Advances in the biodiesel industry have resulted in the production of large amounts of glycerol as a byproduct of the transesterification of vegetable oils. Adding value to surplus glycerol by chemical transformations has attracted significant attention in recent decades [1]. Among the various transformation routes such as dehydration [5, 6], oxidation [7, 8], and reforming [9, 10], the selective hydrogenolysis of glycerol to 1, 3-propanediol (1, 3-PD) is highly desirable, owing to the widespread use of 1, 3-PD in the polyester industry for the production of polytrimethylene terephthalate (PTT) [17]. Whereas the synthesis of 1, 2-propanediol (1, 2-PD) can be accomplished with a wide range of catalysts, only Pt-W catalysts [18] and Ir-Re catalysts [19] can selectively cleave the secondary -OH group of the glycerol molecule to obtain high yields of 1, 3-PD. Very recently, our group developed a novel Pt single/pseudo-single atom catalyst deposited on a mesoporous tungsten oxide [20, 21]. The optimized interface between the Pt and the WOx, as well as the mesoporous structure of the material, result in outstanding activity by this catalyst under relatively low H2 pressures (i.e. 1MPa), as well as a high space-time yield of 1, 3-PD. Even under mild reaction conditions, however, the undesirable formation of 1-propanol as a result of over-hydrogenolysis is still the dominant reaction, with approximately 50% selectivity for this product. Therefore, there is a requirement to further enhance the selectivity for 1, 3-PD by suppressing the formation of 1-propanol, and the addition of small amounts of promoters to the present Pt/WOx catalyst appears promising in this regard.
The introduction of a promoter is generally used to improve catalyst performance and stability via controlling and/or modulating the electronic structure of the noble metal [22], covering unfavorable sites [11], or modifying the surface chemistry of the support and its consequent interaction with the noble metal [17]. Because the selective hydrogenolysis of glycerol to 1, 3-PD is a synergistic process based on concerted dehydration and hydrogenation, modification of the catalyst with a promoter could definitely tune both the catalytic performance and the distribution of products. Therefore, an investigation of promoter effects on Pt-W catalysts will be crucial to future breakthroughs in catalyst design. In previous studies, AlOx promoters [11] and Al2O3 supports [17] have both demonstrated promising promotional effects during the selective hydrogenolysis of glycerol to 1, 3-PD (90% and 100% glycerol conversions with 44% and 66% 1, 3-PD selectivities, respectively). It is likely that the AlOx support inhibits the over-hydrogenolysis reaction, although the mechanism is still unclear. Moreover, in contrast to Pt/WOx, supported catalysts are more desirable in practical applications owing to their favorable mechanical and shaping properties. Despite this, positive promoters that work for Pt/WOx might not function with supported catalysts, because the different chemical properties of Al2O3 and WOx may affect the action of the promoters. To examine such effects, the present work employed both Pt/WOx and Pt/WOx/Al2O3 as the mother catalysts, and introduced transition metal (such as Re, La, Fe, Zr, Sn and Ce) oxides and noble metals (such as Ru, Ir and Rh) as promoters, and investigated the role of the promoters in modifying the hydrogenolysis reactivity. Among these catalysts, La and Fe showed positive promotional effects on Pt/WOx, while Re showed a similar effect on Pt/WOx/Al2O3.
The Pt/WOx was prepared according to a previously reported method [17]. Briefly, 3 g of WCl6 was added to 100 mL of ethanol with stirring at 500 r/min for 20 min, and then transferred to a Teflon-lined autoclave and heated in an oven at 433 K for 36 h. After cooling to room temperature, the solution was filtered and the recovered WOx was washed with ethanol and water, then dried at 323 K for 6 h under vacuum. This material is denoted simply as WOx herein. Pt/WOx was prepared by impregnating this material with an aqueous solution of H2PtCl6, followed by drying at 323 K for 6 h under vacuum.
