Efficient Dehydration of C6-10-α,ω-Alkanediols to Alkadienes as Catalyzed by Aliphatic Acids.

The aliphatic-acid-mediated dehydration of C6-10-α,ω-alkanediols to alkadienes proceeds in a stepwise manner: C6-10-α,ω-alkanediols react with aliphatic acids first to generate diesters; subsequent pyrolysis of the latter produces alkadienes. The highest yields of 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene were up to 70.3, 74.8, and 90.3%, respectively. It turned out that pyrolysis favors the diester with a longer carbon chain more, while acetic acid outperformed the other aliphatic acids in the pyrolysis step that a relatively lower temperature was enough for a high yield of alkadienes.

Introduction

Alkadienes play an important role in chemical research, as they serve as essential build blocks in synthetic rubbers, co-crosslinkers, petroleum mixture, and starting materials for macrocycle synthesis.[1−6] There exist quite a few methods for preparing alkadienes, such as dehydration of alkanediol, reductive coupling reaction of halogenated olefins, catalytic decarboxylation of alkanedioic acid, or Grignard reaction of halogenated hydrocarbons.[7−14] Among all these methods, catalytic dehydration of alkanediol is the most commonly used one due to several advantages like low cost of raw materials, short reaction route, convenient post-treatment, and less pollution. As to the dehydration of alcohol, a series of organic and inorganic acids can catalyze this process, while some inorganic materials are capable of this task as well. For example, acids like sulfuric acid, phosphoric acid, p-toluenesulfonic acid, and inorganic materials like salts, molecular sieves, solid superacid, ion exchange resins, are proven to be active.[15−19] Acid-catalyzed dehydration of alkanol to alkene is generally accompanied by the migration of the double bond and the intermolecular and/or the intramolecular etherification, resulting in an unsatisfactory selectivity.[20]

Further, pyrolysis of esters may also produce olefins, and this route benefits from less side reactions such as rearrangement and isomerization.[4,21,22] In 1959, Froemsdorf reported the first example of pyrolysis of n-butyl acetate to 1-butene at 500 °C.[23] Up till now, however, there are only a few reports on the preparation of alkadiene via the pyrolysis of ester. Herein, we report a novel approach to continuously pyrolyze diester to alkadiene as performed in a tubular reactor; furthermore, no catalyst was involved. Several dienes like 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene from 1,6-hexanediol, 1,8-octanediol, and 1,10-decanediol, respectively, were obtained in high yields. Factors such as the lengths of carbon chain and the aliphatic acid used are discussed regarding their influence on pyrolysis.

Results and Discussion

Esterification

Water-separation devices consisting of a water divider and a condensation tube were used in the synthesis of diester from alkanediol and organic acid. Toluene, cycloheptane, and propyl acetate are ideal reagents to form azeotrope with water, thus removing it from the systems. Thus, toluene was employed as a water carrier. Excessive acids were used for uncatalyzed esterification. After esterification, the excessive acid and toluene in the reactor were recycled via evaporation. To further examine the influence of acid on the pyrolysis step, 1,10-decanediol was reacted with formic acid, acetic acid, propionic acid, butyric acid, and isobutyric acid to prepare the corresponding esters. It turned out that the weaker the aliphatic acid, harsher the condition required in esterification, in line with previous findings.[24] More details are shown in Table .

Table 1

Ester Synthesis from Aliphatic Acida

entry	n	R-groups	T (°C)	product	yield (%)
1	6	methyl	108	1,6-diacetoxyhexane	96.5
2	8	methyl	108	1,8-diacetoxyoctane	97.9
3	10	methyl	108	1,10-diacetoxydecane	97.2
4	10	H	94	1,10-dimethyloxydecane	97.8
5	10	ethyl	118	1,10-bis(propionyloxy)decane	96.3
6	10	propyl	140	1,10-bis(butyryloxy)decane	98.0
7	10	isopropyl	140	1,10-bis(isobutyryloxy)decane	95.5

The molar ratio of alkanediol to acid is 1:4.

Pyrolysis of Esters

The pyrolysis of esters is actually an eliminating process. For a general mechanism, the reaction proceeds according to the coordination mechanism of cyclic transition state, which features for a cis-elimination in stereochemistry. The reaction mechanism can be seen in Scheme .

Scheme 1

Pyrolytic Elimination

Gaseous species in a tubular reactor usually flows in a turbulent form. Assuming that temperature, velocity, or concentration does not diffuse in the axial direction and there is no radial gradient, the flow in the tubular reactor can be regarded as plug flow. Thus, different feed rates only result in different residence times. When the length and the filling fraction of the reactor are constant, the yield of the product increases with the decreasing feed rate (or the increasing residence time) at constant temperature. By adjusting the reaction temperature and the feed rate of the material, both the target product alkadiene and the byproduct monoene can be obtained (Scheme ).

