Tianmiao
Wang
a,
Yoshinao
Nakagawa
*ab,
Masazumi
Tamura‡
ab,
Kazu
Okumura
c and
Keiichi
Tomishige
*ab
aDepartment of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan. E-mail: yoshinao@erec.che.tohoku.ac.jp; tomi@erec.che.tohoku.ac.jp
bResearch Center for Rare Metal and Green Innovation, Tohoku University, 468-1 Aoba, Aramaki, Aoba-ku, Sendai, 980-0845, Japan
cDepartment of Applied Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji, Tokyo 192-0015, Japan
First published on 11th May 2020
Efficient and reusable catalysts were developed for one-pot reduction of 1,4-anhydroerythritol (1,4-AHERY), which is a promising biomass-derived C4 platform chemical, into 1,4-butanediol (1,4-BuD) with H2. First, various ReOx catalysts on oxide supports were tested for reductive conversion of 2,5-dihydrofuran (2,5-DHF) to 1,4-butanediol (1,4-BuD). ReOx/WO3–ZrO2 showed the best performance, and ReOx catalysts supported on other oxides were much less active in the isomerization of 2,5-DHF to 2,3-DHF which is the first step of the 2,5-DHF conversion to 1,4-BuD. The ReOx/WO3–ZrO2 catalyst was combined with the ReOx–Au/CeO2 catalyst for deoxydehydration (DODH) of 1,4-AHERY into 2,5-DHF to develop a one-pot conversion system of 1,4-AHERY to 1,4-BuD. The highest 1,4-BuD yield of 55% from 1,4-AHERY was obtained. Even though the yield was lower than that obtained over the combination of ReOx/C and ReOx–Au/CeO2 catalysts in our previous study, the regeneration of the combination of ReOx/WO3–ZrO2 and ReOx–Au/CeO2 is possible: calcination at 573 for 3 h of the used catalyst mixture increased the activity to the fresh level. THF was the major by-product in both 1,4-AHERY and 2,5-DHF conversions, which was due to hydrogenation of DHF, disproportionation of DHF and/or dehydration of 1,4-BuD. The W amount in WO3–ZrO2 greatly affected the catalytic performance of ReOx/WO3–ZrO2: too much W above the monolayer level on the ZrO2 support sharply decreased the activity in 2,5-DHF isomerization. On the other hand, WO3–ZrO2 with a tetragonal ZrO2 structure prepared by co-precipitation showed comparable performance to WO3–ZrO2 with a monoclinic ZrO2 structure as the support of the ReOx catalyst, demonstrating that the crystal structure of ZrO2 has little effect on the catalytic performance. The Re species were suggested to be highly dispersed on the WO3 (sub)monolayer on ZrO2 based on the effect of the Re loading amount. The dispersed Re species on monolayer WO3 species on ZrO2 can be the active sites for 2,5-DHF disproportionation to 2,3-DHF.
(1) |
(2) |
Recently, we have reported that 1,4-BuD could be produced from 1,4-AHERY over the combination of ReOx/C and ReOx–Au/CeO2 catalysts in a one-pot reaction.23 The yield of 1,4-BuD was around 90% which was the highest yield of 1,4-BuD from biomass-derived products so far even including succinic acid. In this co-catalyst system, ReOx–Au/CeO2 firstly catalyzed the reduction of 1,4-AHERY to 2,5-dihydrofuran (2,5-DHF), and ReOx/C catalyzed the further reduction of 2,5-DHF to 1,4-BuD (Scheme 1). The former step is the conversion of cis-vicinal OH groups to a CC double bond which is called deoxydehydration (DODH), and this reaction is typically catalyzed by homogeneous Re(VII) catalysts with organic reducing agents such as 3-pentanol and PPh3.24–26 Development of solid catalysts for DODH has been a hot topic in biomass conversion.27–31 Our laboratory has developed a heterogeneous ReOx–Au/CeO2 catalyst for DODH using H2 as the reducing agent.32–35 The highly dispersed monomeric Re oxide species on the crystalline CeO2 surface can be the active sites of DODH.32,33,36–38 Au serves as the activation site of H2, and the activated hydrogen species move on CeO2 probably as pairs of protons and electrons to Re species. Au was selected among various noble metals based on its low CC hydrogenation activity.32 The stabilization of high valent Re species by CeO2 and the capability of hydrogen species to move on the CeO2 surface are the keys of the DODH activity. The latter step catalyzed by ReOx/C consists of the following steps: the isomerization of 2,5-DHF to 2,3-dihydrofuran (2,3-DHF), the hydration of 2,3-DHF to 2-hydroxytetrahydrofuran (2-HTHF), and the reduction of 2-HTHF or its straight chain form 4-hydroxybutanal to 1,4-BuD. In the most recent research, we also found that the ReOx/CeO2 catalyst without an Au promoter still could catalyze the DODH reaction in the presence of ReOx/C, enabling the reduction of 1,4-AHERY to 1,4-butanediol with the mixture of ReOx/CeO2 and ReOx/C.39 The Re species on carbon could activate H2 like an Au promoter for the reduction of Re species on the CeO2 surface which are the active sites in the conversion of 1,4-AHERY. Furthermore, CeO2 without Re loading combined with ReOx/C even could convert 1,4-AHERY to 1,4-BuD, because the Re species on the carbon surface could move to CeO2 surfaces to form ReOx/CeO2. The highest yield of 1,4-BuD reached 85% over the ReOx/CeO2 + ReOx/C system, which was comparable to that in the co-catalyst system with an Au promoter. As above, ReOx/C plays many roles in this system: CC bond movement (isomerization), H2 activation, hydrolysis and CO hydrogenation. Versatile roles of ReOx/C have been also reported in other catalytic reactions with multiple steps such as reductive lignin depolymerization.40,41
The largest drawback of these mixed catalyst systems was catalyst deactivation. The conversion of 1,4-AHERY sharply dropped from 100% to 65% (ReOx–Au/CeO2 + ReOx/C) or 64% (ReOx/CeO2 + ReOx/C) after the first run in reuse tests, indicating the deactivation of ReOx(–Au)/CeO2. The selectivity to 1,4-BuD also decreased, indicating the deactivation of ReOx/C. The activity of noble-metal-modified ReOx/CeO2 catalysts for DODH has been also reported to be deactivated during the reaction; however, they can be regenerated by calcination to recover the activity.32 On the other hand, due to the combustibility of the carbon support, it is not feasible to regenerate the recycled catalyst mixture by calcination. Therefore, in this study, a new Re catalyst with a regenerable support was studied as an alternative to ReOx/C in the co-catalyst system with ReOx–Au/CeO2. Among various oxide-supported ReOx catalysts, ReOx/WO3–ZrO2 is a possible alternative to ReOx/C combined with ReOx–Au/CeO2 in the production of 1,4-BuD from 1,4-AHERY. While the yield of 1,4-BuD over the combination of ReOx/WO3–ZrO2 + ReOx–Au/CeO2 was lower than that over the co-catalyst with a carbon support, the activity of the used catalyst mixture of ReOx/WO3–ZrO2 and ReOx–Au/CeO2 could be regenerated by calcination, which could be comparable to the fresh one.
