Ring Opening of Biomass-Derived Cyclic Ethers to Dienes over Silica/Alumina

Article abstract of DOI:10.1021/acscatal.7b01436

We show that cyclic ethers, such 2-methyltetrahydrofuran (2-MTHF), can undergo dehydration to produce pentadienes over SiO₂/Al₂O₃. The catalyst exhibited reversible deactivation due to coke deposition, with the yield to pentadienes decreasing from 68% to 52% at 623 K over 58 h time on stream. A reaction network for 2-MTHF dehydration was proposed on the basis of the results of space time studies. Pentadienes can be produced directly by a concerted hydride shift and dehydration of carbenium intermediates or indirectly through dehydration of pentanal and pentenol. Reaction kinetics studies were performed at temperatures ranging from 573 to 653 K and 2-MTHF partial pressures from 0.21 to 2.51 kPa. The apparent activation energy barrier for 2-MTHF conversion to pentadienes and the reaction rate order for ring opening were determined to be 74 kJ mol^-1 and 0.24, respectively, indicating strong interaction between 2-MTHF and the SiO₂/Al₂O₃ surface. Other solid acids such as γ-Al₂O₃, H-ZSM-5, and Al-Sn-Beta were found to be active for 2-MTHF dehydration to pentadienes. The rate of ring opening decreased in the order 2,5-dimethyltetrahydrofuran > 2-MTHF > tetrahydropyran > tetrahydrofuran. Over SiO₂/Al₂O₃, the dehydration of 2,5-dimethyltetrahydrofuran resulted in 75% yield to hexadiene isomers. (Figure Presented).

Full text of DOI:10.1021/acscatal.7b01436

Article
Ring Opening of Biomass-derived Cyclic Ethers to Dienes over Silica/Alumina
Mrunmayi D. Kumbhalkar, J. Scott Buchanan, George W. Huber, and James A. Dumesic
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01436 • Publication Date (Web): 28 Jun 2017
Downloaded from http://pubs.acs.org on June 28, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Ring Opening of Biomassꢀderived Cyclic Ethers to
Dienes over Silica/Alumina
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Mrunmayi D. Kumbhalkar,† J. Scott Buchanan, ‡ George W. Huber,† and James A. Dumesic†,*
†Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI
53706, USA.
‡ExxonMobil Research and Engineering, Annandale, NJ, 08801, USA.
Corresponding Author
*Eꢀmail: jdumesic@wisc.edu
ABSTRACT
We show that cyclic ethers, such 2ꢀmethyltetrahydrofuran (2ꢀMTHF), can undergo dehydration
to produce pentadienes over SiO2/Al2O3. The catalyst exhibited reversible deactivation due to
coke deposition, with the yield to pentadienes decreasing from 68% to 52% at 623 K over 58 h
time on stream. A reaction network for 2ꢀMTHF dehydration was proposed based on the results
of space time studies. Pentadienes can be produced directly by concerted hydrideꢀshift and
dehydration of carbenium intermediates or indirectly through dehydration of pentanal and pentꢀ
enꢀol. Reaction kinetics studies were performed at temperatures ranging from 573 to 653 K and
2ꢀMTHF partial pressures from 0.21 to 2.51 kPa. The apparent activation energy barrier for 2ꢀ
1
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 43
1
2
3
4
5
6
7
8
9
MTHF conversion to pentadienes and the reaction rate order for ring opening were determined to
be 74 kJ molꢀ1 and 0.24, respectively, indicating strong interaction between 2ꢀMTHF and the
SiO2/Al2O3 surface. Other solid acids such as γꢀAl2O3, HꢀZSMꢀ5, AlꢀSnꢀBeta were found to be
active for 2ꢀMTHF dehydration to pentadienes. The rate of ring opening decreased in the order:
2,5ꢀdimethyltetrahydrofuran > 2ꢀMTHF > tetrahydropyran > tetrahydrofuran. Over SiO2/Al2O3,
the dehydration of 2,5ꢀdimethyltetrahydrofuran resulted in 75% yield to hexadiene isomers.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