Scheme 2

Pyrolysis Process of Diester

The quartz Raschig rings with a filler type of 3 × 7 mm2 were filled into the tubular reactor to afford a desired residence time and a uniform radical distribution of temperature; the filler void fraction was about 0.7. The optimum reaction conditions as well as the associated results for the pyrolysis of various esters are shown in Table ; the selected reactions proceed with good to excellent yields of alkadienes. Further, the influence of temperature, the acid used, and the residence time on the pyrolysis processes was investigated with 1,10-decanediol esters (Figure ).

Table 2

Pyrolysis of Esters to Alkadienea

entry	reactant	product	yield (%)
1	1,6-diacetoxyhexane	1,5-hexadiene	70.3
2	1,8-diacetoxyoctane	1,7-octadiene	74.8
3	1,10-diacetoxydecane	1,9-decadiene	90.3
4	1,10-dimethyloxydecane	1,9-decadiene	8.9
5	1,10-bis(propionyloxy)decane	1,9-decadiene	78.9
6	1,10-bis(butyryloxy)decane	1,9-decadiene	67.3
7	1,10-bis(isobutyryloxy)decane	1,9-decadiene	71.5

All of the reactions are at the temperature of 450 °C and the flow rate of 15 g/h.

Figure 1

Pyrolysis of (a) 1,10-dimethyloxydecane, (b) 1,10-diacetoxydecane, (c) 1,10-bis(propionyloxy)decane, and (d) 1,10-bis(isobutyryloxy)decane.

As shown in Figure , in general, the weaker the aliphatic acid used in the esterification, harsher is the condition required in the pyrolysis process except for 1,10-dimethyloxydecane; the latter is more prone to dissociate to 1,10-decanediol and CO by hydrogen transfer from the formaldehyde moiety to the α-oxygen. Conceivably, the weaker the aliphatic acid is, lower the electron density located around the carbonyl moiety is; accordingly, the oxygen of the carbonyl moiety is less favorable to serve as the hydrogen donor. In addition, a weaker aliphatic acid used for the ester facilitates a low diene/monoene ratio. Further, temperature affects the pyrolysis of esters both kinetically and thermochemically in a concert manner. For example, the higher the reaction temperature, the faster the reaction proceeds and higher the ratio of diene/monoene results. An endothermic nature of the pyrolysis processes can thus be justified. In addition to these general trends, indications can be found regarding how to tune the product distribution by combining temperature/aliphatic acid/residence time properly. Thus, if only the diene product is desired, high temperature/strong acid/long residence time should be adopted (Figure a); if both the diene and the monoene products are wanted equivalently, the combination high temperature/weak acid/short residence time is ideal. However, if only the monoene product is the target, a weak acid can be affirmed first, while the temperature and the residence time should be selected subtly and specifically for each monoene; to obtain 9-decen-1-ol, 1-acetate with a relatively high selectivity, the combination 400 °C/isobutyric acid/(15 g/h) should be used. Such a delicate selection of the reaction system provides a tactic for the selective preparation of diene/enol from alkanediols. In fact, selective monodehydration of alkanediols to enols has been reported previously by Yamanaka;[20] however, palmitic or stearic acid has to be used.

It should be noted that during the catalytic cycle, the amount of acetic acid recycled is always less than the theoretical value. This is because some of the acetic acid decomposes into ethenone and water at high temperatures. Similarly, propionic acid, butyric acid, and isobutyric acid were partially recycled. Under the conditions of 450 °C and 15 g/h feed rate, the ratio of organic acids recovered from ester pyrolysis is shown in Table . Obviously, the weaker the acid is, the less amount decomposes in the pyrolysis.

Table 3

Recovery Ratio of Aliphatic Acids

entry	organic acids	recovery ratio (%)
1	formic acid	0
2	acetic acid	32.7
3	propionic acid	44.5
4	butyric acid	62.3
5	isobutyric acid	65.1

Finally, the pyrolysis of another two diacetic esters, 1,8-diacetoxyoctane and 1,6-diacetoxyhexane, was performed at different temperatures and feed rates (Figure ). Generally, 1,7-octadiene and 1,5-hexadiene were also obtained with high yields. Notably, when the feed rate was reduced to 7.5 g/h, the yields of 1,8-diacetoxyoctane and 1,6-diacetoxyhexane decreased as compared to the ones at 15 g/h. This is due to the fact that short-chain alkadiene is more easily polymerized at high temperatures. Accordingly, though the yields of 1,9-decadiene, 1,7-octadiene, and 1,5-hexadiene are similar during 350–400 °C, the yields of 1,9-decadiene are significant as compared to the other two dienes when the temperature rises above 400 °C.