Noble metal promoted catalysts were prepared by impregnating the Pt/WOx in RhCl3·3H2O, RuCl3·3H2O, or H2IrCl6 solutions overnight, with subsequent drying at 383 K for 12 h. The as-prepared catalysts were calcined at 673 K for 1 h at a heating rate of 2 K/min.
Transition metal promoted catalysts were prepared using the incipient wetness impregnation method. The metal precursors used included NH4ReO4, La(NO3)3·6H2O, Zr(NO3)2·5H2O, Al(NO3)3·9H2O, Fe(NO3)3·9H2O, Zn(NO3)2·6H2O, Ce(NO3)3·6H2O, SnCl4·5H2O, Ga(NO3)3·4.5H2O, and ammonium paratungstate (APT). After impregnation, the samples were dried at 383 K for 12 h and then calcined at 673 K for 1 h at a heating rate of 2 K /min. These materials are denoted as M/Pt/WOx. The M loading was fixed at 0.1 wt% and was calculated based on the mass of the mother catalyst. All the catalysts were reduced in H2 at 573 K prior to reaction trials.
The Pt/WOx/Al2O3 catalyst was prepared using a sequential impregnation method. WOx/Al2O3 was initially synthesized according to a previously published procedure [25]. In this process, 1.248 g of APT was dissolved in distilled water at 363K. The solution was then cooled to room temperature, after which 10 g of Al2O3 was added, followed by additional stirring at room temperature for 16 h. The solid was subsequently filtered off, dried at 383 K for 12 h, and calcined at 1073 K for 3 h to obtain WOx/Al2O3. This material was then impregnated with an aqueous solution of H2PtCl6·6H2O, followed by drying at 383 K for 12 h and calcination at 573 K for 3 h to obtain the Pt/WOx/Al2O3 catalyst. Analysis by inductively coupled plasma-atomic emission spectroscopy showed that the catalyst contained 7.0 wt% tungsten and 2.0 wt% platinum.
The introduction of promotors to the Pt/WOx/Al2O3 catalyst was performed in a manner similar to that applied in the case of the Pt/WOx, except for the use of a calcination temperature of 573 K. The promoted catalysts are referred to herein as M/Pt/WOx/Al2O3, where the M loading was fixed at 0.1 wt% as calculated based on the mother catalyst.
For comparison purposes, M-Pt/WOx/Al2O3 catalysts were also prepared by co-impregnation of WOx/Al2O3 using a solution of H2PtCl6·6H2O and the corresponding metal precursor. The resulting solids were dried at 383 K and calcined at 573 K for 3 h. In addition, Pt/M/WOx/Al2O3 catalysts were prepared by sequential incipient wetness impregnation of WOx/Al2O3 with solutions of the corresponding metal precursor and H2PtCl6·6H2O.
The WOx/Pt/Al2O3 catalyst was prepared via a sequential impregnation method. The initial Pt/Al2O3 catalyst was prepared by impregnation of Al2O3 samples with an aqueous solution of H2PtCl6·6H2O, after which the impregnated sample was dried at 383 K for 12 h and subsequently calcined at 673 K for 3 h. WOx/Pt/Al2O3 was prepared by impregnation of Pt/Al2O3 samples with an aqueous solution of ammonium metatungstate (AMT). Impregnated samples were dried at 383 K for 12 h and subsequently calcined at 823 K for 3 h. The platinum content was fixed at 2 wt%, while the tungsten content was varied between (0.1 and 8) wt%.
Glycerol hydrogenolysis trials over Pt/WOx were conducted in a 75-mL autoclave with a Teflon lining. Typically, 0.3 g of catalyst and 12 g of an aqueous glycerol solution (5 wt%) were transferred into the autoclave, after which the chamber was flushed several times with H2 and then filled with H2 to a pressure of 1.0 MPa. The reactor was subsequently sealed and heated to 413 K and the reaction proceeded at a stirring speed of 800 r/min for 12 h. The same conditions were applied in the performance tests over Pt/WOx/Al2O3, except using a 3 wt% glycerol solution, an initial H2 pressure of 5.0 MPa and a reaction temperature of 453 K.
