 315) under 1.0 MPa O2 at 80 ℃. After purging with oxygen for six times, the reactor was pressurized to 1.0 MPa at room temperature. Zero reaction time was taken by switching on the stirrer (900 r/min) as soon as the autoclaved solution temperature reached 80 ℃ (the reaction temperature); heating the reaction solution in the autoclave from room temperature to this reaction temperature in our reaction set up took ca. 30 min. Preliminary experiments conducted with widely altered stirring speeds (600-1100 r/min) showed no effect of diffusion limitation under our chosen reaction conditions. Termination of the reaction was done by switching off the stirrer and immediately cooling the autoclave reactor with an ice-water bath. The gases in the cooled reactor were collected with a gas-bag and the liquids were separated from the solid catalyst by filtration, respectively. The collected gases and liquids were then analyzed, respectively, by gas chromatography (GC) and high-performance liquid chromatography (HPLC). Special attention was paid to discriminate DHA from GLD and/or GL in HPLC analysis and ascertain no formation of GLD during the reaction, as detailed earlier [3]. The carbon balance of the catalytic reactions was always higher than 93%.
The composition by x and textural properties for all the MgO-Al2O3 samples are reported in Table 1. The number x as measured by ICP-AES was ca. 10%-40% lower and ca. 20%-40% higher than the theoretic number according to the composition of the starting preparation solutions at x £ 0.5 and x ³1.0, respectively, indicating that the major component (Mg2+ or Al3+) in the starting solution became enriched in the solid product of the co-precipitation preparation. The sample specific surface area was in the range of 190-290 m2/g, showing a decreasing trend with the increase in x. The sample pore volume (0.28-0.73 cm3/g) and average pore diameter (4-13 nm) increased up to x = ca. 0.4 and remained almost unaltered on further increase in x. These textural data were also close to those reported by Poupin et al. [13], whose samples were prepared by a different method.
NH3-and CO2-TPD experiments are conducted to measure the acid-base property of the MgO-Al2O3 samples, the results are shown in Fig. 1. The NH3-TPD profiles (Fig. 1(a)) show similar shapes, featuring only one broad peak with the maximum at 170-200 ℃. The continued shift towards lower temperature of the NH3-TPD peak with increasing x clearly indicates a weakening of the surface acidity when the material contains more MgO (less Al2O3) or has a higher x. Quantitatively, the sample with a lower x showed a bigger NH3-TPD peak or a larger amount of chemisorbed NH3, signifying that increasing the content of MgO in the MgO-Al2O3 material resulted in lower surface acidity of the material. The CO2-TPD profiles (Fig. 1(b)) also feature one broad peak, showing the maximum desorption rate at 160-180 ℃. The basicity measured as the number of CO2 molecules desorbed during the CO2-TPD experiment also increased with x. The surface acidity and basicity data determined from these NH3-and CO2-TPD measurements are given numerically in the last two columns of Table 1. Apparently, increasing x in the sample led to continued decrease by ca. 5 folds in the acidity from 0.94 to 0.20 μmol/m2 but increase by ca. 15 folds in the basicity from 0.05 to 0.80 μmol/m2, demonstrating that a successful tuning of the surface acid-base property of the supporting MgO-Al2O3 material by varying the composition x [14, 15].
To understand if the surface acidity and basicity of the Au-loaded samples (Au/MgO-Al2O3(x)) would differ from those of the supporting MgO-Al2O3(x) materials, the NH3-and CO2-TPD experiments were also conducted on the Au/MgO-Al2O3(x) samples. Their NH3-and CO2-TPD profiles appeared almost the same as those shown in Fig. 1 for their counterpart MgO-Al2O3 samples (not shown), as shown quantitatively by the parenthesized numbers in the last two columns of Table 1 for the Au/MgO-Al2O3(x) samples. Clearly, the loading of Au had no effect on the surface acid-base property of MgO-Al2O3(x). This is not surprising as in these Au/MgO-Al2O3(x) samples (Au loading: 0.7-0.9 wt%) the Au NPs could only occupy a few percent of the support surface area.
Figure 2 presents the powder XRD patterns for the Au/MgO-Al2O3(x) samples, which resembled those for their corresponding MgO-Al2O3(x) supports (not shown). The weak and relatively broad diffractions observed at 2θ = 37.6°, 45.9° and 67.0° for Au/Al2O3 (Fig. 2(1)) are, respectively, characteristic of the (311), (400) and (440) faces of crystalline γ-Al2O3 (JCPDS No. 10-0425). The curve (9) is the pattern for a reference MgO (cubic), which produced diffractions at 2θ = 36.9°, 42.9°, 62.3° and 78.6° associated with the (111), (200), (220) and (222) faces (JCPDS No. 45-0946). The patterns for the samples of x < 0.5 are like magnesia-incorporated alumina or Mg2+-doped alumina while the patterns for the other samples (x > 1.4) more like alumina-incorporated magnesia or Al3+-doped magnesia. Diffractions characteristic of crystalline Au particles, which should appear at 2θ = 38.0°, 44.4° and 64.8° [12, 16], were not discernible except for the samples of x < 0.5, hinting that Au NPs in these samples are smaller than ca. 5 nm. However, the week diffraction at 2θ = 38.0° for the other samples (x >1.4) would be associated with the presence of larger Au NPs. These inferences are consistent with the Au particle sizes by TEM measurements (Fig. 3), which are listed in Table 2 as the number-averaged Au sizes by randomly measuring at least 200 Au NPs in TEM images for every Au/MgO-Al2O3 sample. A slight enlargement of the Au NPs on the supports of x £ ca. 0.4 but more significant enlargement on the supports of higher x was indicated.