The activity test was conducted with an autoclave reactor equipped with an inner glass cylinder. After the reaction, both gas and liquid phases were analyzed by FID-GC. The carbon balance (C.B.) of each analysis result was calculated using eqn (3). The sum of the detected but unidentified products is denoted as “others” in the results. The FID sensitivity of “others” was assumed to be the same as that of 1,4-butanediol. When the C.B. is in the range of 100 ± 10% considering the experimental error, the conversion and selectivity on the carbon basis are calculated using eqn (4) and (5), respectively, and the data of C.B. are not shown in each result. In contrast, when the C.B. is clearly lower than 100% (<90%), the conversion is calculated using eqn (6). The selectivity to “others” is the same as the above case. The data of C.B. are shown in each result when clearly below 100%. The yield was calculated using eqn (7) and (8) for normal cases and cases with low C.B., respectively. The TOF was calculated using eqn (9) with the increase of conversion (Δconversion) from that at 0 h reaction because the reaction proceeded significantly during heating.
(3) |
(4) |
(5) |
(6) |
(7) |
(8) |
(9) |
For reuse tests, the catalyst mixture was collected by filtration, washed with 1,4-dioxane and dried for 12 h. The dried catalyst was used directly or after calcination for the next activity test. The calcination conditions were described in each result. The weight loss during the recovery and drying process was about 20%. Typically, multiple runs were carried out simultaneously to collect a sufficient amount of catalyst for the next use, and the number of runs was decreased during reuses (1st use: 4 runs; 2nd use: 3 runs, and so on).
The BET surface area was measured with a Micromeritics Gemini instrument. XRD patterns were obtained with a Rigaku MiniFlex600 diffractometer. H2-TPR profiles were obtained with a home-made apparatus equipped with a fixed-bed quartz reactor, frozen acetone trap and TCD. The sample weight was about 50 mg, and the sample was reduced with 5% H2 in Ar from room temperature to 1173 K at a heating rate of 10 K min−1. Temperature-programmed desorption of NH3 (NH3-TPD) profiles were obtained with a MicrotracBEL BELCAT-II instrument. The sample (50 mg) was first treated with He at 773 K for 1 h, and then NH3/He (5/95) was passed through the sample at 323 K for 30 min. After purging NH3 with He at 323 K, the sample was heated at 10 K min−1 under flowing He. The desorbed NH3 was analyzed with MS at the m/z = 16 signal. XAFS spectra were measured at the BL01B1 station of SPring-8 (Proposal No. 2019A1369). Detectors for Re L3-edge spectra were ion chambers filled with N2/Ar = 85/15 and N2/Ar = 50/50 for I0 and I, respectively. Analysis of data was performed using the REX2000 ver. 2.6 program to obtain the XANES spectra. TG-DTA profiles were obtained with a Rigaku Thermo Plus EVO-II instrument using a 10 mg sample and α-Al2O3 reference under static air.
