Mengpan Wanga,
Hanmin Yangb,
Yinzheng Xiea,
Xiaohui Wua,
Chen Chena,
Wenbo Maa,
Qifeng Donga and
Zhenshan Hou*ac
aKey Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China. E-mail: houzhenshan@ecust.edu.cn; Tel: +86 21 64251686
bKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission, Ministry of Education, South-Central University for Nationalities, Wuhan 430074, Hubei Province, China
cDepartment of Chemical and Environmental Engineering, Xinjiang Institute of Engineering, Urumqi 830011, China
First published on 16th March 2016
The one-pot hydrogenolysis of biomass-derived glycerol to 1-propanol has been investigated over sequential two-layer catalysts in a continuous-flow fixed-bed reactor. The zirconium phosphate layer was packed in the upper layer for dehydration of glycerol into acrolein and the supported Ru catalysts were in the second layer for the sequential hydrogenation of acrolein to 1-propanol. It was observed that the second layer catalyst with the strong acid sites would cause the formation of glycerol degradation products such as methanol, ethanol, methane and carbon dioxide etc., while 2%Ru/SiO2 with weak acid sites afforded the highest selectivity for 1-propanol. The sequential packing of zirconium phosphate and the 2%Ru/SiO2 catalytic system can give full glycerol conversions at 77% selectivity of 1-propanol, as well as exhibiting long-term stability (80 h). Carbonaceous deposits were a main reason for deactivation and the deactivated catalysts can be regenerated conveniently by calcinations in air. The present approach afforded an effective one-pot hydrogenolysis of glycerol to biopropanol, which could bring about the benign development of the biodiesel industry.
Recently, 1-PO could be obtained by a process that can convert glycerol-derived PDOs to 1-PO.32–34 For example, RhReOx/SiO2 (Re/Rh = 0.5) catalyst gave high yields of 1-PO (66%) by the hydrogenolysis of 1,2-PDO at 393 K and 8.0 MPa initial H2 pressure.32 In addition, amorphous zirconia-supported niobium catalysts also exhibited selectivity favouring propanol (approximately 39%) at 85.0% conversion of 1,2-PDO at 290 °C under 1 atm N2.33 When 1,3-PDO was used as feed and hydrogenolyzed at 230 °C and 6 MPa H2 pressure, 1-PO was found to be the main product over Ni/Al2O3 and Ni/SiO2 catalysts.34
Regarding the synthetic route above, the one-pot hydrogenolysis of glycerol to biopropanols would be more preferential when concerned sustainability and energy efficiency, in comparison with the processes based on petroleum-derived ethylene, propylene or glycerol-derived PDOs. It has been reported that Pd/C catalyst gave a promising 1-PO selectivity (85 mol%) under batch-wise conditions through direct hydrogenolysis of glycerol solutions, but the conversion of glycerol was very low (2 mol%).35 The continuous-flow fixed-bed reactors are the preferred reactors for heterogeneous catalysis where the reactions take place on solid catalyst surfaces. Recently, the one-step production of long-chain hydrocarbons from waste-biomass-derived chemicals using bi-functional heterogeneous catalysts has been developed.36 Furthermore, a sequential two-layer catalytic system containing Hβ zeolite and Ni/Al2O3 catalyst in a fixed-bed reactor has also been reported for the hydrogenolysis of glycerol into 1-PO in high selectivity (69%) at high glycerol conversions (about 100%).37
In the previous studies, our group has indicated that zirconium phosphate (ZrP) catalyst was very efficient and selective for gas phase dehydration of glycerol to acrolein.38 Although acrolein is an important intermediate for the further industrial production, it is highly toxic and hard to storage. In this work, we attempted to use a sequential packing of the ZrP and hydrogenation catalysts in a fixed-bed reactor for hydrogenolysis of glycerol. The ZrP catalyst layer was packed before the hydrogenation catalyst layer. Considering that Ru component is highly efficient for unsaturated CC and C
O hydrogenation, the different supported Ru catalysts have been prepared and screened as the second layer for transforming acrolein produced by the first layer ZrP catalyst into 1-PO in a fixed-bed continuous flow reactor.