The systematically varied surface acid-base property of the supporting MgO-Al2O3 materials enables us to study the effect of the support acid-base property on the catalysis of their carrying Au NPs for the aerobic oxidation of GL in base-free water. The catalytic reaction was carried out in an autoclave at 80 ℃ with a molar ratio of GL/Au = 315 and an oxygen pressure of 1.0 MPa. As blank reaction tests, the supporting materials without Au (MgO-Al2O3) were also employed, as alternatives to the Au/MgO-Al2O3 catalyst for the reaction under otherwise the same conditions, and they showed either no activity or effected GL conversion of less than 1% after a period of 12 h, uncovering that Au NPs are indispensable to the catalytic oxidation of GL. The results for GL conversion and product distribution over the different Au/MgO-Al2O3 catalysts are presented in Table 2. For rigorous comparison of the reaction rate and product selectivity, the conversion levels of GL were controlled in the range of 11%-16% by adjusting the reaction duration. With no surprise that the obtained product identities are typical for those reported in the literature; for instance, GLA, DHA, LA and TTA but no GLD are identified as the C3 products [2, 3, 4, 5, 6]. The main products are DHA, GLA and GCA over all of the Au/MgO-Al2O3 catalysts. The most significant findings here are that the most acidic catalyst (Au/Al2O3, x = 0) showed the highest selectivity (82%) for the formation of DHA but the lowest selectivity (4.4%) for the production of GLA, the latter was usually the most prevailing product of Au-catalyzed aqueous GL oxidation with the presence of NaOH [2, 4, 6]. The selectivity of DHA was lowered but that of GLA improved when the composition of the supporting material was changed to involve more and more MgO or when x in the Au/MgO-Al2O3(x) catalysts was increased; the selectivity of DHA became as low as 11% but the selectivity of GLA as high as 50% over the catalyst of x = 4.8. The selectivity change of TTA seemed to be in parallel with that of GLA, which is easily understandable as TTA would be reasonably regarded as an immediate secondary product from GLA.
As the change in x of the supporting MgO-Al2O3 material resulted in variation of the support surface acid-base property (Fig. 1, Table 1) that could directly impact the catalysis of Au for the GL oxidation reaction, an attempt is made in Fig. 4 to correlate the selectivity data of DHA and GLA over the supported Au NPs with the support surface basicity. Apparently, the support surface acid-base property inversely impacted the selectivity of DHA and GLA. It is therefore that Au NPs supported by the less acidic but more basic MgO-Al2O3 tend to produce less DHA but more GLA, demonstrating that the surface acidity/basicity of the supporting material is a key to the selectivity control of their carrying Au NPs for GL oxidation in base-free water.
It should be noted that the above discussion of the catalytic selectivity has taken no consideration of the differences in the average Au particle size of the Au/MgO-Al2O3(x) catalysts. To gain an insight into the effect of Au particle size on the product selectivity without varying the support acid-base property, the calcination temperature of the Au/MgO-Al2O3(0.2) catalyst (x = 0.2) was also changed to 300 and 500 ℃, respectively. The average Au particle size was smaller (2.8 nm) for the sample prepared by calcination at 300 ℃ but larger (6.6 nm) for the sample prepared by calcination at 500 ℃ than the Au NPs (3.2 nm) for the “normal” Au/MgO-Al2O3(0.2) sample in Table 2, whose calcination temperature was 400 ℃. Compared in Table 3 are the catalytic performance of these three Au/MgO-Al2O3(0.2) catalysts for the GL oxidation reaction. Evidently, the change in the Au particle size (from 2.8 to 6.6 nm) produced essentially no effect on the product selectivity, thus giving a strong further support for the role of the surface acidity/basicity to the selectivity control of Au-catalyzed GL oxidation in base-free water. This is certainly interesting as the product selectivity of Au-catalyzed GL oxidation in NaOH-containing water was also found insensitive to the Au particle size [2, 17].
Assuming no significant Au size effect could allow us to establish in Fig. 5 a correlation between the catalytic reaction rate by GL turnover frequency (TOF) over the present Au catalysts and the surface basicity of the MgO-Al2O3 support. This correlation discloses an important effect of the support basicity on the catalytic activity of Au NPs for GL oxidation in base-free water. The first three points in Fig. 5 are in fact based on virtually unchanged Au sizes (3.1-3.2 nm, as given in Table 2) on the surfaces of varying basicity, their associated TOF numbers differed by as high as nine times (Table 2). Thus, the volcano curve in Fig. 5 demonstrates that the intrinsic Au activity can be boosted by several times when the supporting material carrying the catalytic Au NPs has an appropriate surface basicity though a disturbance due to Au particle size effect could not yet be excluded for those data pertaining to the surface basicity higher than 0.3 µmol/m2.