Entry | Catalyst | Re loading amount/wt% | Conv./% | Selectivity/% | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1,4-BuD | THF | 2,3-DHF | 1-BuOH | GBL | Furan | 2-HTHF | Acetal A | Acetal B | Others | ||||
a Reaction conditions: 2,5-DHF = 0.15 g, water = 0.04 g, catalyst = 0.15 g (Re = 1 or 3 wt%), 1,4-dioxane = 4 g, PH2 = 8 MPa, T = 413 K, t = 4 h. b Reported in ref. 23. c TiO2 = 30 wt%. d SiO2 = 10 wt%. e CeO2 = 50 wt%. f WO3 = 10 wt%. g ReOx/WO3–ZrO2 was reduced in solvent at 413 K for 1 h before the reaction. h W = 5 wt%; homemade supports prepared by impregnation. BuD: butanediol, DHF: dihydrofuran, THF: tetrahydrofuran, BuOH: butanol, GBL: γ-butyrolactone, HTHF: hydroxytetrahydrofuran, acetal A: ; acetal B: . | |||||||||||||
1b | ReOx/C | 3 | 94 | 60 | 14 | 0 | 2 | 12 | 2 | 0 | 0 | 5 | 5 |
2 | ReOx/TiO2 | 3 | 43 | 21 | 22 | 3 | 18 | 6 | 9 | 2 | 2 | 5 | 11 |
3 | ReOx/ZrO2 | 3 | 62 | 19 | 20 | 6 | 3 | 17 | 7 | 4 | 1 | 16 | 6 |
4 | ReOx/Al2O3 | 3 | 23 | 1 | 27 | 11 | 1 | 2 | 26 | 5 | 4 | 1 | 24 |
5 | ReOx/SiO2 | 3 | 25 | 13 | 48 | 11 | 8 | 0 | 9 | 11 | 0 | 0 | 0 |
6 | ReOx/HZSM5 | 3 | 26 | 2 | 45 | 0 | 7 | 0 | 32 | 1 | 0 | 0 | 12 |
7 | ReOx/MgO | 3 | 25 | 0 | 35 | 46 | 0 | 0 | 11 | 8 | 0 | 0 | 0 |
8 | ReOx/TiO2–ZrO2c | 1 | 57 | 21 | 30 | 10 | 2 | 7 | 4 | 4 | 1 | 17 | 4 |
9 | ReOx/SiO2–ZrO2d | 1 | 52 | 22 | 36 | 1 | 3 | 6 | 6 | 2 | 1 | 17 | 2 |
10 | ReOx/CeO2–ZrO2e | 1 | 17 | 2 | 37 | 28 | 0 | 0 | 0 | 21 | 0 | 0 | 12 |
11 | ReOx/WO3–ZrO2f | 1 | 94 | 43 | 38 | 0 | 4 | 5 | 2 | 1 | 0 | 6 | 1 |
12 | ReOx/WO3–ZrO2f,g | 1 | 63 | 18 | 39 | 1 | 3 | 10 | 3 | 3 | 0 | 12 | 9 |
13 | ReOx/WO3/TiO2h | 1 | 26 | 3 | 31 | 0 | 5 | 0 | 28 | 5 | 0 | 0 | 27 |
14 | ReOx/WO3/Al2O3h | 1 | 22 | 9 | 38 | 1 | 2 | 1 | 21 | 10 | 2 | 6 | 9 |
15 | ReOx/WO3/ZrO2h | 1 | 93 | 40 | 45 | 0 | 5 | 3 | 3 | 0 | 0 | 3 | 1 |
16 | None | — | 2 | 0 | 38 | 19 | 0 | 0 | 26 | 0 | 11 | 0 | 6 |
All the ReOx catalysts with a single oxide support showed lower activity and 1,4-BuD selectivity than ReOx/C (Table 1, entry 1). Therefore, the Re catalysts with mixed oxide supports were further tested in the reaction of 2,5-DHF. Zirconia was selected as one of the components because of the variety of available mixed oxides and relatively good performance of ReOx/ZrO2. The ReOx/WO3–ZrO2 catalyst (entry 11) using a commercial WO3–ZrO2 support with 10 wt% WO3 loading showed the highest conversion of 2,5-DHF and the highest selectivity to 1,4-BuD among all the catalysts on non-carbon supports. The conversion over ReOx/WO3–ZrO2 was as high as that obtained over ReOx/C, and the selectivity to 1,4-BuD was slightly lower than that of ReOx/C due to the higher selectivity to THF. Other Re catalysts with mixed oxide supports such as ReOx/TiO2–ZrO2, ReOx/SiO2–ZrO2, ReOx/CeO2–ZrO2, ReOx/WO3/TiO2, and ReOx/WO3/Al2O3 (entries 8–10, 13–14) showed lower activity in the conversion of 2,5-DHF and selectivity to 1,4-BuD than ReOx/WO3–ZrO2. The ReOx/WO3–ZrO2 catalyst reduced before use (entry 12) showed a lower activity than the non-pretreated one, and the selectivity to 1,4-BuD also decreased, while the selectivity to other intermediates such as 2-hydroxytetrahydrofuran (addition product of 2,3-DHF and water) and acetals increased. This decrease might be due to the aggregation of catalytically active Re sites.
Fig. 1 shows the effect of the Re loading amount of ReOx/WO3–ZrO2 in the reaction of 2,5-DHF. Without Re, the conversion was low and the main products were THF and furan which can be produced by disproportionation. More furan was formed than THF, which suggests that some of furan was formed by dehydrogenation of 2,5-DHF. 1,4-BuD was not formed at all over WO3–ZrO2 alone. The conversion and selectivity to 1,4-BuD dramatically increased with the increase of the Re loading amount from 0 to 1 wt%. The optimized Re loading amount was 1 wt% based on the 2,5-DHF conversion and 1,4-BuD selectivity. However, the formation of THF increased when the Re loading amount was over 1 wt%, while the conversion of 2,5-DHF gradually decreased at the same time. The decrease of conversion can be due to the aggregation of catalytically active Re sites to inactive polymeric Re species (in isomerization of 2,5-DHF to 2,3-DHF). The increase of THF selectivity with the increase of the Re amount can be explained by the hydrogenation ability of polymerized Re species with metallic nature.
