Selective Dehydration of 1,2-Propanediol to Propanal over Boron

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Selective Dehydration of 1,2-Propanediol to Propanal over Boron Phosphate Catalyst in the Presence of Steam Ryoichi Otomo, Chiaki Yamaguchi, Daiki Iwaisako, Shun Oyamada, and Yuichi Kamiya ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04594 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Sustainable Chemistry & Engineering

　

Selective Dehydration of 1,2-Propanediol to Propanal over Boron Phosphate Catalyst in the Presence of Steam

Ryoichi Otomo,†,* Chiaki Yamaguchi,‡ Daiki Iwaisako, ‡ Shun Oyamada, ‡ and Yuichi Kamiya†

† Faculty of Environmental Earth Science, ‡ Graduate School of Environmental Science, Hokkaido University, Nishi 5 Kita 10, Kita-ku, Sapporo 060-0810, Japan.

*Corresponding author: Dr. Ryoichi Otomo

Tel: +81-11-706-2259, E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract Catalytic properties of metal phosphates were investigated for gas-phase dehydration of 1,2-propanediol to propanal in the presence and absence of steam in the temperature range of 150 – 340 °C. Boron, aluminum, and nickel phosphates showed promising catalytic activity for the dehydration reaction. Especially, boron phosphate showed outstanding catalytic activity at a low reaction temperature and high selectivity to propanal without formation of competitive dehydration products such as acetone and allyl alcohol. The catalytic activity of boron phosphate was remarkably enhanced in the presence of steam co-fed with 1,2-propanediol. The additional steam was also favorable for promoting hydrolysis of dioxolane, which is a by-product formed through acetalization of propanal, resulting in the high yield of propanal over 95%. Boron phosphate showed more durable catalytic activity and much higher yield of propanal than conventional solid acid catalysts such as ZSM-5, silica-alumina and niobium oxide that have been reported to be active for the dehydration of 1,2-propanediol.

Keywords 1,2-Propanediol, Propanal, Boron Phosphate, Dehydration, Steam, Biomass


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Introduction Plant biomass has attracted much interest as renewable carbon resource both in academics and industries due to the global concern on diminishing reserves of crude oil with a rising price in the future. For the purpose of utilizing lignocellulose, which is the most abundant form of plant biomass in nature, it is essential to convert cellulose into value-added chemicals. The one-pot conversion of cellulosic materials into polyols has been accomplished through hydrogenolysis of cellulose or glucose in efficient and convenient manners.1-4 Furthermore, hydrogenolysis of glycerol, which is a by-product in bio-diesel production, produces lower polyols.5,6 Since propanediols are often main products in the hydrogenolysis reactions,7-10 the production of useful chemicals from propanediols is required toward the utilization of plant biomass as carbon resource. Gas-phase dehydration of 1,2-propanediol to propanal has been studied using various types of heterogeneous catalysts,11-17 because, owing to a reactive formyl group, propanal is a potential intermediate material to obtain bio-based value-added chemicals. In the reaction of 1,2-propanediol, the formation of a variety of by-products such as acetone, allyl alcohol and dioxolane were inevitable but should be suppressed (Scheme 1). Sato et al. reported that silicotungstic acid and tungsten oxide supported on silica showed very high selectivity over 90% to propanal, but the formation of acetone and allyl alcohol became pronounced particularly at high temperatures.11,15 Chadwick et al. investigated the catalytic performance of several zeolites for the dehydration of 1,2-propanediol and found that ZSM-23 and ZSM-5 showed high selectivity to propanal, but a small amount of acetone and allyl alcohol was also produced.12 Furthermore, these zeolites were 3 ACS Paragon Plus Environment


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quickly deactivated in just several hours. Xu et al. reported that Brønsted acid catalysts such as ZSM-5 and Nb2O5 with suitable acid strength (-8.2 < H0 ≤ -3.0) showed high selectivity to propanal (~70%) and that too strong acidity resulted in the formation of by-products and missing carbon balance.14 In this way, various types of solid acid catalysts have been investigated for the dehydration of 1,2-propanediol and successful results have been achieved. However, the development of a novel catalyst that is highly active and selective at low reaction temperature is still a key to the efficient production of propanal from 1,2-propanediol.