After each reaction, the gaseous and liquid products were collected separately. The liquid phase products were analyzed with an Agilent 7890B gas chromatograph with an HP-INNOWAX capillary column (30 m × 0.32 mm × 0.5 m, flame ionization detector) using n-butanol as the internal standard, while the gaseous products were analyzed online using the same instrument with a HayeSep Q packed column (3 m × 1/8", thermal conductivity detector). The conversion of glycerol and the selectivity for each liquid product were calculated using the following equations.
Conversion of glycerol (%) = (moles of glycerol consumed)/(moles of glycerol initially added) × 100.
Selectivity (%) = (moles of carbon in a given product)/(moles of carbon in glycerol consumed) × 100
Deactivation rate (% h-1) = (Conv.max - Conv.final) × 100 / ( t × Conv.max)
High resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning tunneling electron microscopy (HAADF-STEM) images were obtained on a JEM-2100F instrument operating at 200 kV. The samples were prepared by ultrasonically dispersing the finely powdered catalysts in ethanol and the dropping each solution onto a C/Cu TEM grid.
NH3-adsorption data were obtained using a Micromeritics AutoChem II 2920 chemisorber to measure the acidity of catalysts. The samples were degassed in He flow at 393 K for 1 h and then cooled to 373 K prior to pulse adsorption trials. After adsorption saturation, the amount of NH3 adsorption was calculated accordingly.
H2 adsorption trials were conducted with the same instrument to measure the H2 uptake of catalysts. The samples were reduced in H2 flow at 573 K for 1 h after being degassed in Ar flow at 583 K for 30 min, then cooled to 323 K in preparation for pulsed adsorption. After adsorption saturation, the amount of H2 adsorption was calculated accordingly.
As noted, transition metal promoters can drastically change the activity and selectivity of noble metal supported catalysts. Thus the effects of transition metal promoters during the hydrogenolysis of glycerol were examined in the present work, with the results shown in Table 1. In the case of Pt/WOx, the additions of Re, Sn, Zn, and Ga (Entries 5-8) decreased glycerol conversion but increased 1, 3-PD selectivity, while Zr (Entry 4) had the opposite effect. During the sequential glycerol hydrogenolysis reaction, it is usually difficult to simultaneously enhance both glycerol conversion and selectivity for 1, 3-PD, because a highly active catalyst often generates over-hydrogenolysis, thus decreasing the selectivity, and vice versa. However, when promoted by La or Fe (Entries 2 and 3), the glycerol conversion was increased from 37.4% to 39.9% or 41.6%, respectively, while the 1, 3-PD selectivity was improved from 35.1% to 41.3% or 37.8%. These slight enhancements resulted in a higher yield of 1, 3-PD compared with that obtained with Pt/WOx. Moreover, the incorporation of La obviously improved the stability of the mother catalyst. The 0.1La/Pt/WOx had an average 0.13% activity loss per hour during an 85-h stability test (Fig. 1), while the activity loss of the Pt/WOx was 0.33% per hour over a 70-h stability test [22]. In the case of the Pt/WOx/Al2O3, the addition of La or Fe also remarkably enhanced the 1, 3-PD selectivity (from 48.2% to 57.0%) although the glycerol conversion decreased to some extent. We assume that the inherently higher activity of the Pt/WOx/Al2O3 was responsible for the lesser extent of activity enhancement on adding the promoters. It should be noted that improved selectivity at the cost of a slight loss of activity is the most acceptable scenario with regard to practical applications. In this regard, La is the be tter promoter.
To illustrate the effects of promoters on hydrogenolysis performance with or without the Al2O3 support, further investigations were performed, using La as a model promoter.