Even though 1,4-BuD was formed over the ReOx/ZrO2 catalyst, adding WO3 to the ZrO2 support improved the conversion and the selectivity to 1,4-BuD dramatically. In addition to the commercial WO3–ZrO2 (10 wt% WO3) support, WO3/ZrO2 supports with various W amounts were prepared by impregnation and tested as ReOx/WO3/ZrO2 catalysts for the reaction of 2,5-DHF (Fig. 2). Up to 5 wt% W, the activity of the ReOx/WO3/ZrO2 catalyst increased with increasing W amount. The selectivity to 1,4-BuD and its precursors (acetals and 2-HTHF) became higher when the W amount was between 3 and 5%. The THF formation sharply increased and the selectivity to 1,4-BuD decreased at a higher W loading amount than 5 wt%. The conversion of 2,5-DHF also decreased dramatically when the W loading amount was higher than 5 wt%. In tungsten–zirconia supported Re catalysts, this homemade WO3/ZrO2 (W = 5 wt%) support (Table 1, entry 15) showed a similar activity to the commercial WO3–ZrO2 (WO3 = 10 wt%) support (entry 11). The best W amount in the homemade support (W = 5 wt%) was similar to the W amount of the commercial support (WO3 = 10 wt%; W = 7.9 wt%) based on the surface area (ZrO2: 62 m2 g−1; WO3–ZrO2: 103 m2 g−1). We also prepared a tungsten–zirconia support by co-precipitation (cpWO3–ZrO2), because the tungsten–zirconia prepared by co-precipitation is known to have a tetragonal ZrO2 structure43 while pure standard ZrO2 has the monoclinic phase. The activity of ReOx/cpWO3–ZrO2 (W = 5 wt%) was lower than those of ReOx/WO3–ZrO2 (WO3 = 10 wt%) with a commercial support and ReOx/WO3/ZrO2 with the same W amount (5 wt%). We used the commercial WO3–ZrO2 (WO3 = 10 wt%) support in the following studies due to the convenience of accessibility, unless noted. The structure–performance relationship will be discussed in a later section based on the characterization data of various tungsten–zirconia supports.
Fig. 3 shows the effect of H2 pressure in the reaction of 2,5-DHF over ReOx/WO3–ZrO2 (the detailed data for the reaction rate determination are shown in Fig. S2 and Table S1, ESI†). The conversion and selectivity to 1,4-BuD increased with the increase in hydrogen pressure from 2 to 8 MPa (Fig. 3(a)). Although the THF selectivity increased at the same time, the reaction rate was much higher under 8 MPa hydrogen pressure. The total selectivity to 1,4-BuD + acetals + 2-HTHF + γ-butyrolactone (GBL) as 1,4-BuD and its precursors was similar to the sum of selectivity to furan + THF under different H2 pressures, even though the 1,4-BuD selectivity was lower and the GBL selectivity was higher under lower H2 pressure. Therefore, GBL may be an intermediate of 1,4-BuD like 2-HTHF and acetals in the conversion of 2,5-DHF.
Fig. 3 Effect of H2 pressure in the reaction of 2,5-DHF over ReOx/WO3–ZrO2. (a) Results at 4 h; (b) initial conversion rate. Reaction conditions: 2,5-DHF = 0.15 g, water = 0.04 g, ReOx/WO3–ZrO2 = 0.15 g (Re = 1 wt%, WO3 = 10 wt%), 1,4-dioxane = 4 g, PH2 = 2–8 MPa, T = 413 K, t = 4 h (a), 0–2 h (b). The detailed data of (b) are shown in Table S1, ESI.† DHF: dihydrofuran, BuD: butanediol, THF: tetrahydrofuran, BuOH: butanol, GBL: γ-butyrolactone, HTHF: hydroxytetrahydrofuran, acetal A: ; acetal B: . |
The reaction rate dependence on H2 pressure was also determined based on the reaction results at low conversion levels. The reaction rate of 2,5-DHF increased with increasing H2 pressure over ReOx/WO3–ZrO2. The reaction order with respect to hydrogen pressure was 0.94 (Fig. 3(b)), indicating that the rate-determination step was a reaction involving hydrogen species. Although the conversion of 2,5-DHF to 2,3-DHF does not consume hydrogen, the conversion can proceed by addition of a hydrogen atom to the 4-position of 2,5-DHF and removal of a hydrogen atom at the 2 position (eqn (10)). The rate-determining step might be the activation of H2 or the addition of hydrogen species to the substrate.