Scheme 1 Reaction pathways in dehydration of 1,2-propanediol.


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Metal phosphate is a class of heterogeneous catalysts that are active for various types of acid-base and redox reactions. Because of a wide variety of structures and compositions, metal phosphates with tunable acid properties have been applied as catalysts for dehydration reactions.18-20 It was reported that metal phosphates were highly active for the dehydration of 2-propanol and that their catalytic activity strongly depended on the kind of metal componenets; Boron and iron phosphates showed high activity for the dehydration of 2-propanol to propene.18,21 Considering that the formation of propanal from 1,2-propanediol would follow the reaction similar to 2-propanol, though there is the difference in whether the reactant has a primary hydroxyl group or not, we intended to study the dehydration of 1,2-propanediol by using metal phosphate catalysts. In this study, we investigated the catalytic properties of metal phosphates for selective dehydration of 1,2-propanediol to propanal. Among the metal phosphates, boron phosphate showed high catalytic activity for the reaction. In addition, we found that the additional co-fed steam significantly enhanced catalytic activity of boron phosphate, leading to the selective formation of propanal in high yield over 95%.

Experimental Preparation of Catalyst Samples. Boron phosphate was prepared by a evaporation method.22 Aluminum, iron, cobalt, nickel zinc, and lanthanum phosphates were prepared by a precipitation method.23-28 The detailed procedure for the preparation is shown in Supporting Information. Metal phosphate samples were 5 ACS Paragon Plus Environment


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designated as MP, where M means a kind of metal element. As a comparison, the conventional solid acids including ZSM-5, SiO2-Al2O3, Nb2O5, and Al2O3 were also tested for the reaction. All the catalysts except for ZSM-5 were calcined at 350 °C for 3 h in air prior to use. ZSM-5 was calcined at 550 °C for 3 h in air.

Characterization of Catalyst Samples. Elemental analysis of the samples was performed by ICP-AES using a Shimadzu ICPE-9000 analyzer. Powder X-ray diffraction (XRD) patterns of the samples in the range of 10 – 80° were collected on a Rigaku MiniFlex diffractometer using a Cu Kα radiation (30 kV, 15 mA). Nitrogen adsorption isotherms were measured using a BEL-mini analyzer (MicrotracBEL) at -196 °C and Brunauer–Emmett–Teller (BET) surface area was calculated in the P/P0 range of 0.1 – 0.3. Prior to the measurements, powder samples were pretreated at 350 °C for 1 h in nitrogen flow. Acid properties of catalyst samples were examined by the n-butylamine titration method with several Hammett indicators, following the procedure reported elsewhere.29,30 The number of acid sites estimated by this method had difference of 0.02 mmol/g, when a titer of a sample was smaller than 0.2 mmol/g. When it was larger than 0.2 mmol/g, the number of acid sites was bracketed to 0.05 mmol/g.

Catalytic Dehydration of 1,2-Propanediol. Gas-phase dehydration of 1,2-propanediol was conducted at 150 – 340 °C under 6 ACS Paragon Plus Environment

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atmospheric pressure using a vertical fixed-bed flow reactor. The catalysts were pretreated at 350 °C for 1 h in H2 flow (60 mL/min). Pure 1,2-propanediol or its aqueous solution was fed by a calibrated syringe pump and mixed with H2. Since the use of H2 as carrier gas was reported to be effective for retarding deactivation of catalysts,13,15 H2 was used as carrier gas in this study. Unreacted 1,2-propanediol and products in the effluent gas were collected with ethyl acetate solvent in an ice bath and the obtained solution was analyzed on a GC-FID using a SHIMADZU GC-2025 equipped with a SHIMADZU SH-Rtx-Wax capillary column (30 m × 0.25 mm × 0.5 μm). Products were identified on the basis of mass fragmentation profiles using a gas chromatograph-mass spectrometer (SHIMADZU GCMS-QP2010SE). 1,2-Propanediol and main products including propanal, acetone, allyl alcohol, 2-ethyl-4-methyl-1,3-dioxolane, and 1-propanol were quantified by using drawn calibration curves. 1,2-Propanediol, propanal, and 2-ethyl-4-methyl-1,3-dioxolane are designated as 1,2-PDO, PAL, and DXO, respectively. The sum of other C3 by-products are represented as “Other C3”.