The supported Ru catalysts including 2%Ru/SiO2, 2%Ru/Al2O3, 2%Ru/Nb2O5, 2%Ru/Hβ and 2%Ru/HZSM-5 were prepared by incipient wetness method. As a typical example, 0.054 g RuCl3·3H2O was dissolved in 1 mL H2O, and this solution was joined to 1.0 g SiO2. Then the dispersion was stirring vigorously for 1 h, stewing 24 h and dried at 120 °C for 12 h. The resulting material was reduced in situ at 400 °C for 1 h prior to reaction or various characterizations and denoted as 2%Ru/SiO2. The other reduced Ru catalysts such as 2%Ru/Al2O3, 2%Ru/Nb2O5, 2%Ru/Hβ and 2%Ru/HZSM-5 have also been prepared in a similar method.
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Fig. 1 The characterization of ZrP and ZrP-R catalysts (a) TG curves; (b) XRD patterns; (c) NH3-TPD profiles; (d) pyridine-adsorbed FT-IR spectra. |
Catalysts | Surface area (m2 g−1) | Pore size (nm) | Pore volume (cm3 g−1) | NH3 desorbed before 350 °C (mmol NH3 g−1) | NH3 desorbed after 350 °C (mmol NH3 g−1) | Total acidity (mmol NH3 g−1) |
---|---|---|---|---|---|---|
ZrP | 101.3 | 1.7 | 0.22 | 0.85 | 0.18 | 1.03 |
ZrP-R | 75.8 | 1.2 | 0.14 | 0.63 | 0.04 | 0.67 |
The acidic properties of calcined ZrP samples were investigated by temperature-programmed desorption of ammonia analysis and infrared spectroscopy of adsorbed pyridine, and the results were shown in Fig. 1(c) and (d), respectively. The sample showed a little desorption of ammonia in the temperature range of 50–200 °C, indicating the presence of little numbers of weak acid sites (Fig. 1(c)). However, it was clear that the amounts of the medium strength acid sites (desorption in the temperature range of 200–350 °C) were much more than that of the strong acid sites (desorption in the temperature range of (>350 °C)) (Table 1). Total acidity (calculated from total area under the peaks in Fig. 1(c)) for ZrP was 1.03 mmol g−1 (Table 1), which was close to that of previous report.41
In the next step, the type of acidic site on ZrP catalysts was determined by infrared spectroscopy of adsorbed pyridine and the spectra in the range of 2000–1200 cm−1 are shown in Fig. 1(d). There are mainly three bands in this region. Pyridine adsorbed FT-IR spectra of the ZrP materials show sharp bands at 1543 cm−1, which are the characteristic bands of the typical pyridinium ion (PyH+) confirming the presence of Brønsted acid sites. This Brønsted acidity within the catalysts could be due to the presence of a reasonable amount of P(OH) groups in the ZrP. Further, the band at 1492 cm−1, which could be attributed to the adsorption of pyridine in Brønsted and Lewis sites is also present in ZrP samples at the same time. The band at about 1453 cm−1, corresponding to the adsorbed pyridine at the Lewis acid site (PyL) is clearly observed.42,43
In this work, the Ru-supported catalysts were functioned as hydrogenation and normally packed in the layer after the ZrP catalyst. Fig. 2 showed the XRD patterns of all supports and these supported Ru catalysts. The presence Fig. 2(a) of one broad peaks in the ranges of 10–30° indicated that SiO2 owns amorphous nature.44 The (020), (121), (420), (110) and (424) planes are corresponding to the typical pattern of the Al2O3 (Fig. 2(b)) (JCPDS 46-1131).45 The (001), (180), (201), (331), (381) and (382) planes are corresponding to the typical pattern of the Nb2O5 (Fig. 2(c)) (JCPDS 27-1003).46 The Fig. 2(d) shows the characteristic signals of the parent Hβ catalyst at 2Theta angles of 7–8° and 21–22° (JCPDS 45-0406).47 The Fig. 2(d) shows the characteristic signals of the parent HZSM-5 catalyst at 2Theta angles of 7–10° and 23–25° (JCPDS 42-0024).48 Additionally, compared to the standard XRD pattern of metal state Ru (JCPDS 65-7646), no obvious Ru(0) characteristic diffraction peaks of (100), (002), (101) (102), (110) and (103) planes over all supported Ru catalysts were observed in Fig. 2(a–e), which was ascribed to the low contents of metals or the high dispersion of the metal particles.