(10) |
As above, the optimized reaction conditions for 1,4-BuD production from 2,5-DHF were a temperature of 413 K and a H2 pressure of 8 MPa, which were the same reaction conditions used in the reaction of 1,4-AHERY over the combination of ReOx–Au/CeO2 (or ReOx/CeO2) and ReOx/C catalysts as reported in our previous reports.23,39
The optimized ReOx/WO3–ZrO2 catalyst was applied to the reaction of 1,4-AHERY combined with the ReOx–Au/CeO2 catalyst for DODH of 1,4-AHERY to 2,5-DHF under the standard reaction conditions. The time course of the reaction of 1,4-AHERY over the mixture of ReOx–Au/CeO2 and ReOx/WO3–ZrO2 is shown in Fig. 4 (the detailed data are shown in Table S2, ESI†). The selectivity to 1,4-BuD and THF increased with the increase of conversion. At the beginning of the reaction, 2,5-DHF as an intermediate and the acetal from 4-hydroxybutanal and 1,4-AHERY were detected, and they were gradually converted with time. Unlike the time course of 1,4-AHERY reduction over the combination of ReOx–Au/CeO2 and ReOx/C where only the acetal was detected as the main intermediate,23 2,5-DHF as an intermediate was detected in a significant amount at a shorter reaction time. This difference means the lower activity of the ReOx/WO3–ZrO2 catalyst in the isomerization of 2,5-DHF to 2,3-DHF than that of ReOx/C. These results are somewhat inconsistent with the similar activity of ReOx/WO3–ZrO2 alone to the ReOx/C catalyst alone in 2,5-DHF reduction (Table 1, entries 1 and 11). The difference can be explained by the movement of Re species from WO3–ZrO2 to the CeO2 support as described below. The DHFs were quickly converted over the ReOx/C catalyst, and the rate-determining step of the ReOx(–Au)/CeO2 + ReOx/C system for 1,4-AHERY reduction was the DODH catalyzed by ReOx(–Au)/CeO2. In the system of ReOx–Au/CeO2 + ReOx/WO3–ZrO2, the rate-determining step in the conversion of 1,4-AHERY was not clear: the DODH step and isomerization step had similar reaction rates. The highest yield of 1,4-BuD was 55% obtained at 32 h, which was significantly lower than the 90% over the co-catalyst of ReOx–Au/CeO2 and ReOx/C, due to the high selectivity to THF in the reaction. Because THF was formed from several pathways (DHF hydrogenation, DHF disproportionation, and 1,4-BuD dehydration), the improvement of the 1,4-BuD yield in the reaction of 1,4-AHERY over the mixture catalyst of ReOx–Au/CeO2 and ReOx/WO3–ZrO2 might be difficult. The TOFRe-total based on the conversion and total Re amount of the mixture of ReOx–Au/CeO2 + ReOx/WO3–ZrO2, which means the TOFRe-total of the first DODH step, is calculated to be 3 × 10 h−1 (conversion of 45% → 90% from 1 to 4 h, Fig. 4 and Table S2†). Although this value could have considerable error because of the limited data points and high conversion levels, this value is comparable to the TOFRe-total of CeO2 + ReOx/C (Re 2 wt%) in 1,4-AHERY reduction to 1,4-BuD (26 h−1)39 at the same temperature and H2 pressure. According to our previous works in DODH using ReOx/CeO2 + promoter catalysts, the DODH activity is controlled by the amount of monomeric ReOx species on CeO2, while Re species on C and other oxide supports are inactive in DODH.39 Therefore, the TOFRe-total value of mixture catalysts is reflected by the ratio of the Re amount on CeO2 to the total Re amount. It was reported that ReOx–Au/CeO2 (1 wt%) + ReOx/C (3 wt%) showed a much lower TOFRe-total (5 h−1) because the number of active Re sites on CeO2 for DODH is fixed by the rapid reduction of both Re species on CeO2 and C to insoluble species. In the case of CeO2 + ReOx/C, the Re species move from the C support to the CeO2 support before reduction to form a large number of active Re sites for DODH. The similar high TOFRe-total value of ReOx–Au/CeO2 + ReOx/WO3–ZrO2 to that of CeO2 + ReOx/C suggests that some of the Re species on WO3–ZrO2 moved to the CeO2 support. On the other hand, the decrease of the Re amount in ReOx/WO3–ZrO2 would lower the activity in 2,5-DHF conversion when it was mixed with ReOx–Au/CeO2.
Fig. 4 Time course of the reaction of 1,4-AHERY over ReOx–Au/CeO2 + ReOx/WO3–ZrO2. Reaction conditions: 1,4-AHERY = 0.3 g, ReOx–Au/CeO2 = 0.15 g (Re = 1 wt%, Au = 0.3 wt%), ReOx/WO3–ZrO2 = 0.15 g (Re = 1 wt%, WO3 = 10 wt%), 1,4-dioxane = 4 g, PH2 = 8 MPa, T = 413 K, t = 1–48 h. The detailed data are shown in Table S2, ESI.† AHERY: anhydroerythritol, BuD: butanediol, THF: tetrahydrofuran, DHF: dihydrofuran. |
Entry | Substrate | Catalyst | Conv./% (C. B./%) | Selectivity/% | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1,4-BuD | THF | 2,5-DHF | 2,3-DHF | 1-BuOH | GBL | Furan | 2-HTHF | Acetal A | Acetal B | Others | ||||
