Changji Wang†
a,
Feng Jiang†b,
Guangzheng Zuo†b,
Bing Liub,
Hanxu Li*a and
Xiaohao Liu*b
aCollege of Earth and Environment, Anhui University of Science and Technology, 232001 Huainan, China
bDepartment of Chemical Engineering, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China. E-mail: liuxh@jiangnan.edu.cn
First published on 9th December 2019
Highly dispersed tungsten species with an isolated tetrahedral WOx species structure are substantially beneficial for the metathesis reaction of ethylene and 1-butene to propene. The conventional impregnation method always leads to the formation of inactive crystalline WO3 thereby notably decreasing the amount of active sites. In this study, we synthesized a highly dispersed W-MCM-41 catalyst using the one-step precipitation method with a Si/W ratio of 30. The prepared catalyst showed excellent catalytic performance with a 1-butene conversion of 92.7% and a propene selectivity of 80.8%. In contrast, the impregnated catalyst with the same W loading as the one-step precipitation method resulted in a much lower 1-butene conversion of 76.5% and propene selectivity of 34.1%. Various characterization techniques including XRD, XPS, ICP-OES, UV-vis DRS, TEM, and Raman spectroscopy were applied to confirm that the one-step precipitation method can efficiently prepare well-dispersed W-MCM-41 catalysts with the desired structure in spite of the fact that the ideal dispersive structure was strongly dependent of the Si/W ratio and stirring time of the reaction mixture of tungstic acid and TEOS. In addition, the introduction of an upstream catalyst onto the W-MCM-41 catalyst could not obviously improve the 1-butene conversion and propene selectivity, which might be due to fast 1-butene isomerization easily occurring on the abundant Si–OH of the W-MCM-41 catalyst. This work provides new insights for the design of metathesis catalysts and reaction processes to efficiently convert ethylene and 1-butene into propene.
Supported WO3 catalysts exhibit better activity, excellent stability, anti-poisoning property, and low price and thus are highly desirable for industrial application.16–21 It is widely accepted that the highly dispersed WOx species and its existing state on supports play critical roles in the catalytic performance.22 The properties of supports and the preparation methods have a significant effect on the structure and dispersion of WOx species, and thus significantly influence the catalytic performance.23 Mesoporous materials, such as HMS, SBA-15, MCM-41, and FDU-12, have uniform pores, large BET surface areas and pore volumes, which are beneficial for the dispersion of active species and the transportation of reactants and products.24–29 Thus, mesoporous materials have been considered as promising catalyst supports in olefins metathesis.
In our previous study, we investigated the structure of tungsten oxide supported on SBA-15 and its catalytic performance in the metathesis reaction of 1-butene and ethylene to propene.30 It was found that the propene production through the metathesis reaction consists of three steps, including 1-butene isomerization, W–carbene formation, and metathesis reaction. The silanol group (Si–OH) in SBA-15 is acting as the active site for 1-butene isomerization. The W–carbene (WCH–CH3) species are formed via the partial reduction of isolated tetrahedral WOx species which contain WO or W–OH bonds. The W–carbene species play a crucial role in the metathesis reaction. Furthermore, the W/SBA-15 pretreated by H2O leads to a decrease of the metathesis activity. This is mainly attributed to the sintering of isolated WOx species, forming an inactive crystalline WO3 phase. These results suggest that the nature of the supported tungsten oxide phase significantly affects the catalytic performance of the olefin metathesis reaction. Establishing the structure–activity/selectivity relationships for metathesis reaction would be helpful for the rational design of more efficient catalysts.
In present investigation, MCM-41 molecular sieve with two-dimensional pore structure, smaller pore size and larger surface area was used as the catalyst support to study the olefin metathesis reaction. The WOx species were anchored onto the framework of MCM-41 by one-step precipitation method, which can improve the dispersion of tungsten active species and thus might have a significant impact on the catalytic activity and selectivity. The effects of stirring time and Si/W ratio on the catalyst structures and corresponding reaction performances were systematically investigated. Moreover, the effect of upstream isomerization catalyst on the improvement in the catalytic activity and selectivity was also studied.