The textural and surface properties of the samples are shown in Table 2, where acid density was derived from the profile of NH3-TPD (Fig. 3) and the mean size of Ru particles from the TEM images (Fig. 4). Obviously, the supports with higher surface area could be favourable for forming smaller Ru particles (Table 2, entries 1–5). In addition, 2%Ru/SiO2 catalyst has the maximum amount of weak and medium strong acid site (0.85 mmol g−1), but has the minimal total amount acid sites among all catalysts due to the absence of strong acid site (Table 2, entries 1–5). In contrast, the HZSM-5 catalyst has the maximum amounts of the strong acid site (1.04 mmol g−1) and total acid sites (1.7 mmol g−1). It can be seen clearly that the amounts of the strong acid sites (desorption after the temperature of 350 °C) decreased in the order: 2%Ru/HZSM-5 > 2%Ru/Nb2O5 > 2%Ru/Al2O3 > 2%Ru/Hβ > 2%Ru/SiO2.
Entries | Catalysts | Surface area (m2 g−1) | Pore size (nm) | Pore volume (cm3 g−1) | Ru size (nm) | NH3 desorbed before 350 °C (mmol NH3 g−1) | NH3 desorbed after 350 °C (mmol NH3 g−1) | Total acidity (mmol NH3 g−1) |
---|---|---|---|---|---|---|---|---|
1 | 2%Ru/SiO2 | 258.6 | 10.7 | 0.69 | 2.8 | 0.85 | 0.01 | 0.86 |
2 | 2%Ru/Al2O3 | 165.2 | 9.4 | 0.37 | 5.0 | 0.66 | 0.37 | 1.03 |
3 | 2%Ru/Nb2O5 | 99.0 | 5.6 | 0.13 | 12.0 | 0.65 | 0.64 | 1.29 |
4 | 2%Ru/Hβ | 302.3 | 2.3 | 0.18 | 5.5 | 0.71 | 0.28 | 0.99 |
5 | 2%Ru/HZSM-5 | 298.8 | 2.3 | 0.17 | 3.0 | 0.66 | 1.04 | 1.70 |
6 | 2%Ru/SiO2-R | 175.6 | 1.2 | 0.14 | 4.1 | 0.43 | 0.00 | 0.43 |
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Fig. 3 NH3-TPD profiles of different catalysts. (a) 2%Ru/SiO2; (b) 2%Ru/Al2O3; (c) 2%Ru/Nb2O5; (d) 2%Ru/Hβ; (e) 2%Ru/HZSM-5. |
Entries | Catalysts | Con. % | Sel. % | Carbon yield in liquid products % | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
The first layer | The second layer | 1-POb | 1,2-PDOc | Acrolein | Acetol | Ethanol | Methanol | Othersd | |||
a Hydrolysis of glycerol using ZrP catalyst (1.0 g) as the first layer, and the supported Ru catalysts (1.0 g) as the second layer in a fixed-bed reactor. Temperature: 315 °C; H2 pressure: 2 MPa; flow rate of glycerol aqueous solution (10% wt): 0.04 mL min−1; flow rate of H2 = 30 mL min−1.b 1-Propanol.c 1,2-Propanediol.d Others gas and condensation products.e N2 (0.1 MPa).f H2 (0.1 MPa).g 10% 1,2-PDO aqueous solution as a feed.h 10% acrolein aqueous solution as a feed. | |||||||||||
1 | ZrPe | None | 100 | 0.0 | 0.0 | 82.0 | 10.3 | 0.0 | 0.0 | 7.7 | 92.3 |
2 | Zrpf | None | 100 | 0.0 | 0.0 | 81.7 | 10.8 | 0.0 | 0.0 | 7.6 | 92.5 |
3 | ZrP | None | 100 | 0.0 | 0.0 | 81.3 | 11.5 | 0.0 | 0.0 | 7.2 | 92.8 |
4 | Zrp | 2%Ru/HZSM-5 | 100 | 13.2 | 2.6 | 2.3 | 1.4 | 16.2 | 8.9 | 55.4 | 44.6 |