a Reaction conditions: 1,4-AHERY = 0.3 g, catalyst = 0.15 g (or 0.15 g + 0.15 g), 1,4-dioxane = 4 g, PH2 = 8 MPa, T = 413 K, t = 24 h. b Reported in ref. 23. c 1,4-AHERY = 0.5 g. d 2,5-DHF = 0.15 g, water 0.04 g, t = 4 h. e 2,3-DHF = 0.15 g, water 0.04 g, t = 4 h. f MgO = 0.15 g. C. B.: carbon balance; only described when it was clearly different from 100% (±10%).AHERY: anhydroerythritol, BuD: butanediol, THF: tetrahydrofuran, DHF: dihydrofuran, BuOH: butanol, GBL: γ-butyrolactone, acetal A: , acetal B: . | ||||||||||||||
1 | 1,4-AHERY | ReOx–Au/CeO2 + ReOx/WO3–ZrO2 | 100 | 53 | 35 | 3 | 0 | 2 | 1 | 0 | 0 | 0 | 1 | 6 |
2 | ReOx/CeO2 + ReOx/WO3–ZrO2 | 1 | 0 | 0 | 64 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 36 | |
3b,c | ReOx–Au/CeO2 | 64 | 0 | 1 | 89 | 5 | 0 | 0 | 0 | 1 | 0 | 0 | 3 | |
4 | ReOx/WO3–ZrO2 | 1 | 0 | 0 | 0 | 0 | 0 | 63 | 0 | 0 | 0 | 0 | 37 | |
5b,d | 2,5-DHF | ReOx–Au/CeO2 | 45 | 4 | 34 | — | 54 | 2 | 0 | 2 | 2 | 0 | 0 | 1 |
6d | ReOx/WO3–ZrO2 | 94 | 43 | 38 | — | 0 | 4 | 5 | 2 | 1 | 0 | 6 | 1 | |
7d | WO3–ZrO2 | 7 | 0 | 30 | — | 1 | 0 | 0 | 56 | 7 | 3 | 0 | 3 | |
8d | ZrO2 | 5 | 0 | 36 | — | 5 | 1 | 4 | 29 | 8 | 2 | 0 | 16 | |
9d | None | 2 | 0 | 38 | — | 19 | 0 | 0 | 26 | 0 | 11 | 0 | 6 | |
10b,e | 2,3-DHF | ReOx–Au/CeO2b | 49 (71) | 8 | 6 | 0 | — | 0 | 3 | 2 | 41 | 23 | 2 | 13 |
11e | ReOx/WO3–ZrO2 | 100 (20) | 32 | 6 | 0 | — | 0 | 15 | 0 | 3 | 0 | 15 | 30 | |
12e | WO3–ZrO2 | 98 (21) | 9 | 6 | 0 | — | 0 | 21 | 0 | 18 | 5 | 18 | 23 | |
13e | ZrO2 | 76 (62) | 4 | 0 | 0 | — | 0 | 12 | 0 | 30 | 18 | 16 | 19 | |
14e,f | MgO | 32 (79) | 0 | 0 | 3 | — | 0 | 5 | 0 | 64 | 23 | 0 | 6 | |
15e | None | 27 (80) | 0 | 0 | 0 | — | 0 | 0 | 4 | 76 | 10 | 0 | 10 |
In the conversion of 2,5-DHF, single WO3–ZrO2 (Table 2, entry 7) or ZrO2 (entry 8) without Re loading showed very low activity, and the main products were THF and furan, which were co-produced by disproportionation of DHFs. Generally, furan is easily hydrogenated to THF over a metal catalyst and H2; therefore, furan is an unfavorable by-product. The higher THF selectivity of ReOx/WO3–ZrO2 was due to the activity of the WO3–ZrO2 support in disproportionation of 2,5-DHF to some extent. However, the yield of THF over ReOx/WO3–ZrO2 (entry 6) was higher than that of THF + furan over WO3–ZrO2 (entry 8), suggesting that another route existed for THF formation such as hydrogenation of DHFs. The formation of 2,3-DHF and 1,4-BuD was negligible over the WO3–ZrO2 support without Re, indicating that the Re species have the function of catalyzing the conversion of 2,5-DHF to 2,3-DHF which was the first step in the production of 1,4-BuD from 2,5-DHF. Although ReOx–Au/CeO2 could catalyze a part of the conversion of 2,5-DHF to 2,3-DHF (entry 5), it is difficult to catalyze the hydration of 2,3-DHF to 2-HTHF. The total yield of 1,4-BuD and its precursors over ReOx/WO3–ZrO2 was higher than that over ReOx–Au/CeO2, indicating that the isomerization step of 2,5-DHF to 2,3-DHF in 1,4-AHERY conversion over ReOx–Au/CeO2 + ReOx/WO3–ZrO2 was mainly catalyzed by the Re species on the WO3–ZrO2 support.
The reaction of 2,3-DHF was investigated next (Table 2, entries 10–15). Similar to the reaction of 2,5-DHF, although ReOx–Au/CeO2 could partially catalyze the hydration of 2,3-DHF to 2-HTHF and the acetals (entry 10), the conversion activity was lower than that of ReOx/WO3–ZrO2 (entry 11) or WO3–ZrO2 (entry 12), suggesting that the hydration of 2,3-DHF was mainly catalyzed by WO3–ZrO2. Acidity is probably necessary in the hydration of 2,3-DHF to 2-HTHF and its derivatives. Basic supports such as MgO (entry 14) showed much lower conversion than non-basic supports like ZrO2 (entry 13) and WO3–ZrO2 (entry 12), and the results were similar to the case without a catalyst (entry 15). Furthermore, the addition of WO3 to ZrO2 increased the conversion probably due to the stronger acidity of WO3–ZrO2 than ZrO2. The carbon balance over ReOx/WO3–ZrO2, WO3–ZrO2, and ZrO2 (entries 11–13) was low, which might be due to the formation of polymers from 2,3-DHF as well as the evaporation of 2,3-DHF (boiling point: 328 K). A similar low carbon balance in the reaction of 2,3-DHF has been also reported over the ReOx/C catalyst.23 The low concentration of 2,3-DHF in the reaction of 1,4-AHERY and the presence of basic ReOx–Au/CeO2 can suppress the loss of carbon balance in the reaction of 1,4-AHERY over ReOx–Au/CeO2 + ReOx/WO3–ZrO2. The selectivity to 1,4-BuD was dramatically increased when the Re species were loaded on WO3–ZrO2. This indicates that the hydrogenation of 2-HTHF-derived products to 1,4-BuD was mainly catalyzed by the Re species on WO3–ZrO2, although the W species in WO3–ZrO2 have some activity in hydrogenation (or transfer hydrogenation) of 2-HTHF-derived products to 1,4-BuD.