As a comparison of the one-step prepared W-MCM-41(30) catalyst, the impregnated 18W/MCM-41 is also prepared with the same loading of W in the two catalysts. The catalyst support, MCM-41, is prepared as the above W-MCM-41(z) catalyst without the addition of W precursor. The W species was impregnated into the MCM-41 support with the W loading of 18 wt% by an incipient wetness impregnation method as our previous work.30
The UV-vis DR spectra can provide reliable information about the local molecular coordination and bonding of W species. As shown in Fig. 2, all the samples show a typical peak at ∼220 nm and a weaker peak at ∼270 nm indicating the presence of the isolated tetrahedral structure and the oligomeric octahedral structure of WOx species, respectively.33 Moreover, the peak at ∼400 nm assigned to the d–d band of crystalline WO3 is not observed in all the samples.31 These results suggest that the WOx species are highly dispersed on the surface of support and no crystalline WO3 is formed.
Raman spectra were further carried out to reveal the structural properties of these samples. As shown in Fig. 3, the vibrational bands at 493 and 608 cm−1 are attributed to the D2 and D1 defect modes related to tri- and tetra-cyclosiloxane rings of the support, respectively.34 The 970 cm−1 Raman band arises from the terminal vs. (WO) of surface WOx.35 The band intensity of support firstly decreases and then increases as the increase of stirring time and the maximum intensity is obtained with stirring time of 10 h. Combined with the BET results, it's deduced that the good crystallinity of the support favors the high surface area. There is slight influence of stirring time on the peak intensity of WO bond in the sample as the stirring time of 5 h shows a slight higher peak which indicates the presence of more surface WOx species.
These samples were tested for the metathesis reaction of 1-butene and ethylene to propene and the results are shown in Fig. 4 and Table 2. It can be seen from Fig. 4a that all the catalysts show stable 1-butene conversion after reaction for 3 h except the cases with stirring time longer over 10 h which show slight deactivation with time on stream. As shown in Fig. 1b, some larger pores at about 4 nm are formed at a longer stirring time over 10 h. The catalyst with stirring time of 5 h shows the highest 1-butene conversion of 88.3% and propene selectivity of 70.4% (Table 2). The higher conversion and selectivity over the catalyst with stirring time of 5 h is assigned to the higher amount of WOx species with the isolated tetrahedral structure and WO bond (Fig. 2 and 3). The detailed relationship between catalyst structure and catalytic performances will be discussed later. Thus, a suitable stirring time should be selected to prepare the excellent metathesis catalyst for the efficient conversion of ethylene and 1-butene to propene. In the following part, the stirring time is assigned to 5 h for all the prepared W-MCM-41 catalysts.
Fig. 4 Time-dependence of the 1-butene conversion (a) and propene selectivity (b) over W-MCM-41(50) catalyst with different stirring time. |
Stirring time (h) | Conversion (%) | Selectivity (%) | ||||
---|---|---|---|---|---|---|
Propene | C5+ | trans-2-Butene | cis-2-Butene | Iso-2-butene | ||
a Reaction condition: T = 450 °C, P = 0.1 MPa, catalyst weight = 0.5 g, WHSV (E + B) = 1.8 h−1, n(E)/n(B) = 2. | ||||||
1 | 88.1 | 69.5 | 3.9 | 15.1 | 11.0 | 0.5 |
5 | 88.3 | 70.4 | 4.1 | 14.5 | 10.5 | 0.5 |
10 | 82.7 | 62.2 | 4.8 | 18.8 | 14.0 | 0.2 |
15 | 84.8 | 65.5 | 5.1 | 16.8 | 12.4 | 0.2 |
20 | 86.3 | 67.5 | 5.2 | 15.6 | 11.4 | 0.3 |
Table 3 shows the BET surface area, pore volume and pore size of the W-MCM-41 catalysts prepared with different Si/W ratio. It is clear that the sample without the addition of tungstic acid shows the largest BET surface area of up to 1111 m2 g−1. When the Si/W ratio is 70 and 100, there is no obvious change in the BET surface area but the pore volume slightly increases probably due to the addition of H2O2 which is used to dissolve tungstic acid and has the effect of the enlargement of pore volume.31 The amount of W loading is tested by both XPS and ICP-OES and the results are also listed in Table 3. As characterized by the ICP-OES, the amount of W loading increases from 2.6% to 27.0% as the ratio of Si/W decreases from 100 to 10. The amount of W loading detected by XPS is always lower than the value by ICP-OES. This discrepancy might be resulted from that the result of ICP-OES represents the total amount of W in the catalyst while that of XPS represents the surface amount of W on the catalyst.