5 | ZrP | 2%Ru/Nb2O5 | 100 | 29.7 | 8.5 | 38.4 | 6.8 | 2.2 | 1.7 | 12.7 | 87.4 |
6 | ZrP | 2%Ru/Hβ | 100 | 66.3 | 4.3 | 2.6 | 3.1 | 3.9 | 3.4 | 16.4 | 83.5 |
7 | ZrP | 2%Ru/Al2O3 | 100 | 66.2 | 5.9 | 5.4 | 2.9 | 1.7 | 1.0 | 16.9 | 83.1 |
8 | ZrP | 2%Ru/SiO2 | 100 | 76.5 | 7.5 | 1.6 | 1.2 | 0.7 | 0.6 | 11.9 | 88.0 |
9 | None | 2%Ru/SiO2 | 18.1 | 2.5 | 48.0 | 0.0 | 4.2 | 9.7 | 4.2 | 33.9 | 67.0 |
10 | None | 2%Ru/SiO2g | 42.0 | 57.4 | — | 0.0 | 0.0 | 5.8 | 6.5 | 30.3 | 65.1 |
11 | None | 2%Ru/SiO2h | 99.5 | 96 | — | — | — | 1.4 | 2.2 | 0.4 | 98.2 |
Next, the gas phase hydrogenolysis of glycerol was carried out over the present two-layer catalysts. As shown in Table 3, entries 4–8, the main products included 1-PO and 1,2-PDO and acrolein, while acetol, ethanol, and methanol were detected as minor side products in liquid phase. Other small amount of gaseous products such as CO2, CO and CH4, which possibly resulted from the decomposition of cleavage of C–C bonds, can be also detected by GC with TCD. Despite these gaseous products, the carbon yield in liquid phase was always over 80% except that 2%Ru/HZM-5 used as the second layer catalyst gave only 44.6% carbon yield (Table 3, entry 4). The further GC analysis proved that the tail gas contained a large amount of CO2, CO and CH4.
According to the characterization results of catalysts as shown in Table 2, we can see that 2%Ru/SiO2, 2%Ru/Al2O3, 2%Ru/Hβ and 2%Ru/HSM-5 have a relatively high BET surface area while 2%Ru/Nb2O5 have a low BET surface area, which resulted in the larger Ru particles (about 12 nm) on Nb2O5, as compared with that of others. The larger Ru particles could show low hydrogenation ability for acrolein,51 leading to high selectivity to acrolein (38.4%) in the products (Table 3, entry 5). In contrast, those catalysts with small Ru particles showed very low selectivity to acrolein. For example, the second layer catalysts like 2%Ru/Al2O3, 2%Ru/Hβ which have a close Ru particles size and acid properties give 66.3% and 66.2% of selectivities to 1-PO, respectively (Table 3, entry 6 and 7). Nevertheless, the second layer Ru/HZSM-5 catalyst afforded relatively high selectivity to methanol and ethanol and also the carbon yield in liquid phase is very poor because a large amount of gases was formed likely due to the presence of a large amount of strong acid sites (Table 2, entry 4), resulting in the cleavage of carbon–carbon bond and thus the very high selectivities to gaseous products, methanol and ethanol. The present results is similar with that of previous research,52 which indicated that the acidity of HZSM-5 has a significant effect on the activity of Ru catalysts and products electivity in the hydrogenolysis of glycerol.