GRAPHICAL ABSTRACT
KEYWORDS
2ꢀmethyltetrahydrofuran, dehydration, pentadiene, reaction network, solid acids, reversible
deactivation
2
ACS Paragon Plus Environment

Page 3 of 43
ACS Catalysis
1
2
3
4
INTRODUCTION
5
6
7
8
9
Lignocellulosic biomass has recently received considerable interest as a renewable
feedstock for the production of platform molecules which can subsequently be converted to fuels
and valueꢀadded chemicals.1,2 One such molecule, 2ꢀmethyltetrahydrofuran (2ꢀMTHF) has
gained attention as a green solvent or as a gasoline additive.3,4 2ꢀMTHF can be obtained from
biomassꢀderived intermediates such as levulinic acid and furfural.5 The pentoses in
lignocellulose can be converted to furfural using acid hydrolysis.6ꢀ8 Furfural can undergo
hydrodeoxygenation to 2ꢀMTHF in high yield in a single step over silicaꢀsupported Pd and Cu
catalysts.9 2ꢀMTHF is also a monofunctional intermediate produced during aqueous phase
hydrodeoxygenation of sorbitol over catalysts such as PtꢀRe/C and PdꢀAg/WOxꢀZrO2.10,11 In our
onꢀgoing work of aqueous phase hydrodeoxygenation of sorbitol over Co/TiO2, these
monofunctional intermediates consist mainly of alcohols (69% of the carbon in monofunctionals)
and heterocyclic species (23% of the carbon in monofunctionals). These monofunctional
compounds can then undergo CꢀC bond formation followed by hydrodeoxygenation to produce
jet and diesel range blendstocks.2,12 The monofunctionals can also be fed to an acidic catalyst to
produce olefins, which can then be converted into jet and diesel fuel by oligomerization and
hydrogenation reactions. In our current work, we remove the oxygen from these monofunctional
compounds via dehydration using a solid acid catalyst. In addition to the monoalkenes formed by
dehydration of the alcohol fraction, dienes were also observed in the effluent stream. We then
performed studies over this solid acid using 2ꢀMTHF as the model heterocyclic and observed the
formation of 1,4ꢀpentadiene and 1,3ꢀpentadiene isomers. There is limited work reported on the
dehydration of cyclic ethers to dienes. The objective of this work, therefore, is to understand the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 43
1
2
3
4
5
6
reaction network as well as the factors that affect the activity of the solid acid for this acidꢀ
catalyzed reaction.
7
8
9
1,3ꢀPentadiene, commercially known as piperylene, is a reactive linear hydrocarbon.
Piperylenes are byꢀproducts of the petroleum industry and are widely used as monomers in the
production of adhesives, plastics, and resins. The conjugated dienes serve as substrates for
conversion into highly functionalized molecules via organic reactions, e.g. DielsꢀAlder,
hydroformylation, and carbonylation.13,14 Currently, the main source of C5 hydrocarbons is steam
cracking of naphtha obtained from crude oil. The separation of C5 hydrocarbons is achieved
using extractive rectification and metalꢀorganic frameworks.15 Besides the crude oil route,
majority of literature for piperylene production involves processes using thermochemical
methods and homogeneous catalysis. Geller and Schniepp observed that pyrolytic decomposition
of 1,5ꢀpentanediol diacetate yielded 1,4ꢀpentadiene with 90ꢀ96% purity at 848 K.16 Brookhart
and coꢀworkers have reported ~40% yield to piperylenes via transfer dehydrogenation of pentane
using iridium pincer complex.17 Recently, Goldman and coꢀworkers have reported high yields
(~70%) to piperylenes for the same reaction catalyzed by pincerꢀligated iridium complexes.18
These homogeneous catalysts, however, have separations costs associated with them and have