Results and Discussion Characterization of Metal Phosphates. Structure and Composition. Judging from their XRD patterns, BP, ZnP, and LaP had crystalline structures of BPO4, Zn3(PO4)2·2H2O, and LaPO4·0.5H2O, respectively (Figure S1). AlP, FeP, CoP, and NiP showed only a halo peak, indicating that these samples were amorphous. Compositional and textural 7 ACS Paragon Plus Environment


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properties of the samples are shown in Tables 1 and S1. BP and ZnP had M/P ratios almost the same as the stoichiometric ones (1.0 and 1.5, respectively), expected from each crystalline structure. LaP had a slightly phosphorous-rich composition, implying the presence of phosphorous species outside the LaPO4 crystallite. AlP, FeP, CoP and NiP, which were amorphous materials, had phosphorous-rich compositions, probably because these samples were precipitated from weakly acidic mother solution and a part of metal cation was likely to remain in the solution. BP, ZnP, and LaP with the crystalline structures had relatively small surface area, while amorphous AlP, FeP, and NiP had large surface area.

Table 1 Compositional, textural and acid properties of catalyst samples. Sample

M/P a (-)

SBET b (m2/g)

Acid site (mmol/g) H0 ≤ -5.6

-5.6 ≤ H0 ≤ -3.0

-3.0 ≤ H0 ≤ +3.3

Total

BP AlP FeP NiP LaP

0.96 0.84 0.84 0.99 0.88

18 205 104 80 57

0.04 0.45 0.15 0.02 0.02

0.06 0.25 0.20 0.13 0.13

0.15 0.20 0.25 0.50 0.40

0.25 0.90 0.60 0.65 0.55

ZSM-5 SiO2-Al2O3 Nb2O5

-

475 654 102

0.20 0.40 0.25

0.25 0.10 0.10

0.05 0.10 0.10

0.50 0.65 0.45

a

Metal/phosphorous atomic ratio determined by ICP-AES. b BET specific surface area.

Acid Properties. Table 1 shows acid properties of metal phosphate samples examined by titration method with Hammett indicators. The acid sites was classified into three groups according to the Hammett


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functions (H0). BP had a small number of total acid sites (0.25 mmol/g), a majority of which was in a class of weak acid sites with -3.0 ≤ H0 ≤ +3.3. In contrast, AlP had a large number of total acid sites and strong acid sites with H0 ≤ -5.6 accounted for a half of them. The other samples had similar numbers of acid sites (~0.60 mmol/g). For FeP, the numbers of strong, medium, and weak acid sites were almost equal each other. NiP and LaP had mainly weak acid sites but only a negligible number of strong acid sites was present.

Catalytic Properties of Metal Phosphates. Catalytic Properties in the Absence of Co-Fed Steam. Catalytic properties of metal phosphate samples were investigated in the dehydration of 1,2-PDO using pure 1,2-PDO solely as feed (Figure 1). The dehydration reactions were conducted with the reaction temperature varied in a stepwise manner from 150 to 340 °C, at each of which, reaction products were collected and analyzed (Scheme S1). None of metal phosphate catalysts showed any catalytic activity at 150 °C. BP gave 41% conversion at 200 °C, where a predominant product was DXO, which is formed by acetalization of PAL with 1,2-PDO.11,12 Upon increasing the reaction temperature to 220 °C, BP showed the complete conversion of 1,2-PDO and gave 65% yield of PAL. Further increase in the reaction temperature resulted in the gradual increase of PAL yield. Finally, the yield of PAL reached 95% at 300 °C. For the other metal phosphate catalysts, catalytic activity first appeared at 240 or 260 °C, implying the high catalytic activity of BP. 9 ACS Paragon Plus Environment


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Figure 1 Dehydration of 1,2-propanediol over metal phosphate catalysts in the absence of co-fed steam at different temperatures: (a) conversion of 1,2-propanediol, (b) yield of propanal, (c) yield of dioxolane, and (d) yield of other C3 products. Reaction conditions: catalyst, 0.2 g; 1,2-propanediol, 0.12 mmol/min; total flow rate (H2 balance), 60 mL/min; W/F, 27 g·h/mol; temperature, 150 – 340 °C; time, 0.5 h.