Based on these results, we propose a reaction mechanism of 1,4-AHERY to 1,4-BuD over the combination of ReOx–Au/CeO2 and ReOx/WO3–ZrO2 (Scheme 2) similar to the one we proposed for the mixture of ReOx–Au/CeO2 and ReOx/C.23,39 It is composed of ReOx–Au/CeO2-catalyzed DODH of 1,4-AHERY to 2,5-DHF and ReOx/WO3–ZrO2-catalyzed reduction of 2,5-DHF to 1,4-BuD. In the conversion of 2,5-DHF, the Re species of ReOx/WO3–ZrO2 firstly catalyzed the isomerization of 2,5-DHF to 2,3-DHF, and then WO3–ZrO2 catalyzed the hydration of 2,3-DHF to 2-HTHF-derivatives; finally the Re species on ReOx/WO3–ZrO2 catalyzed the hydrogenation of the 2-HTHF-derived intermediates to 1,4-BuD.
Entry | Calcination conditions | Usage times | Conv./% | Selectivity/% | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1,4-BuD | THF | GBL | 1-BuOH | 2,5-DHF | 2,3-DHF | Acetal A | Acetal B | Acetal C | Others | ||||
a Reaction conditions: 1,4-AHERY = 0.3 g, ReOx–Au/CeO2 = 0.15 g (Re = 1 wt%, Au = 0.3 wt%), ReOx/WO3–ZrO2 = 0.15 g (Re = 1 wt%, WO3 = 10 wt%), 1,4-dioxane = 4 g, PH2 = 8 MPa, T = 413 K, t = 24 h. b Recycled ReOx–Au/CeO2 + ReOx/WO3–ZrO2 mixture = 0.3 g. c Recycled ReOx–Au/CeO2 + ReOx/WO3–ZrO2 mixture = 0.24 g, and fresh ReOx–Au/CeO2 = 0.06 g (Re = 1 wt%, Au = 0.3 wt%) was added. d Recycled ReOx–Au/CeO2 + ReOx/WO3–ZrO2 mixture = 0.24 g, and fresh ReOx/WO3–ZrO2 = 0.06 g (Re = 1 wt%, WO3 = 10 wt%) was added. AHERY: anhydroerythritol, BuD: butanediol, THF: tetrahydrofuran, DHF: dihydrofuran, BuOH: butanol, GBL: γ-butyrolactone, HTHF: hydroxytetrahydrofuran, acetal A: , acetal B: , acetal C: . | |||||||||||||
1 | — | 1 | 100 | 53 | 31 | 1 | 2 | 5 | 0 | 0 | 1 | 6 | 0 |
2b | None | 2 | 97 | 25 | 23 | 4 | 1 | 12 | 2 | 0 | 20 | 8 | 5 |
3b | 573 K, 1 h | 2 | 100 | 46 | 38 | 0 | 2 | 1 | 0 | 0 | 1 | 11 | 1 |
4b | 573 K, 1 h | 3 | 84 | 35 | 44 | 0 | 2 | 2 | 0 | 0 | 1 | 13 | 3 |
5b | 573 K, 3 h | 2 | 100 | 49 | 39 | 0 | 2 | 5 | 0 | 0 | 0 | 4 | 1 |
6b | 573 K, 3 h | 3 | 100 | 43 | 34 | 1 | 2 | 13 | 1 | 0 | 3 | 2 | 1 |
7b | 773 K, 3 h | 2 | 80 | 38 | 41 | 0 | 2 | 1 | 1 | 0 | 1 | 14 | 2 |
8b | 773 K, 3 h | 3 | 68 | 26 | 18 | 6 | 1 | 26 | 1 | 0 | 8 | 6 | 9 |
9c | None | 2 | 100 | 51 | 35 | 1 | 2 | 0 | 0 | 0 | 1 | 10 | 1 |
10d | None | 2 | 68 | 32 | 36 | 0 | 2 | 2 | 0 | 0 | 3 | 23 | 2 |
Fig. 5 Re L3-edge XANES spectra and the Re valence calculated using the white line area. (a) Re powder, (b) ReO2, (c) ReO3, (d) Re2O7, and (e) ReOx/WO3–ZrO2 after the reaction (reaction conditions: 2,5-DHF 0.15 g, water 0.03 g, catalyst 0.15 g, 1,4-dioxane 4 g, H2 8 MPa, 413 K, 4 h). The relationship between the white line area and Re valence is shown in Table S4 and Fig. S4, ESI.† |
The H2-TPR profiles of WO3–ZrO2 and ReOx/WO3–ZrO2 are shown in Fig. 6, and Table 4 summarizes the H2 consumption amount in H2-TPR. WO3–ZrO2 showed a broad signal at 623–1000 K, which can be assigned to the reduction of WO3 in WO3–ZrO2. The H2 consumption of this broad signal was 0.0082 mmol (50 mg sample), which corresponded to the valence change of W by 0.74. On the other hand, ReOx/WO3–ZrO2 had strong two bands in the range 500–723 K and a broad band up to 923 K. The broad reduction signal of W was shifted from that in the case without ReOx to lower temperature, probably by the supply of hydrogen species activated on the reduced Re site. We assume that the band for W reduction with the same H2 consumption amount (0.0082 mmol) as WO3–ZrO2 was overlapped with Re reduction bands. With this assumption, the H2 consumption for W reduction in ReOx/WO3–ZrO2 between 500 and 723 K was estimated to be 0.0032 mmol (= 0.0082–0.0050 mmol), and the H2 consumption amount for Re reduction was calculated to be 0.0052 mmol (= 0.084–0.0032 mmol). This consumption amount corresponds to the Re valence change from +7 to +3.1, which agreed well with the XANES results. Although the temperature range of this signal (500–723 K) was higher than the reaction temperature (413 K), the reduction of Re species could take place in the reaction temperature due to the higher H2 pressure.45 Considering the higher temperature range and small H2 consumption amount for the reduction of W, only a small portion of W species in ReOx/WO3–ZrO2 was probably reduced to the +5 valence state during the catalysis, while most of the W species retained the +6 valence state.