Sample | W (wt%) | BET surface area (m2 g−1) | Pore volumea (cm3 g−1) | Pore sizea (nm) | |
---|---|---|---|---|---|
By ICP-OES | By XPS | ||||
a Evaluated by the BJH method. | |||||
MCM-41 | 0 | 0 | 1111 | 0.53 | 2.17 |
W-MCM-41(100) | 2.6 | 2.1 | 1037 | 0.61 | 2.45 |
W-MCM-41(70) | 4.5 | 3.9 | 1096 | 0.62 | 2.36 |
W-MCM-41(50) | 11.9 | 6.2 | 879 | 0.44 | 2.38 |
W-MCM-41(30) | 18.1 | 11.0 | 654 | 0.43 | 2.56 |
W-MCM-41(10) | 27.0 | 21.0 | 499 | 0.43 | 3.34 |
As shown in Fig. 5a, all the samples show type IV isotherms with H1-hysteresis loops suggesting ordered mesoporous channels.12 Especially, for the W-MCM-41(10), the hysteresis loop is much bigger than other samples. As displayed in Fig. 5b, the introduction of W species also increases the pore size of the sample by increasing the amount of pores around 3.5 nm. Over W-MCM-41(10), the amount of pores at about 2 nm is much lower than the other samples while the amount of pores at around 3.5 nm is much higher, which indicates a destroyed zeolite structure over the sample with too much loading of W.
TEM images can clearly reflect the morphology of samples. As shown in Fig. 6, all the samples except W-MCM-41(10) show ordered two dimensional pore structures assigned to the MCM-41 zeolite. Moreover, the pore structures of W-MCM-41(30) are not as obvious as the other samples indicating a less ordered pore structures of the sample. The W-MCM-41(10) with the highest content of W shows no regular pore structure and the presence of amorphous SiO2. Most of WOx species are highly dispersed in the MCM-41 zeolite with a very small particle size of 1–2 nm. However, on the catalyst surface of higher W loading, such as W-MCM-41(10) and W-MCM-41(30) sample, some large WOx particles can also be observed. Especially for the W-MCM-41(10), large amount of WOx particles is observed and the particle size is over 100 nm. Too much amount of W is indeed destroying the structure of the expected MCM-41 zeolite, which is not suitable for obtaining the well dispersed WOx species. For the 18W/MCM-41 sample, the pore structures of MCM-41 is well reserved, but there are only large WOx particles unlike the W-MCM-41(30) catalyst showing highly dispersed WOx species on the catalyst surface. The one-step prepared method highly increased the dispersion of WOx species.
Fig. 6 TEM images of (a1 and a2) W-MCM-41(10), (b1 and b2) W-MCM-41(30), (c1 and c2) W-MCM-41(50), (d1 and d2) W-MCM-41(70), (e1 and e2) W-MCM-41(100), (f1 and f2) 18W/MCM-41 catalysts. |
The information about the local molecular coordination and bonding of W species is also detected by UV-vis DR spectra and the results are shown in Fig. 7. The peak intensity at ∼220 nm for W-MCM-41(100) is much stronger than the peak intensity at ∼270 nm, suggesting that most of the W species exist in terms of tetrahedral structure. The amount of octahedral structure at ∼270 nm increases as the increase of the amount of W. Moreover, the peak at ∼400 nm assigned to crystalline WO3 appears when the ratio of Si/W is 30 and the peak intensity increases as the Si/W ratio further decreases to 10. It indicates that the WOx species are highly dispersed on the catalyst surface except the W-MCM-41(30) and W-MCM-41(10), which is in accordance with the TEM results (Fig. 6).