In principle, hydrogenolysis requires acidic sites for dehydration and metallic sites for hydrogenation, and the selective hydrogenolysis of glycerol to 1, 3-PD involves the concerted functioning of these two types of sites. For this reason, the acidity of a catalyst is widely accepted as playing a crucial role in glycerol conversion and product selectivity [23, 24]. According to the NH3 adsorption results (Table 2), the addition of La did indeed generate a greater quantity of acidic sites on both the 0.1La/Pt/WOxand 0.1La/Pt/WOx/Al2O3. In addition to these surface acid sites, as determined by NH3 adsorption, we propose that acid sites could also have originated from Brnsted acid sites generated in situ by the heterolytic dissociation of H2 on metal sites with subsequent spillover to the support [1]. In addition to the increased acid site concentration, the Brunauer-Emmett-Teller (BET) surface area and pore structure of the original Pt/WOx were both greatly decreased after the introduction of La, possibly due to partial collapse of the mesoporous WOx structure during calcination. In the case of the Pt/WOx/Al2O3, however, the BET surface area and pore structure were almost unchanged after La introduction, indicating the superior mechanical strength of the Al2O3 support compared to the WOx.
TEM and HAADF-STEM images demonstrated aggregations of Pt single/pseudo-single atoms after La introduction (Fig. 2), which may have been caused by the high temperature calcination following impregnation. In contrast, EDS results showed that Pt species were only partially aggregated, and that there were still a large number of single/pseudo-single Pt atoms highly dispersed on the W species. In addition, the majority of the La species were found to be associated with Pt species, in accordance with the deposition sequence of each component.
It has been reported that La possesses unique properties for catalytic cellulose conversion [2]. In the present study, La also exhibited a superior promotional effect for Pt-W catalysts during the selective hydrogenolysis of glycerol to 1, 3-PD, thus it was thought that optimization of the La content might provide opportunities to further improve the hydrogenolysis performance. However, additional evaluation (Table 3) demonstrated that the use of a greater quantity of La both lowered glycerol conversion and decreased 1, 3-PD selectivity, likely due to over-coverage of the active Pt sites by La species. Further investigation of the deposition sequence of La relative to Pt on the WOx/Al2O3 sample showed that La must be introduced after Pt loading, otherwise the activity is greatly decreased upon introduction of the La. This result suggests that the coverage of the highly active Pt sites by a very small amount of La tends to enhance selectivity without much activity loss.
Ru, Ir, and Rh were introduced to Pt/WOx and Pt/WOx/Al2O3 with a loading of 0.1 wt %, respectively, and the promotional effects were examined during the hydrogenolysis of glycerol under optimized reaction conditions (Table 4). In the case of the Pt/WOx, the Ru promoted material showed similar performance to the mother catalyst (Entry 2), while Ir and Rh promoters significantly decreased the glycerol conversion (from 37.4% to approximately 26.3%) but maintained the 1, 3-PD selectivity at approximately 35.0% (Entries 3 and 4). Similarly, no obvious effect was identified when using the Ru/Pt/WOx/Al2O3, indicating that Ru has the same function with and without a support. In contrast, the negative effect of Rh was eliminated when employing Al2O3 as the support, and the yield of 1, 3-PD returned to 32.4%. In addition, introducing Ir to the Pt/WOx/Al2O3 decreased not only the glycerol conversion but also the 1, 3-PD selectivity, indicating the negative effects were more pronounced on supported catalysts. Chemical adsorption results demonstrated that variation in the initial acid amounts over the Al2O3 supported catalysts (from 0.40 to 0.39 mmol/g) was less than that over the Pt/WOx (from 0.65 to 0.55 mmol/g) after Ir introduction, suggesting that the hydrogenolysis reactivity is not highly correlated with the initial acid amount. As well, compared with the transition metal promoted catalysts, that decreased the H2 uptake from 0.067 to the range of 0.046 to 0.049 mmol/g, the use of Ir greatly decreased the H2 uptake, from 0.067 to 0.024 mmol/g. This effect most likely was responsible for the inferior performance of this catalyst during the hydrogenolysis of glycerol to 1, 3-PD.