challenges for large scale applications.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
An alternative and renewable route to produce pentadienes would be to employ
heterogeneous catalysts for conversion of biomassꢀderived platform chemicals. Chia and
Dumesic have studied a process for conversion of 4ꢀhydroxyꢀ6ꢀmethylꢀ2ꢀpyrone to 6ꢀmethylꢀ
5,6ꢀdihydroꢀ2ꢀpyrone and used acidꢀcatalyzed ring opening of the latter compound to produce
2,4ꢀhexadienoic acid and 1,3ꢀpentadiene.19 The advantage of this method is that the starting
material can be made from a renewable precursor, such as biomassꢀderived glucose using
4
ACS Paragon Plus Environment

Page 5 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
recombinant bacteria or yeast. Another process would be the conversion of C5 cyclic ether, 2ꢀ
MTHF, obtained in good yield from lignocelluloseꢀderived pentoses. Oyama and coꢀworkers
have reported ~33% yield of pentadienes during the hydrodeoxygenation of 2ꢀMTHF over
WP/SiO2 at 623 K.20 Johns developed a process consisting of twoꢀstep hydrogenation of furfural
to 2ꢀMTHF using Cu and Ni catalysts and subsequently dehydrating 2ꢀMTHF over a phosphate
salt at temperatures near 600 K to obtain 1,3ꢀpentadienes with small amount of 1,4ꢀpentadiene.21
Norman has reported the dehydration of 2ꢀMTHF to piperylene in high yield over vanadiumꢀ
titaniumꢀphosphorous ternary mixed oxide and boron phosphate.22 Over VꢀTiꢀP oxide, 80% of 2ꢀ
MTHF is converted to 1,4ꢀpentadiene and 1,3ꢀpentadiene with 81.4% total selectivity i.e. 65.1 %
yield at 623 K and atmospheric pressure. The side products reported are 4ꢀpentenꢀ2ꢀol, 3ꢀpentenꢀ
2ꢀol, and an unknown fraction. Diene polymerization leads to formation of coke and subsequent
catalyst deactivation. Dehydration of tetrahydropyran and 3ꢀmethyltetrahydrofuran to pentadiene
and isoprene was carried out, but the activity was lower as compared to that for 2ꢀMTHF.
Recently, the dehydraꢀdecyclization of tetrahydrofuran and 3ꢀmethyltetrahydrofuran to butadiene
and isoprene, respectively, over phosphorous selfꢀpillared pentasil has been reported by
Abdelrahman et al.23,24
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this work, the vaporꢀphase dehydration of 2ꢀMTHF to pentadienes was investigated
over various solid acids, particularly over amorphous SiO2/Al2O3. Silica/alumina has been
demonstrated to be active and stable during dehydration of 1ꢀbutanol and decarboxylation of γꢀ
valerolactone in high water environments.10,25 Dauenhauer and coꢀworkers have observed higher
yield to pentadienes during 2ꢀMTHF dehydration over SiO2/Al2O3 as compared to phosphorusꢀ
containing siliceous selfꢀpillared pentasil.23 Space velocity studies were performed to probe the
reaction network for the ring opening of 2ꢀMTHF. The effects of temperature and partial
5
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 43
1
2
3
4
5
6
7
8
9
pressure of the reactant on the activity of 2ꢀMTHF are also discussed. In addition to 2ꢀMTHF,
ring opening of other cyclic ethers, namely, tetrahydropyran, tetrahydrofuran, and 2,5ꢀ
dimethyltetrahydrofuran to their respective dienes was investigated. This conversion of cyclic
ethers can serve as a potential route to make dienes from biomass.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EXPERIMENTAL DETAILS
Materials and Characterization
The cyclic ethers, 2ꢀmethyltetrahydrofuran (2ꢀMTHF, SigmaꢀAldrich, >99%, 250 ppm
butylated hydroxytoluene as stabilizer), 2,5ꢀdimethlytetrahydrofuran (2,5ꢀDMTHF, Sigmaꢀ
Aldrich, 96%), tetrahydrofuran (THF, SigmaꢀAldrich, >99.9%, BHT as stabilizer), and
tetrahydropyran (THP, SigmaꢀAldrich, 99%) were used as purchased without further
purification.