To further characterize the structure and phase properties, low-angle and wide-angle XRD patterns of these samples are collected and shown in Fig. 8. As shown in Fig. 8a, all the samples except W-MCM-41(10) and W-MCM-41(30) exhibit well-defined (100) reflection indexed on the hexagonal lattice, suggesting the formation of the long-range ordered MCM-41 mesophase in these samples.36 Furthermore, the sample with higher Si/W ratio exhibits higher crystallinity than the one with lower Si/W ratio. This may be due to the differences between Si and W atoms in atomic radius (1.32 versus 1.41 Å), polarizability and possible coordination valancy.37 The W–O–Si likage would distort the vicinal tetrahedral SiO4 units. Thus, the high amount of W is detrimental for the formation of long-range ordered pores. Although the (100) reflection of W-MCM-41(30) is not obvious in the XRD patterns, there are still mesopore structures can be observed in the TEM images (Fig. 6b). It indicates that ordered mesopores but not long-range ordered mesopores are existed in this sample.
Fig. 8 Low-angle XRD patterns (a) and wide-angle XRD patterns (b) of the 18W/MCM-41 and W-MCM-41 catalysts with different Si/W molar ratio. |
Fig. 8b shows the wide-angle XRD patterns of these samples to reveal the dispersion of WOx species. The diffraction peaks assigned to crystal WO3 (PDF#72-1465) appear when the Si/W ratio is 30, which are located at 23.1, 23.6, 24.3, 28.9, 33.3 and 34.1°. The intensity of peaks assigned to WO3 becomes stronger as the Si/W ratio is 10 indicating the less dispersion of W species on the W-MCM-41(10) catalyst. Moreover, the 18W/MCM-41 catalyst shows more enhanced intensity of WO3 compared with W-MCM-41(30) suggesting the poor dispersion of WOx species of 18W/MCM-4, which is consistent with the TEM results. The Raman spectra in Fig. 9 also shows that only characteristic peaks assigned to MCM-41 can be observed when the Si/W ratio is higher than 30 suggesting the highly dispersed W species on the support surface. If the Si/W ratio decreases to 30, the peaks located at 274, 705 and 815 cm−1 can be observed assigned to the vibration of crystal WOx, which is the deformation vibration mode of W–O–W, bending vibration mode of W–O, and symmetric stretching vibration mode of W–O, respectively.38 These results are well in agreement with the XRD results.
The catalytic performances of these samples are shown in Table 4. It is obvious that both the 1-butene conversion and the propene selectivity increase as the decrease of Si/W ratio from 100 to 30 and then both of them decrease slightly as the further decrease of Si/W ratio from 30 to 10. Therefore, the highest 1-butene conversion of 92.7% and propene selectivity of 80.8% is obtained over W-MCM-41(30) catalyst. Interestingly, the 18W/MCM-41 prepared by the conventional impregnation method with the same loading of W as the W-MCM-41(30) shows notably lower 1-butene conversion of 76.5% and propene selectivity of 34.1%.
Sample | Conversion (%) | Selectivity (%) | ||||
---|---|---|---|---|---|---|
Propene | C5+ | trans-2-Butene | cis-2-Butene | Iso-2-butene | ||
a Reaction condition: T = 450 °C, P = 0.1 MPa, catalyst weight = 0.5 g, WHSV (E + B) = 1.8 h−1, n(E)/n(B) = 2. | ||||||
W-MCM-41(100) | 59.2 | 23.0 | 3.3 | 39.7 | 33.8 | 0.2 |
W-MCM-41(70) | 84.6 | 63.7 | 4.5 | 18.3 | 13.3 | 0.2 |
W-MCM-41(50) | 88.3 | 70.4 | 4.1 | 14.5 | 10.5 | 0.5 |
W-MCM-41(30) | 92.7 | 80.8 | 3.0 | 8.7 | 6.3 | 1.2 |
18W/MCM-41 | 76.5 | 34.1 | 2.6 | 35.6 | 27.3 | 0.4 |
W-MCM-41(10) | 91.7 | 78.7 | 3.2 | 9.9 | 7.2 | 1.0 |
In general, the metathesis reaction of 1-butene and ethylene to propene evolves three steps including (1) fast isomerization of 1-butene, (2) carbene formation and (3) metathesis reaction. The isomerization is considered to occur on the Si–OH of MCM-41 support which might act as a weak Brønsted acid site for the isomerization of CC bond (Scheme 1), but lack of the ability of metathesis.39 The followed two steps involving the carbene formation and the metathesis reaction take place on the highly dispersed WOx species with tetrahedral structure because the crystalline WO3 displays a negligible activity for 1-butene isomerization and inactive in metathesis reaction (Scheme 1).30 Compared with the first step, the followed two steps are very slow, which is accordance with our catalytic results in Fig. 4. At the very beginning, the fast formed 2-butene can not be immediately converted into propene for the lack of carbene. The carbene formation takes time, which determines the formation of propene. Therefore, at the very beginning, although the 1-butene conversion is high, the propene selectivity is low and it gradually increases with time on stream at the initial one hour (Fig. 4).