Figure 2

Pyrolysis of (a) 1,8-diacetoxyoctane and (b) 1,8-diacetoxyhexane.

Conclusions

In summary, 1,6-hexanediol, 1,8-octanediol, and 1,10-decanediol were used as raw materials to prepare 1,5-hexadiene, 1,7-octanediene, and 1,9-decanediene, respectively, and aliphatic acids were employed as catalysts. Esterification took place as the preliminary step, which was followed by the pyrolysis of the so-formed esters to eventually afford dienes. Upon comparing reactions at different temperatures and residence times, it was found that a stronger acid performs better in both the esterification and the pyrolysis processes, while the length of the carbon chain of the alkanediols does not affect either the esterification or the pyrolysis much. More importantly, the product distribution (diene/monoene) can be easily accomplished by properly combining different temperatures/aliphatic acids/residence times.

Experimental Section

Experimental Equipment

Main detector: gas chromatography–mass spectrometry (GC–MS): TRACE GC 2000/TRACE MS GC–MS (Thermo Quest Company); GC: Agilent 1790F Hydrogen Flame Detector; NMR: Bruker Avance DRX-400 NMR.

The esters were prepared in a 500 mL three-port flask equipped with a thermometer and a water separator.

The preparation of 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene was carried out in a tubular reactor, as shown in Figure .

Figure 3

Device of ester pyrolysis. (1) The raw material container, (2) injection pump, (3) preheater tube, (4) silica wool, (5) electric heating wire, (6) tubular reactor, (7) temperature tube, (8) thermoelectric thermometer, (9) transformer tank, (10) condenser, (11) receiving flask, and (12) bubbler. a The tubular reactor has a diameter of 3 cm and a height of 70 cm.

Experiment Procedure

Preparation of 1,9-Decadiene

A mixture of 52.2 g (0.3 mol) of 1,10-decanediol, 72 g (1.2 mol) of acetic acid, and 50 g of toluene was added to a 500 mL, three-port flask, which is equipped with a thermometer and a water separator. When the temperature was elevated to 108 °C, the water generated during the reaction is removed by a separator. After 6 h of reaction, the remaining acetic acid and toluene were removed via evaporation at 200 °C to obtain 1,10-diacetoxydecane.

The device of ester pyrolysis is shown in Figure . 1,10-Diacetoxydecane was placed in container bottle 1. The flow rate was adjusted using an injection pump 2 and the temperature was adjusted by the transformer tank 8. The crude 1,9-decadiene was collected from the receiving flask 11. 1,9-Decadiene was purified from crude 1,9-decadiene by vacuum distillation to collect 68 ± 0.5 °C/20 mmHg fraction. 1H NMR (400 MHz, CDCl3) δ 5.80 (ddt, J = 16.9, 10.2, 6.7 Hz, 2H), 5.10–4.76 (m, 4H), 2.04 (ddt, J = 14.4, 6.8 Hz, 4H), 1.45–1.16 (m, 8H). 13C NMR (101 MHz, CDCl3) δ 139.08 (s), 114.14 (s), 33.80 (s), 28.95 (d, J = 9.3 Hz).

Accordingly, 1,8-diacetoxyoctane and 1,6-diacetoxyhexane were prepared in the presence of acetic acid, 1,10-dimethyloxydecane in the presence of formic acid, 1,10-bis(propionyloxy)decane in the presence of propanoic acid, 1,10-bis(butyryloxy)decane in the presence of n-butyric acid, and 1,10-bis(isobutyryloxy) decane in the presence of isobutyric acid. 1,9-Decadiene was prepared from 1,10-dimethyloxydecane, 1,10-bis(propionyloxy)decane, 1,10-bis(butyryloxy)decane, or 1,10-bis (isobutyryloxy)decane; 1,7-octadiene was prepared from 1,8-diacetoxyoctane. 1H NMR (400 MHz, CDCl3) δ 5.80 (ddt, J = 16.9, 10.2, 6.7 Hz, 2H), 5.13–4.68 (m, 4H), 2.24–1.88 (m, 4H), 1.66–1.22 (m, 4H). 13C NMR (101 MHz, CDCl3) δ 138.90 (s), 114.28 (s), 33.64 (s), 28.40 (s); 1,5-hexadiene was prepared from 1,6-diacetoxyhexane. 1H NMR (400 MHz, CDCl3) δ 6.12–5.54 (m, 2H), 5.36–4.78 (m, 4H), 2.39–1.91 (m, 4H). 13C NMR (101 MHz, CDCl3) δ 138.11 (s), 114.64 (s), 33.14 (s).