It should be noticing that the single-layer 2%Ru/SiO2 catalyst only gave moderate selectivity to 1,2-PDO (48.0%) with poor glycerol conversion (18%) (Table 3, entry 9). The main by-products were ethanol (in 9.7% selectivity) and methanol (in 4.2% selectivity). Acetol, CO2, CO and CH4 were also observed as minor products, and the selectivity of 1-PO is only 2.5%. When 1.0 g of ZrP was packed in the layer before the 1.0 g of 2%Ru/SiO2 layer, both of the glycerol conversion and selectivity to 1-PO were improved significantly (Table 3, entry 8). On the other hand, the selectivity to 1,2-PDO was dramatically decreased from 48% to 7.5%. In addition, when 10% acrolein aqueous solution was employed as a feed and converted only by Ru/SiO2 catalyst. It was found that the acrolein conversion was more than 99% and the selectivity to 1-PO reached to 96% (Table 3, entry 11). This result demonstrates that for the two-layer catalysts, acrolein produced from glycerol dehydration over the first layer catalyst (ZrP) could be hydrogenated into 1-PO effectively by the sequential Ru/SiO2 catalyst (Table 3, entries 3, 8 and 11). The addition of the ZrP catalyst directed the hydrodeoxygenation of glycerol toward a higher degree, and thus improved the selectivity to 1-PO.
Since the ZrP, coupling with 2%Ru/SiO2 catalyst exhibited promising catalytic performance, the following investigations were conducted on the two layer catalysts. The influence of temperature on glycerol hydrogenolysis was illustrated in Fig. 5(a), in which showed that glycerol conversion improved from 95% (285 °C) to 100% (300 °C). While, the selectivity to 1-PO increased remarkably to a maximum at 315 °C and then decreased. It can be seen that both acrolein and acetol co-existed in low selectivity due to the high hydrogenation activity over 2%Ru/SiO2 catalyst. Meantime, the main by-product 1,2-PDO which is hydrogenised from acetol is decreased when the temperature increased from 285 °C to 315 °C, indicating that 1,2-PDO would underwent a dehydration and sequential hydrogenation at higher temperature over Ru/SiO2 catalyst (Fig. 5(a)). This can be further evidenced by the fact that when 1,2-PDO was used as a feed and hydrogenated by a single layer 2%Ru/SiO2 catalyst, the conversion of 1,2-PDO is 42.0% and the selectivity to 1-PO reached up to 57.4% (Table 3, entry 10). Moreover, as reaction temperature increasing on ZrP catalyst, the selectivity of acrolein increased, accompanying with that of acetol decreased,37 which thus resulted in a decline of the selectivity of 1,2-PDO derived from acetol hydrogenation over 2%Ru/Al2O3 catalyst accordingly (Fig. 5(a)). On the other hand, the selectivity of acrolein is always below 2.5%, and this meant that the second layer 2%Ru/SiO2 catalyst exhibited promising hydrogenation performance even the temperature is below 315 °C. Whereas the decrease of selectivity to 1-PO at higher temperature (>315 °C) is related to the formation of large amounts of undesired by-products, such as the over hydrogenolysis product propane, and the degradation products methanol, ethanol, methane, carbon dioxide. This is in agreement with the results reported by Esti van Ryneveld et al.34
Fig. 5(b) shows the effect of hydrogen pressure on the catalytic performance of glycerol hydrogenolysis over two layer catalysts at 315 °C. The selectivity of 1-PO enhanced from 32.0% to 76.5% while the selectivity of acrolein decreased sharply from 47.2% to 2.3% as the hydrogen pressure was increased from 0.5 MPa to 2 MPa, revealing that although the increase of the hydrogen pressure does not affect the product distribution on the first ZrP layer (Table 3, entries 2 and 3), the hydrogenation rate of the second layer 2%Ru/SiO2 catalyst increased greatly. This implied that the 2%Ru/SiO2 catalyst was highly active for acrolein hydrogenation to 1-PO, even at a relatively low hydrogen pressure compared to previous report.30 Additionally, as H2 pressure increased to 2 MPa, the selectivity of 1,2-PDO increased slightly from 1.5% to 8.0% while the selectivity to acetol decreased from 11.8% to 2.2%, indicating that 1,2-PDO mainly resulted from the acetol hydrogenation. However, at least the portion of 1,2-PDO was also converted into 1-PO by dehydration and sequential hydrogenation route as discussed above. The further increasing hydrogen pressure above 2.0 MPa did not have an obvious impact on the selectivity of 1-PO Fig. 5(b).