AlP gave high conversion and high yield of PAL at 280 °C or higher temperatures. For NiP, the yield of PAL sharply increased above 280 °C and 94% yield of PAL was attained at 320 °C. It was commonly observed among BP, AlP, FeP, and NiP that the increase in the reaction temperature resulted in the increase of PAL yield with the concurrent decrease in DXO yield. This behavior indicates that DXO was transformed to PAL through subsequent hydrolysis (Scheme 1). LaP showed gradual increase in both conversion of 1,2-PDO and yield of PAL. CoP and ZnP showed poor catalytic activity (Figure S2). 10 ACS Paragon Plus Environment

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Over LaP, the formation of Other C3 including acetone and allyl alcohol was significant, lowering the yield of PAL; the yields of acetone and allyl alcohol were 10% and 15%, respectively at 320 °C. For the other metal phosphate catalysts, the formation of acetone and allyl alcohol was pronounced at high temperatures, where the total yield of Other C3 was approximately 10%. It is noteworthy that BP gave only negligible Other C3 even at high temperatures; the yield of acetone was only 1.5% at 340 °C and the formation of allyl alcohol was under the detection limit. There are several reaction pathways to form by-products (Scheme 1); allyl alcohol and acetone are formed through parallel dehydration reactions, while DXO is produced through the subsequent acetalization of PAL with 1,2-PDO. The formation of such by-products is a cause of lowering the yield of PAL. Although BP as well as the conventional catalysts gave DXO in a moderate yield,11,12,13,15,17 DXO can be converted back to PAL and 1,2-PDO, as shown above. Hence, it is more important to block the irreversible dehydration pathways to allyl alcohol and acetone for achieving a high yield of PAL. BP, which gave a negligible yield of such by-products (below 1%) at low temperature, has an advantage over the conventional catalysts, leading to the high yield of propanal over 95%. The large number of acid sites mainly composed of strong acid sites can account for the high catalytic performance of AlP. FeP, NiP and LaP having the moderate number of acid sites but smaller number of strong acid sites showed lower catalytic activity. Obviously, the catalytic activity of these metal phosphates was governed by their acid properties. However, BP exceptionally exhibited the high catalytic performance despite its small surface area and a small number of acid 11 ACS Paragon Plus Environment


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sites. We assume that precious few particular active sites on BP should have a great contribution to the high catalytic activity.

Catalytic Properties in the Presence of Co-Fed Steam. Since 1,2-PDO is produced through hydrogenolysis of saccharides and glycerol in aqueous solvent, crude 1,2-PDO solution contains plenty of water. In this context, aqueous solution of 1,2-PDO has been used as the feed instead of pure 1,2-PDO and gas-phase catalytic reactions were done in the presence of steam.11,12,14-16 In addition, since steam is a convenient and inexpensive diluent in an industrial process, use of aqueous solution of 1,2-PDO as feed has a great merit in terms of feasibility. Thus, we investigated the influence of the additional steam on the catalytic performance of BP, AlP, and FeP, which showed high catalytic activity in the absence of additional steam (Figure 2). The solution of 1,2-PDO with different water contents was used to co-feed steam (10 and 20 vol.%). The feed rate of 1,2-PDO and the total volume of the gas flow were kept constant by changing the flow rate of H2. The reaction runs were done at different temperatures with respect to each catalyst for adjustment of the conversion of 1,2-PDO to 20 – 40% without addition of steam. The influence of the steam differed among these phosphate catalysts. The reaction results did not change for AlP. In contrast, for BP and FeP, the addition of steam increased the conversion of 1,2-PDO and the yield of PAL. Especially, BP was highly sensitive to steam and its activity was significantly enhanced by steam at 200 °C. 12 ACS Paragon Plus Environment