Fig. 6 H2-TPR profiles of (a) WO3–ZrO2 (WO3 = 10 wt%) and (b) ReOx/WO3–ZrO2 (Re = 1 wt%, WO3 = 10 wt%). Measurement conditions: sample weight 50 mg, H2/Ar = 5/95, 10 K min−1. The H2 consumption amount is summarized in Table 4. |
Sample | Sample weight/mg | Re amount/mmol | W amount/mmol | H2 consumption amount/mmol (temperature range/K) |
---|---|---|---|---|
(a) WO3–ZrO2 | 50 | — | 0.022 | 0.0082 (500–1000) |
(b) ReOx/WO3–ZrO2 | 50 | 0.0027 | 0.022 | 0.0084 (500–723) |
0.0050 (723–923) |
An NH3-TPD experiment was carried out to determine the acidity of catalysts. The NH3-TPD profiles of WO3–ZrO2, carbon and Re catalysts on these supports are shown in Fig. S5, ESI.† The desorbed NH3 amount from WO3–ZrO2 was larger than that from the carbon support, indicating that the acidity of WO3–ZrO2 is stronger than that of carbon. However, the performance of ReOx/C combined with ReOx–Au/CeO2 in the reaction of 1,4-AHERY is much higher than that of ReOx/WO3–ZrO2. As mentioned above, while acidity is probably necessary in the hydration of 2,3-DHF to 2-HTHF-derivatives, it might not be a significant factor on the overall reaction of 1,4-AHERY to 1,4-BuD since DODH of 1,4-AHERY and 2,5-DHF isomerization to 2,3-DHF are slower steps than hydration of 2,3-DHF. In addition, mixing a basic material (CeO2 in ReOx–Au/CeO2) with ReOx/WO3–ZrO2 in the reaction media can affect the acidity. Also considering the low carbon balance in the reaction tests of 2,3-DHF (Table 2), we do not give a decisive conclusion for the acidity–performance relationship from these NH3-TPR data.
The XRD patterns of the tungsten–zirconia-supported Re catalysts are shown in Fig. 7. The Re species did not show peaks due to the low loading amount and/or low crystallinity on the support surface for all the catalysts. The catalyst with commercial WO3–ZrO2 which has been mainly used as a support in this study (Fig. 7(b)) had mainly a monoclinic structure of ZrO2 (characteristic peaks at around 28 and 32°) and a small amount of tetragonal ZrO2 structure (characteristic peak at 30°). In order to determine which phase is more active, the crystalline structures of the series of catalysts with home-made supports were determined and the catalytic performances were correlated with the structures. The commercial ZrO2 had a monoclinic crystalline structure (Fig. 7(a)). WO3/ZrO2 prepared by impregnation of ZrO2 showed also a monoclinic ZrO2 structure while there were small peaks of WO3 when the loading of W was high (≥10 wt%) (Fig. 7(c–e)). The ZrO2 of cpWO3–ZrO2 prepared by the co-precipitation method formed a tetragonal crystalline structure (t-ZrO2) (Fig. 7(f)), as reported in the literature.43 As shown in Fig. 1, all the ReOx/tungsten–zirconia catalysts except ReOx/WO3/ZrO2 with high W loading (≥10 wt%) showed activity in 2,5-DHF reduction to 1,4-BuD. The activity difference can be simply explained by the surface concentration of W species, because WO3–ZrO2 prepared by impregnation should have a higher surface concentration of W than cpWO3–ZrO2 prepared by co-precipitation with a similar surface area (actual surface area of supports: 62 and 68 m2 g−1, respectively) and the same W amount (5 wt%). The performance of ReOx/cpWO3–ZrO2 corresponded to that of ReOx/WO3/ZrO2 with 2–3 wt% W loading. Therefore, the crystalline structure of ZrO2 does not have a critical role in the catalysts. On the other hand, ReOx/WO3/ZrO2 with high W loading had very low activity (Fig. 2). The presence of WO3 crystallites seemed to deactivate the Re species. The similar crystal structure of WO3 and ReO3 (partially reduced rhenium oxide) might be related to the low activity, by incorporation of Re species into the WO3 crystal. In the literature, WO3 species on ZrO2 with lower loading can form monolayer or sub-monolayer species composed of a two-dimensional plane of corner-shared WO6 octahedra,46 and the loading amount per surface area (5 wt% on 62 m2 g−1 support; 2.7 W atoms per nm2) was slightly lower than the monolayer coverage (5 W atoms per nm2).47 The similar catalytic performance of ReOx/WO3–ZrO2 (commercial) to ReOx/WO3/ZrO2 (5 wt% W) and similar W loading amount per surface area (7.9 wt% in 103 m2 g−1 WO3–ZrO2) suggest that the good catalytic performance of ReOx/WO3–ZrO2 is also derived from surface submonolayer WO3 on ZrO2. Also considering the similar structure of WO3 and ReO3, Re species during reduction can spread over the WO3 (sub)monolayer and high dispersion of Re species can be obtained. Considering that both 2,5-DHF isomerization and 4-hydroxybutanal hydrogenation, which were catalyzed by the Re species in ReOx/WO3–ZrO2, involve hydrogen species, the highly dispersed Re0 species on the WO3–ZrO2 support can be the active sites for these steps.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0re00085j |
‡ Current address: Research Center for Artificial Photosynthesis, The Advanced Research Institute for Natural Science and Technology, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi, Osaka, 558-8585, Japan. |
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