Amorphous SiO2/Al2O3 (13.7 wt% alumina content) obtained from Grace Davision
(Grade 135) was used for the ring opening of various heterocyclics to dienes. The catalyst was
calcined ex situ at 773 K (2 K minꢀ1, 2 h) in flowing air (Airgas, 50 cm3 minꢀ1). The BET surface
area was obtained from the nitrogen adsorption isotherms carried out in a volumetric adsorption
unit (Micromeritics, ASAP 2010). The catalyst was degassed at 423 K in vacuum for 6 h prior to
the measurements. The surface area and mean pore diameter for the catalyst were calculated to
be 450 m2 gꢀ1 and 5.4 nm, respectively. Ring opening of 2ꢀMTHF was also investigated over
various solid acids like HꢀZSMꢀ5 (SiO2:Al2O3 = 23), HꢀMordenite (HꢀMOR, SiO2:Al2O3 = 20),
HꢀBeta (HꢀBEA, SiO2:Al2O3 = 25) (Zeolyst International), γꢀAl2O3 (Strem Chemicals), AlꢀSnꢀ
6
ACS Paragon Plus Environment

Page 7 of 43
ACS Catalysis
1
2
3
4
5
6
Beta (AlꢀSnꢀBEA, Si/Sn=200, Si/Al= 50, Haldor Topsøe A/S, Denmark), NbOPO4, and Nb2O5
(CBMM, Brazil).
7
8
9
The total and Brønsted acid site densities on the catalyst were estimated using ammonia
temperatureꢀprogrammed desorption (NH3ꢀTPD) and isopropylamine temperatureꢀprogrammed
desorption (IPAꢀTPD), respectively (Micromeritics, Autochem II 2920). Approximately 100 mg
of sample was loaded in a quartz cell and pretreated in He at 773 K. For NH3 adsorption, the
sample was cooled to 423 K after pretreatment, saturated with NH3 for 1 h and then flushed with
He for 1 h to remove the physisorbed ammonia. The TPD was performed using a temperature
ramp of 10 K minꢀ1 from 423 K to 973 K under flowing He. The amount of ammonia desorbed
was calculated by integrating the thermal conductivity detector signal. For IPAꢀTPD, the IPA
was dozed in pulses over the sample at 323 K until saturation after which the cell was purged
with He for 1 h. During TPD, the propene evolved via IPA decomposition was quantified to
determine the Brønsted acid site density.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Reactivity Measurements
The ring opening of various heterocyclics was studied in gasꢀphase in a fixedꢀbed downꢀ
flow reactor (Supporting Information, Figure S1) at atmospheric pressure and temperatures
varying from 573 to 653 K. The catalyst was packed in a stainless steel tubular reactor (6.35 mm
OD) between plugs of quartz wool and quartz beads. A wellꢀinsulated furnace (Applied Test
Systems) was used as the heat source and aluminium blocks were used around the reactor to
ensure isothermal operation. Reactor temperature was monitored at the reactor wall using a Kꢀ
type thermocouple (Omega) and 16A series programmable PID controller (Love Controls). For
the first set of reactions using dodecane as the solvent, 1ꢀ20 wt% of 2ꢀMTHF in dodecane was
introduced in the reactor using an HPLC pump (Lab Alliance Series I). Helium (Airgas) was
7
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 43
1
2
3
4
5
6
7
8
9
used as the carrier gas, and the flow rates were adjusted using a mass flow controller (Brooks
Instruments). The reactor effluent was passed through a gasꢀliquid separator immersed in an iceꢀ
water bath to ensure that condensation of lowꢀboiling products does not take place in the tubing
connected to the gas GC. Gasꢀphase products were purged from the separator by flowing He and
were analysed using an online GCꢀFID (Agilent) and a GCꢀ2014 (Shimadzu) equipped with a
TCD using He as the reference gas. Periodic liquid drains were collected from the separator and
injected in a GCꢀ2014 (Shimadzu). Total carbon balances are typically closed to within 10%.
Both the gas and liquid products were identified using a GCꢀMS (Shimadzu, QPꢀ2010).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A control experiment was carried out to ensure that the solvent was inert. Using pure
dodecane as the feed and the same reaction conditions over SiO2/Al2O3, we did not observe any
dodecane conversion. Similarly, to verify that the conversion of 2ꢀMTHF to pentadiene is indeed
catalytic, a reaction was performed at 623 K using a control bed in the absence of a catalyst and
no 2ꢀMTHF conversion was observed. The reaction kinetics studies were performed in the
absence of external and internal mass transfer limitations and the criteria for the same are
reported in the Supporting Information.