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Figure 2 Effect of co-fed steam on dehydration of 1,2-propanediol over metal phosphate catalysts: (a) conversion of 1,2-propanediol, (b) yield of propanal, and (c) yield of dioxolane. Reaction conditions: catalyst, 0.2 g; 1,2-propanediol, 0.12 mmol/min; H2O, 0, 0.24, or 0.48 mmol/min; total flow rate (H2 balance), 60 mL/min; W/F, 27 g·h/mol; temperature, 200, 240, and 260 °C for boron, aluminum, and iron phosphates, respectively; time, 0.5 h.

The difference in the response to steam among the catalysts implies that the co-fed steam altered the acid properties on BP and FeP, leading to the enhanced catalytic activity. It has been reported that adsorbed water on boron phosphate hydrolyzes P-O-B bonds to form P-OH and B-OH on the surface and that these groups are decreased through reversible condensation by heating.31 For other metal phosphates, by contact with steam, P-O-M bonds on the metal phosphates are transformed to P-OH and M-OH.32 The formed P-OH can act as Brønsted acid sites toward pyridine confirmed by IR observation and are catalytically active sites for a condensation reaction in the presence of steam.32 Thus, similar structural changes are possible for BP and FeP during the dehydration of 1,2-PDO in the presence of steam, leading to the increase in the number of active sites and enhancement of the catalytic activity. Because boron phosphates with B-rich compositions have been reported to behave as 13 ACS Paragon Plus Environment


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Lewis acid catalysts,33-35 we prepared boron phosphate having a B-rich composition (B/P = 1.2) and compared the catalytic performance with that of BP in the absence of co-fed steam (Figure S3). The B-rich boron phosphate showed less catalytic activity than BP. The result supported the above presumption that BP can act as a Brønsted acid catalyst for the dehydration. Improvement of the reaction results by the addition of steam has been reported for other types of catalysts.11,12,15 Sato et al. reported the improvement of the reaction results by diluting 1,2-PDO feed with water for silicotungstic acid and supported tungsten oxide catalysts.11,15 For zeolite catalysts, Chadwick et al. reported the elongation of catalyst life by coexisting steam due to the suppression of coke deposition.12 Considering that crude 1,2-PDO with a large amount of water should be processed, it is desirable for potential catalysts to be highly tolerant against coexisting steam. All of three phosphates showed enhanced or constant catalytic activity for the dehydration of 1,2-PDO in the presence of co-fed steam. Water-compatible acid properties of metal phosphates are favorable for processing 1,2-PDO in a feasible process. Thus, BP is a potentially promising material as a catalyst that could convert crude 1,2-PDO derived from real biomass.

Catalytic Properties of Boron Phosphate under Different Reaction Conditions. The effect of co-fed steam on product distribution was investigated by changing contact time (W/F) in the absence and presence of co-fed steam, where the mass of BP catalyst was varied at a constant rate of 1,2-PDO feed. The reaction results at different contact times are shown in Figure 3. Conversion of 1,2-PDO was increased along with the W/F both in the absence and 14 ACS Paragon Plus Environment

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presence of co-fed steam. The co-fed steam was effective for enhancing the catalytic activity in the whole W/F range.

Figure 3 (a) Conversion of 1,2-propanediol, and (b) product distribution in dehydration of 1,2-propanediol over boron phosphate at different contact times in the absence (blank) and presence (filled) of co-fed steam. Reaction conditions: catalyst, 0.1 – 0.6 g; 1,2-propanediol, 0.12 mmol/min; H2O, 0 or 0.48 mmol/min; total flow rate (H2 balance), 60 mL/min; W/F, 9 – 54 g·h/mol; temperature, 200 °C; time, 0.5 h.