Scheme 1 Illustrative drawing of the metathesis reaction of ethylene and 1-butene to propene over W-MCM-41 catalysts prepared by one-step precipitation method and conventional impregnation method. |
For the one step synthesized W-MCM-41 catalysts, most of the WOx species are highly dispersed and the higher Si/W ratio corresponds to lower W loading. Although the first isomerization step is not rate-determining for the abundant Si–OH on the MCM-41 support, the followed two steps can be limited for the lack of WOx species on the catalyst of higher Si/W ratio. Since the metathesis reaction is a kind of tandem reaction, the suppressing of the second and third steps will hinder the first step to continue. Thus, the conversion of 1-butene should be lower as the W-MCM-41(100) catalyst shows a very lower conversion of 59.2%. It is not hard to understand that the insufficient WOx species is also not favorable for the metathesis reaction of 2-butene formed from the first isomerization step and ethylene, which thereby leads to a lower selectivity to metathesis product propene but a higher selectivity to isomerization product, 2-butene. Indeed, the propene selectivity is only 23% and the 2-butene selectivity is higher than 70% over the W-MCM-41(100) catalyst. As the Si/W ratio decreases lower to 10, WOx species is prone to form inactive crystal WO3 due to the higher W content, smaller surface area and larger pore size of the synthesized amorphous SiO2 rather than the highly ordered MCM-41 (Fig. 6, 8, 9 and Table 3). Therefore, both the 1-butene conversion and propene selectivity slightly decrease as the Si/W ratio decreases from 30 to 10. Over the impregnated 18W/MCM-41 catalyst, the dispersion of WOx species is much less than the one step preparation method (Scheme 1), which results in less active sites for the second and third step similarly as the case of W/MCM-41(10). It is reasonable that 18W/MCM-41 catalyst exhibits quite lower 1-butene conversion of 76.5% and propene selectivity of 34.1%. In general, the one step precipitation method is an efficient approach to obtain the highly dispersed WOx species with desired tetrahedral structure on MCM-41 support, which is responsible for the higher 1-butene conversion and propene selectivity due to the enhanced metathesis reaction of 2-butene and ethylene (Scheme 1).
Catalyst loading | Conversion (%) | Selectivity (%) | ||||
---|---|---|---|---|---|---|
Propene | C5+ | trans-2-Butene | cis-2-Butene | Iso-2-butene | ||
a Reaction condition: T = 450 °C, P = 0.1 MPa, mW-based catalyst = 0.5 g, mupstream catalyst = 0.5 g, WHSV (E + B) (W-based catalyst) = 1.8 h−1, n(E)/n(B) = 2. | ||||||
No upstream catalyst + W-MCM-41(30) | 92.7 | 80.8 | 3.0 | 8.7 | 6.3 | 1.2 |
W-MCM-41(30) + W-MCM-41(30) | 93.1 | 81.3 | 2.9 | 8.2 | 6.0 | 1.6 |
MgO + W-MCM-41(30) | 93.1 | 82.7 | 2.5 | 8.1 | 5.9 | 0.8 |
SBA-15 + W-MCM-41(30) | 91.4 | 78.0 | 3.0 | 10.5 | 7.5 | 1.0 |
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2019 |