Following on from our finding and previous report,37 the possible reaction route involved in glycerol hydrogenolysis is proposed in Scheme 1. The acid-catalyzed dehydration of glycerol over the first ZrP catalyst layer initially formed acrolein as a main product and acetol as minor by-product, which can subsequently hydrogenate into 1-PO and 1,2-PDO over supported Ru catalysts, respectively. The intermediate such as 1,2-PDO can also underwent a dehydration and sequential hydrogenation route on the supported Ru catalyst to give 1-PO. Meantime, both of dehydration of glycerol and subsequent hydrogenation process would generate minor gas by-products including CO2, CO and methane due to C–C bond cleavage.
The long-term performance of glycerol hydrogenolysis over two layer catalysts was conducted at 315 °C and 2.0 MPa, and the results are shown in Fig. 6. The catalytic activity did not decline obviously within 80 h under the present reaction conditions. Meanwhile, the product distribution did not show any appreciable change during the whole test. This result demonstrated that the two layer catalytic system was rather durable under hydrothermal conditions for glycerol hydrogenolysis and could be suitable for practical applications. The superior long-term performance of ZrP and 2%Ru/SiO2 was mainly not only attributed to the large amount of middle strong acid sites on ZrP catalyst which can convert glycerol to acrolein, and also the highly active Ru sites dispersed on SiO2 which can convert acrolein hydrogenation to 1-PO rapidly. As shown in Fig. 6, the conversion and selectivity to 1-PO reached to 100% and 77%, respectively, and both of them can keep stable as long as 80 h although they decreased over 80 h on stream. However, when the catalyst was regenerated by calcining at 500 °C for 4 h after reaction for 100 h, it can be reused for another 100 h in continuous flow process. The conversion of glycerol still achieved a high level, although the selectivity to 1-PO (around 72%) decreased slightly.
It was observed that the ZrP catalyst after reaction for 100 h showed a slight dark colour, indicating the possible presence of a considerable amount of carbonaceous deposits on the surface of catalyst. From profiles of TG of catalysts in air as shown Fig. 1(a) and 7, we can see that both the spent ZrP catalyst (ZrP-R) and the spent 2%Ru/SiO2 catalysts (2%Ru/SiO2-R) are all have an obvious weight loss due to coking removal around 500 °C, as compared to the fresh one. The rise of the weight below 300 °C could be attributed to the oxidation of Ru(0). This indicated that the carbon deposition can be removed simply by calcinations in air, which allowed regenerating catalyst and thus making a longer performance of the catalyst.
Sequentially, the various carbon deposits of the spent catalysts were analysed by Raman spectroscopy and the results are shown in Fig. 8. All spectra showed two major peaks at about 1346 cm−1 and 1573 cm−1. The peak at 1573 cm−1 represented graphitic, designated as G-band and the peak at 1346 cm−1 was attributed to defects present in the structural units of graphite designated as D-band. This meant that the type of coke species on different catalysts was almost same.
The carbonaceous deposits pointed to a deactivation mechanism due to carbonaceous deposits blocking the catalytically active sites. The BET surface area of ZrP-R decreased to 75.8 m2 g−1, as compared with the fresh one (101.3 m2 g−1) (Table 1). In addition, the acid amounts of catalysts also decreased from 1.03 mmol NH3 g−1 to 0.67 mmol NH3 g−1 (Table 1). What's more, the medium strength acid sites have decreased dramatically compared with the fresh catalyst, which led to the decrease of the selectivity of acrolein. Thus, selectivity of the 1-PO was decreased in a small degree. Meantime, the BET surface area of 2%Ru/SiO2-R decreased from 258.6 m2 g−1 to 175.6 m2 g−1 and the acid amount of 2%Ru/SiO2-R catalyst also decreased from 0.86 mmol NH3 g−1 to 0.43 mmol NH3 g−1. It should be noticing that the sizes of Ru particle on SiO2 increased from 2.8 nm to 4.1 nm after 100 h on stream from TEM images (Table 2, entries 1 and 6). This might be the reason why the regenerated catalyst afforded slightly lower selectivity to 1-PO, but higher selectivity to acrolein, in comparison with the fresh one in the first run (Fig. 6).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02682f |
This journal is © The Royal Society of Chemistry 2016 |