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In the absence of co-fed steam, DXO was predominantly produced and PAL was produced only in very low yield. In the presence of co-fed steam, the yield of DXO was increased in low and moderate conversion region, passing through the maximum, and then decreased at high conversion along with steep increase in the yield of PAL. This behavior suggests a subsequent reaction mechanism for the formation of PAL through DXO. The yield of PAL was higher in the presence of the steam at any conversion, while the yield of DXO was higher in the absence. The difference in product selectivity was determined by the promotional effect of steam on the hydrolysis of DXO to PAL. In separate experiments with 2,2,4-trimethyl-1,3-dioxolane, the promotional effect of co-fed steam on the hydrolysis was confirmed (Figure S4). Additionally, the co-fed steam would possibly disturb the contact of PAL with 1,2-PDO, leading to suppression the formation of DXO. The co-feeding steam with 1,2-PDO was favorable both for enhancing the catalytic activity and for promoting the hydrolysis of DXO to PAL over BP. The spent BP samples commonly showed diffraction patterns similar to the fresh one regardless of additional co-fed steam, confirming no change in the bulk structure by co-fed steam (Figure S5). IR spectroscopy with pyridine as a probe molecule demonstrated that quite a small amount of pyridine was adsorbed on the fresh BP and that the adsorbed pyridine was readily removed by simply flowing nitrogen even at 30 °C (not shown), indicating that BP had only a small amount of acid sites and their acid strength was very weak. To gain information on difference in the surface structure, the fresh and spent BP samples were characterized by X-ray photoelectron 16 ACS Paragon Plus Environment

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spectroscopy (Figure S6). Nevertheless, these samples showed similar spectra. Unfortunately, we do not yet have clear evidence for the change in surface structure and acid properties of BP caused by the presence of steam. The difficulty in characterizing our BP sample is probably due to its small surface area which limits a number of surface functional groups and the amount of probe molecules adsorbed. For the purpose of estimating the amount of water adsorbed and dissociated on the surface during the reaction, the spent BP samples were characterized by TG-DTA (Figure S7). If a large number of functional groups like B-OH and P-OH is present on a sample, weight loss due to the condensation of such groups releasing water would be observed. The spent BP sample used in the absence of steam showed 2.2% weight loss from 100 to 800 °C, while the weight loss for the fresh sample was almost the same (2.0%). The sample used in the presence of steam showed slightly larger weight loss (3.3%) in the temperature range. As a control, the fresh BP was treated in 20 vol.% H2O/H2 stream at 200 °C without feeding 1,2-PDO. The treated sample showed only 1.8% weight loss. Hence, the larger weight loss after the use in the presence of steam cannot be definitely assigned to the loss of water released by the condensation of hydroxyl groups. Still, we speculate that a limited number of particular active sites on BP can greatly enhance the high catalytic activity. Additional works on synthesis of boron phosphate with high surface area as well as in-situ or operando characterization are ongoing for resolving the true structure of active sites. We will report them in the future. The dehydration of 1,2-PDO was compared in the presence and absence of co-fed steam at 17 ACS Paragon Plus Environment


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different temperatures (Figure 4). The conversion of 1,2-PDO was always 100%, except for the reaction at 200 °C in the absence of the co-fed steam. At 220 – 300 °C in the presence of co-fed steam, the yield of PAL was over 90% and higher than that in the absence of steam. At higher temperatures, the yield of PAL was decreased regardless of the additional co-fed steam because of deposition of organic compounds on the catalyst and as well possibly because of change of the surface structure with water released, as observed in the weight loss above 300 °C for the fresh BP (Figure S7). These reaction results achieved using BP more or less exceeded those so far reported on other types of catalysts (Table S2). The high selectivity to PAL for BP was attributed to the hydrolysis of DXO and the suppression of Other C3.

Figure 4 Dehydration of 1,2-propanediol over boron phosphate at different temperatures in the absence (blank) and presence (filled) of co-fed steam. Reaction conditions: catalyst, 0.2 g; 1,2-propanediol, 0.12 mmol/min; H2O, 0 or 0.48 mmol/min; total flow rate (H2 balance), 60 mL/min; W/F, 27 g·h/mol; temperature, 200 – 340 °C; time, 0.5 h.