The reaction network was probed using space velocity measurements where the product
selectivities versus reciprocal space velocity (equal to space time) reveal the sequence of product
formation. The space times were varied by altering the catalyst amount and the reactant flow
rate. Weight hourly space velocity (WHSV) was defined as the mass of the feed molecule per
mass of catalyst per hour and the space time was defined as inverse of WHSV (Equation (1)).
The helium flow rate was chosen such that the molar ratio of 2ꢀMTHF:He was kept constant. The
rate of production of various products was plotted against the space time to determine the
8
ACS Paragon Plus Environment

Page 9 of 43
ACS Catalysis
1
2
3
4
5
6
primary and secondary products. The space time experiments were carried out in the kinetic
regime over SiO2/Al2O3 at a constant temperature of 623 K.
7
8
9
1
Mass of catalyst (g)
Space time (h) =
=
(1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
WHSV (h−1) 2ꢀMTHF flow rate (g h−1)
Gaussian09 software26 was used for thermodynamic calculations. Geometry
optimizations and subsequent frequency calculations were performed using B3LYP/6ꢀ
311+G(2d,p). Frequency calculations provided estimates for standard changes of enthalpy,
entropy, and Gibbs free energy, which were used in estimating thermodynamic properties over
the range of temperatures used in this study.
The apparent activation energy for pentadiene formation was calculated over the
temperature range from 573 to 653 K. The reaction rate order with respect to 2ꢀMTHF was
obtained by varying its partial pressure. All solid acid catalysts were observed to undergo
deactivation as the reaction progressed. Each data set was collected within 10 h of the start of the
experiment and the initial rates were obtained by extrapolation to time zero. For SiO2/Al2O3, we
observed that initial activity could be restored completely upon calcination in air at 773 K.
For the second set of reactions without using any solvent, low flow rates of the cyclic
ethers were achieved using a syringe pump (Harvard Apparatus, PHD Ultra) and helium was
used as a carrier gas. The products were analysed by an online GC equipped with a barrier
discharge ionization detector (GCꢀBID, Shimadzu). All the lines from the reactor to the gas GC
were heat traced and were maintained at 423 K to prevent condensation of the products and
reactants.
9
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 43
1
2
3
4
RESULTS AND DISCUSSION
5
6
7
2ꢀMTHF Conversion over SiO2/Al2O3
8
9
The dehydration of 2ꢀMTHF over SiO2/Al2O3 was investigated at 623 K and atmospheric
pressure. We observed the formation of cis and transꢀ1,3ꢀpentadiene, 1,4ꢀpentadiene, pentꢀenꢀ
ols, pentanal, pentꢀenꢀals, butenes as well as C2ꢀC3 alkenes. The conversion of 2ꢀMTHF to transꢀ
1,3ꢀpentadiene is an endothermic reaction with ꢁHº of 45.8 kJ molꢀ1 and is highly favourable at
623 K (ꢁGº = ꢀ76.8 kJ molꢀ1). A reaction at high conversion of 2ꢀMTHF was performed to
determine the yield to pentadienes. The catalyst undergoes continuous deactivation and the 2ꢀ
MTHF conversion decreases from 100% to 77% over a period of 58 h (Figure 1). The yield
towards pentadiene decreases from 67.8% to 51.8%. The initial yield to pentadienes is
comparable to that observed by Norman over VꢀTiꢀP oxide.22 The 1,3ꢀpentadiene (cis and trans
combined) and 1,4ꢀpentadiene yields at 58 h are 41.5% and 10.3%, respectively. Brønsted acid
catalysts are known to catalyze double bond isomerization and the higher yield towards the
conjugated diene, 1,3ꢀpentadiene can be explained owing to its higher stability as compared to
1,4ꢀpentadiene.27
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
100
90
80
70
60
50
40
30
20
2ꢀMTHF Conversion
Pentadienes Yield
10
0
0
10
20
30
40
50
60
Time on stream (h)
10
ACS Paragon Plus Environment

Page 11 of 43
ACS Catalysis
1
2
3
4
Figure 1. 2ꢀMTHF conversion and yield to pentadienes over SiO2/Al2O3. Reaction conditions:
5
6
1.0 g catalyst, 10 wt% 2ꢀMTHF in dodecane, 30 cm3 minꢀ1 He, 623 K, 1 atm, WHSV: 0.18 hꢀ1.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Reaction Pathways
The reaction network for ring opening of 2ꢀMTHF to pentadiene was probed using space
time studies. Figure 2 shows the effect of space time on the selectivity of various products
identified in the reaction. The pentadienes (Figure 2(a)) show nonꢀzero initial product selectivity,
indicating that they are primary products and are formed directly from 2ꢀMTHF. The selectivity
for production of pentadienes also increases with time on stream, suggesting that they can also be
formed as secondary products. The selectivity to pentꢀenꢀol and pentanal (Figure 2(b)) at low
space times are also nonꢀzero, exhibiting primary product behaviour. The selectivity to pentꢀenꢀ
ol decreases from 19.1% to 7.9% as the space time increases from 10.7 s to 101.3 s, showing that
pentꢀenꢀol undergoes further conversion. The selectivity for 1ꢀbutene increases with space time,
showing an apparent secondary product behaviour. In addition to these products, the formation
of pentenes, pentꢀenꢀals and C2ꢀC3 alkenes was observed.