Electron microscopy (Fig. 3) shows significant aggregation of Pt on the Pt/WOx after Ir introduction, consistent with the H2 adsorption results, and presumably responsible for the low activity of this material during the hydrogenolysis reaction. The Pt particle sizes were maintained over the supported catalysts, indicating that the presence of an Al2O3 support may facilitate the stabilization of these particles. Taking the chemical H2 adsorption results into account, the significant decrease in the H2 uptake over the Ir promoted Pt/WOx/Al2O3 is attributed to the change in electronic structure, rather than particle size, of the active Pt species, which greatly influenced the performance of this material during the selective hydrogenolysis of glycerol.
Previous studies have demonstrated that the addition sequence of the AlOx promoter strongly influences the hydrogenolysis reactivity and the product selectivities [3]. Therefore, taking Pt/WOx and Pt/WOx/Al2O3 as reference catalysts, the effects of varying the Al and W impregnation sequence were investigated (Table 5). The introduction of either W or Al species to the Pt/WOx resulted in lower reactivity (glycerol conversion decreased from 37.4% to 27.1 % or 17.0%, respectively), owing to the coverage of active sites, while no significant improvements were identified in the selectivity for 1, 3-PD. Similar effects were observed when introducing W to the Pt/WOx/Al2O3 catalyst. These results imply that merely covering the active Pt sites by other species cannot enhance the selectivity for 1, 3-PD, although it does decrease the glycerol conversion. To probe the interactions between Pt and WOx, we deposited a small amount of WOx species on the Pt/Al2O3 mother catalyst (Entries 6 and 7). In this case, the predominant product from glycerol hydrogenolysis was 1, 2-PD rather than 1, 3-PD. However, further increasing the amount of WOx (Entries 8 and 9) led to more 1, 3-PD and less 1, 2-PD, akin to the results obtained from a typical Pt/WOx/Al2O3 catalyst. These results strongly suggest that the interface between the Pt and WOx determines the selective hydrogenolysis of glycerol to give 1, 3-PD, irrespective of their deposition sequence. Therefore, future work may focus on the design of more selective catalysts by maximizing the Pt/WOx interface.
In summary, a diverse range of promoters, including transition and noble metals, was introduced to Pt/WOx and Pt/WOx/Al2O3. La was the most effective promoter, and the introduction of 0.1% La improved catalytic activity and selectivity for 1, 3-PD, as well as the stability of Pt/WOx. However, the significant increase in 1, 3-PD selectivity came at the cost of a slight activity loss in the case of the Pt/WOx/Al2O3. In some instances, the same promoter exhibited different effects with or without an Al2O3 support. Fe promoted Pt/WOx and Re promoted Pt/WOx/Al2O3 both demonstrated superior 1, 3-PD productivity relative to the unpromoted catalysts, while the opposite effect was observed when varying the mother catalysts. In addition, some other promoted catalysts, such as Zr and Sn promoted Pt/WOx and Fe and Zr promoted Pt/WOx/Al2O3, showed slightly decreased production of 1, 3-PD but still have significant potential in practical applications. This is because the introduction of promoters sometimes enhances the stability of the catalysts, which might be even more significant than the reactivity itself. Characterization of selected catalysts implied that the promotion mechanism likely results from modifying the electronic structure of the active Pt species. Hydrogenolysis performance is not highly correlated with original acid amounts, and therefore measurements of acidic sites generated in situ will be of great importance in future work to provide insight into the reaction mechanism and allow the rational design of such catalysts.
2016,
Vol. 37 Issue (9): 1513-1520 
PDF
2016,
Vol. 37 Issue (9): 1513-1520 PDF