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Comparison of Boron Phosphate with Conventional Catalysts. Catalytic performance of BP was compared with those of the conventional catalysts that have been reported to be highly active and selective for propanal (Figure 5).14 The reactions were conducted at 220 °C in the presence of co-fed steam. BP constantly gave 100% conversion of 1,2-PDO throughout 10 h. Initially the yield of PAL was 95%, but thereafter gradually decreased and became constant at ~70%. The deactivation of BP was probably due to deposition of organic compounds on the catalyst, as observed in weight loss with exothermic peaks in TG-DTA profile of the spent catalyst (Figure S8).

Figure 5 Time course of (a) conversion of 1,2-propanediol, and (b) yield of propanal in dehydration of 1,2-propanediol over boron phosphate and conventional acid catalysts in the presence of co-fed steam. Reaction conditions: catalyst, 0.2 g; 1,2-propanediol, 0.12 mmol/min; H2O, 0.48 mmol/min; total flow rate (H2 balance), 60 mL/min; W/F, 27 g·h/mol; temperature, 220 °C.



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ZSM-5 gave ~100% conversion and 81% yield of PAL at 0.5 h on stream. However, ZSM-5 severely deactivated and the yield of PAL decreased to 25% at 10 h. SiO2-Al2O3 initially attained ~70% conversion of 1,2-PDO and gave 20% and 40% yield of PAL and DXO, respectively. Other C3 such as acetone and allyl alcohol were also produced in 5 – 10% selectivity all the time on stream. SiO2-Al2O3 deactivated throughout the reaction and at 5 h the yield of PAL and DXO was only 10% and 24%, respectively. Nb2O5 showed poor catalytic activity, resulting in low conversion of 1,2-PDO and low yield of PAL (~10%). Al2O3 did not show any catalytic activity. Obviously, BP showed higher and more durable activity with higher selectivity to PAL than the conventional catalysts. Catalytic performance was compared among BP, ZSM-5, and SiO2-Al2O3 in the absence of co-fed steam (Figure S9). All of three catalysts showed low conversion and low yield of propanal initially at 0.5 h, compared to the presence of the steam. BP and ZSM-5 showed rapid decreased in the conversion and yield of products due to severe deactivation of the catalysts. As has been reported on zeolite catalysts,12 the co-fed steam was also effective for suppressing the deactivation of BP. The catalytic activity of the conventional catalysts was highly dependent of the nature of acid sites; ZSM-5 with strong Brønsted acidity showed high catalytic activity, while Al2O3 with Lewis acidity was totally inactive for the dehydration. Similarly, superiority of Brønsted acids to Lewis acids has been reported for this reaction.14 Based on its high catalytic activity, BP is likely to act as Brønsted acid catalyst for the dehydration of 1,2-PDO to PAL. 20 ACS Paragon Plus Environment

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Conclusions Several metal phosphates were tested as catalysts for the gas-phase dehydration of 1,2-PDO to PAL. BP showed high catalytic activity at low temperature and high selectivity to PAL without formation of competitive dehydration products such as acetone and allyl alcohol, giving high yield of PAL over 95%. Co-feeding steam together with 1,2-PDO was favorable both for enhancing the catalytic activity of BP and for promoting the hydrolysis of DXO, resulting in the significant increase in the yield of PAL. BP showed higher and more durable activity with higher selectivity to PAL than the conventional catalysts such as ZSM-5, SiO2-Al2O3, Nb2O5, and Al2O3.

Acknowledgements The analyses of ICP-AES and XPS were conducted with the instruments at the Institute for Catalysis, Hokkaido University.

Supporting information Detailed experimental procedure, reaction results for CoP and ZnP, and characterization of spent catalysts are available in Supporting Information.

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This paper describes an efficient conversion of bio-based 1,2-propanediol to propanal through selective dehydration over boron phosphate catalyst.


Selective Dehydration of 1,2-Propanediol to Propanal over Boron

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