70
60
50
40
30
20
10
0
24
20
16
12
8
Pentꢀenꢀol
Butene
Pentanal
(b)
(a)
4
Pentadiene
0.025 0.030
0
0.000
0.005
0.010
0.015
0.020
0.000
0.005
0.010
0.015
0.020
0.025
0.030
1/WHSV (h)
1/WHSV (h)
11
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 43
1
2
3
4
5
6
7
8
9
Figure 2. Product selectivities vs space times for 2ꢀMTHF ring opening over SiO2/Al2O3.
Reaction conditions: 20 wt% 2ꢀMTHF in dodecane, 623 K, 1 atm, WHSV: 35.5ꢀ 336 hꢀ1. (a)
Carbon selectivity to primary product, pentadiene, versus reciprocal space velocity; (b) Carbon
selectivities to minor products pentꢀenꢀol, butene, and pentanal versus reciprocal space velocity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
There are two positions where the CꢀO bond can undergo cleavage on 2ꢀMTHF, forming
either 2ꢀpentanone or pentanal. The GCꢀMS fragmentation patterns obtained for carbonyl
compounds in the liquid drain confirmed the presence of pentanal. Primary and secondary
carbenium ions can be formed during formation of 2ꢀpentanone and pentanal, respectively. Since
a secondary carbenium is more stable than a primary carbenium, pentanal formation is favored
over that of 2ꢀpentanone. To probe the rate of formation of dienes from pentanal, we conducted
experiments with pentanal in dodecane feed, maintaining the same partial pressure as that for 2ꢀ
MTHF during the space time studies. The specific rate of pentadiene formation from pentanal
was 26 µmol gꢀ1 minꢀ1 which is 30 times lower compared to the pentadiene formation from 2ꢀ
MTHF (740 µmol gꢀ1 minꢀ1) under similar reactions conditions. Also, pentꢀenꢀol formation was
not observed during pentanal conversion.
As seen in Figure 2, butene selectivity increases with space time as that of pentꢀenꢀol
decreases. We propose that a fraction of pentꢀenꢀols is converted to butene and formaldehyde via
reverse Prins reaction over the Lewis acid sites in SiO2/Al2O3. The amount of formaldehyde
could not be quantified using GCꢀFID, but the presence of formaldehyde was confirmed using
GCꢀMS. For quantification, moles of formaldehyde were assumed to be equal to the moles of
butene. The Prins reaction involves condensation of an aldehyde with an alkene, resulting in the
formation of unsaturated alcohols, alkyl dioxanes, and dienes. This acidꢀcatalyzed reaction is
traditionally catalyzed by homogeneous mineral acids, Lewis acids like SnCl4, as well as solid
12
ACS Paragon Plus Environment

Page 13 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
Lewis acid catalysts. Our findings are also consistent with Travin and coꢀworkers who report
reverse Prins reaction of 3ꢀmethylꢀ2ꢀbutenꢀ1ꢀol to form isobutylene and formaldehyde over
H2SO4 catalyst.28
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The changes in carbon selectivity with time on stream for the conversion of pure 2ꢀ
MTHF over SiO2/Al2O3 are shown in Figure 3. The conversion based on the products formed
decreases from 6.6% at 2 h to 3.8% at 6.1 h time on stream. Since the catalyst undergoes
deactivation, the product selectivities at different times during the reaction can be used to
understand the reaction network. The pentadiene selectivity decreases with time on stream,
whereas pentꢀenꢀol selectivity increases. Based on space time studies, pentꢀenꢀols are primary
products, and over a solid acid catalyst we anticipate the enols to undergo dehydration to produce
the pentadienes; however, since the catalyst is deactivating, increasing amounts of enols are
observed as the reaction progresses. It is important to note that the catalyst is continuously
deactivating and the activity shown by a fresh catalyst with a high active site density may be
different than the activity shown by a partially deactivated catalyst.