The three data points linked by the dashed red line at the surface basicity of 0.25 µmol/m2 in Fig. 5 refer respectively to the TOFs for the three Au/MgO-Al2O3(0.2) catalysts of different calcination temperatures (300, 400 and 500 ℃, Table 3). Note that the average Au particle sizes for these three catalysts are 2.8, 3.2 and 6.6 nm, respectively. The invariant surface basicity and different Au particle sizes of these Au/MgO-Al2O3(0.2) catalysts entitle us to gain an insight into the Au particle size effect on the Au activity for the GL oxidation reaction in base-free water. Surprisingly, the smaller Au NPs appeared to be intrinsically less activity than the bigger Au NPs.
Therefore, both the support surface basicity and Au particle size are identified as the keys to the Au activity for the GL oxidation reaction in base-free water. However, a careful inspection of the catalytic data tabulated in Tables 2 and 3 would further reveal that the support surface basicity can be more influential than the Au particle size. This conclusion is clearly supported when one compares the activity of Au/MgO-Al2O3(2.5) (Table 2) with that of the Au/MgO-Al2O3(0.2) prepared by calcination at 500 ℃ (Table 3), as these two catalysts had comparably sized Au NPs (6.6 nm) and quite different surface basicity.
There are two papers in literature that investigated the effect of support acid-base property on the performance of bimetallic AuPt nanoalloy catalyst for GL oxidation, though information on catalysis of monometallic Au nanocatalyst is yet not available. By immobilizing alloyed AuPt NPs (6.2-7.5 nm) for GL oxidation in base-free water with supporting materials of widely varied surface acid-base property (H-Mordenite, SiO2, MCM-41, sulfated ZrO2, NiO and MgO), Prati et al. [18] showed very recently that AuPt NPs carried by the basic supports (MgO, NiO) were more active for the overall oxidation reaction but less selective for the formation of the desired C3 products while those AuPt NPs carried by the acidic supports (H-Mordenite, sulfated ZrO2) appeared less active but more selective for the C3 products. In the other paper, Xu et al. [19] attempted to correlate the support effects of TiO2, CeO2, MgO, ZnO, CaCO3 and Al2O3 on the catalysis in the same reaction of alloyed small AuPt NPs (1-3 nm) with the acid-base property of the supporting oxides. They showed that AuPt NPs carried by the support oxide having stronger basic sites produced a higher selectivity for TTA but much lower selectivity for GLD. However, the effect on the catalysis of AuPt NPs for DHA and GLA production of the widely varied supporting material and their surface acid-base property remained unclear in these two papers [18, 19]. In the work of Prati et al. [18], the selectivity for DHA of the AuPt catalysts was hardly affected by changing the support material but the more basic support led to formation of much more formic acid in the product. In the work of Xu et al. [19], the formation of DHA was not even detectable over the small AuPt NPs, and the relatively more acidic TiO2 and CeO2 were shown as the effective supports in promoting the formation of GLD. However, formation of GLD has never been observed in our study of monometallic Au catalysts for aqueous GL oxidation, either in the presence or absence of NaOH [3].
Comparison of the catalytic results of Au/MgO-Al2O3(x) in this study and those of AuPt NPs immobilized on different supporting materials seems difficult at this stage. This difficulty could arise, in addition to the size of the catalytic metal nanoparticles, from differences in: (1) nature of the catalytic site on monometallic Au and bimetallic AuPt, (2) method of catalyst preparation (sol immobilization vs. deposition-precipitation), (3) nature and amount of stabilizer contamination [20, 21, 22], (4) components in the supporting materials, and even (5) conditions of the catalytic reactions. However, the clear correlations gained by using MgO-Al2O3 as the supporting material and by changing its surface acid-base property with systematic variation of its composition (x) in this present study (Figs. 4 and 5) would provide a starting basis for better understanding the surface catalysis involved in the oxidation of aqueous GL (and other bio-derivative polyols) over Au and Au-based NPs supported/immobilized on acid-base oxides.
The use of MgO-Al2O3 of different compositions as the support for monometallic Au NPs have enabled us to gain insight into the effect of support acid-base property on the catalysis of Au for GL oxidation in base-free water. The most acidic and least basic MgO-Al2O3(0.1) support resulted in the highest activity for GL activation and highest selectivity for DHA production. Increasing the surface basicity or lowering the acidity of the MgO-Al2O3 support by increasing the percentage of MgO led to steadily improved selectivity for GLA but lowered selectivity for DHA formation. These correlations would provide a starting basis for better understanding the surface catalysis involved in the oxidation of GL and other bio-derivative polyols in base-free water over nanosized Au and Au-containing catalysts on different supporting materials.
2015, Vol. 36 Issue (9): 1543-1551
PDF (1381 KB)
2015, Vol. 36 Issue (9): 1543-1551
PDF (1381 KB)