Pentadiene
Butene
60
Pentanal
Pentꢀenꢀol
Pentꢀenꢀal
Pentene
50
40
30
20
10
0
2.0 h
4.1 h
6.1 h
Time on stream
13
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 43
1
2
3
4
Figure 3. Product selectivities with time on stream for a reaction with pure 2ꢀMTHF feed.
5
6
7
8
Reaction conditions: 0.01 g SiO2/Al2O3, pure 2ꢀMTHF feed, 30 cm3 minꢀ1 He, T: 623 K; P: 1
atm.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Based on our findings, we propose a plausible reaction network for 2ꢀMTHF ring
opening as shown in Scheme 1. Attack by a proton from the silicaꢀalumina surface can lead to
the formation of a secondary carbenium ion (Scheme 1, species A) or a primary carbenium ion
with a hydroxyl group attached to it (Scheme 1, species B). The majority of pentadienes are
produced via the primary pathway involving concerted hydrideꢀshift, dehydration and
deprotonation. Owing to the rearrangement via hydrideꢀshift within the carbenium intermediate,
it is possible to form the diene directly without passing through either the enol or aldehyde
species. We also propose a secondary pathway to pentadienes through pentanal formation based
on the reactivity data obtained from reacting pentanal over SiO2/Al2O3. A separate secondary
pathway proceeding via pentꢀenꢀol formation and subsequent dehydration is also possible.
In studies involving hydrodeoxygenation of 2ꢀMTHF over metal phosphide catalysts,
Oyama et al. used space time studies to probe the reaction network.20,29,30 On WP/SiO2, the
primary products were 1,4ꢀpentadiene, cis and transꢀ1,3ꢀpentadiene, and furan. The formation of
pentꢀenꢀols and butene was not observed. The authors propose that to produce pentadienes and
pentanal, 2ꢀMTHF is bound to two active sites on the catalyst. The 2ꢀMTHF first adsorbs via the
ether oxygen atom on the catalyst surface after which a surface nucleophile attacks a carbon in
the alpha position to the oxygen to form a doubly bound intermediate species. In the subsequent
step, the nearby vacant sites induce βꢀhydride elimination and release pentadienes. They also
observed small amounts of pentanal formed by αꢀhydride elimination from the primary alkoxide
species of the same doubly bound intermediate. Though the details of the mechanism outlined by
14
ACS Paragon Plus Environment

Page 15 of 43
ACS Catalysis
1
2
3
4
Oyama et al. and in this study are different, both mechanisms explain that dienes and aldehyde
5
6
come from the same initial intermediates.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Proposed reaction network for 2ꢀMTHF ring opening to pentadiene based on space
time studies over SiO2/Al2O3
15
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 43
1
2
3
4
5
6
Effect of Temperature
7
8
9
Reactions were performed in the kinetically controlled regime to determine the apparent
activation energy for pentadiene formation over SiO2/Al2O3. Low conversion of 2ꢀMTHF was
achieved by decreasing the catalyst amount in the reactor and increasing the feed flow rate. 2ꢀ
MTHF ring opening reactions were performed at four different temperatures, 573, 603, 623, and
653 K, and collected initial rate data within 10 h for each temperature. As the catalyst undergoes
deactivation, the initial rate of pentadiene formation was used to determine the apparent
activation energy, Ea. Figure 4 summarizes the effect of temperature on the rate of pentadiene
formation. The Ea for pentadiene formation is calculated to be 74 ± 7 kJ molꢀ1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
1.50
1.55
1.60
1.65
1.70
1.75
1000/T (Kꢀ1)
Figure 4. Rates of pentadiene formation at various reaction temperatures. Reaction conditions:
0.01 g SiO2/Al2O3; 10 wt% 2ꢀMTHF in dodecane, 90 cm3 minꢀ1 He, WHSV: 54 hꢀ1; P: 1 atm.
Reaction Rate Order
16
ACS Paragon Plus Environment

Page 17 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
The partial pressure of 2ꢀMTHF was varied from 0.21–2.51 kPa to obtain the rate order
dependence for the ring opening reaction. This variation in partial pressure was achieved by
varying the 2ꢀMTHF composition in dodecane as well as the He flow rate. A fractional rate order
of 0.24 ± 0.06 with respect to 2ꢀMTHF was obtained as shown in Figure 5. The rate of 2ꢀMTHF
consumption increases only slightly with increasing 2ꢀMTHF concentration. This behavior can
be attributed to a strong interaction between species derived from 2ꢀMTHF and the surface of
SiO2/Al2O3, which is in accord with the low experimentally calculated apparent activation
energy.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8.0
7.5
7.0
6.5
6.0
5.5
5.0
ꢀ6.5
ꢀ6.0
ꢀ5.5
ꢀ5.0
ꢀ4.5
ꢀ4.0
ꢀ3.5
ln(P2ꢀMTHF (bar))
Figure 5. Rates of 2ꢀMTHF ring opening over SiO2/Al2O3 at various partial pressures of 2ꢀ
MTHF at 623 K and 1 atm.
Catalyst Regenerability
17
ACS Paragon Plus Environment

ACS Catalysis
Page 18 of 43
1
2
3
4
5
6
7
8
9
Catalyst deactivation with time on stream was observed for all the reactions in this study
due to coke formation, likely from polymerization of pentadienes. Catalyst regeneration was
achieved by calcining the catalyst at 773 K (4 K minꢀ1 ramp from room temperature to 773 K, 2 h
hold at 773 K) in flowing air. The CO2 evolution was monitored during calcination using a GCꢀ
TCD. The CO2 peak area increased with temperature and eventually decreased to zero suggesting
that coke removal was complete. After calcination, reaction kinetics studies were performed with
the same operating conditions. As seen in Figure 6, 2ꢀMTHF conversion and the rate of
pentadiene formation were similar for the fresh and regenerated catalyst, indicating that the
deactivation is reversible. Thus, SiO2/Al2O3 is regenerable upon calcination, with no observable
loss in initial catalytic activity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7.2
6.8
6.4
6.0
5.6
5.2
100
80
60
40
20
0
(a)
(b)
Fresh
RG
Fresh
RG
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
Time (h)
Time (h)
Figure 6. (a) 2ꢀMTHF conversion and (b) semiꢀlog plot of pentadiene production rate versus
time on stream over fresh (■) and regenerated (■) SiO2/Al2O3. Reaction conditions: 0.05 g
SiO2/Al2O3; 10 wt% 2ꢀMTHF in dodecane feed; T: 623 K; P: 1 atm; WHSV: 11.5 hꢀ1; 90 ml minꢀ
1 He. RG: Regeneration in air at 773 K for 2 h.
18
ACS Paragon Plus Environment

Page 19 of 43
ACS Catalysis
1
2
3
4
2ꢀMTHF conversion over various solid acids
5
6
7
8
9
The conversion of 2ꢀMTHF was further investigated over HꢀZSMꢀ5, HꢀMOR, HꢀBeta, γꢀ
Al2O3, and AlꢀSnꢀBeta. Pentadiene formation was observed over all these catalysts. No
conversion of the dodecane solvent was seen during a control experiment over amorphous
SiO2/Al2O3 at 623 K and 1 atm. However, we observed conversion of dodecane to form
aromatics compounds such as pꢀxylene, 1ꢀethylꢀ2ꢀmethylꢀbenzene, and 1,2,4ꢀtrimethylꢀbenzene,
and cracking products, such as hexane and heptane, over HꢀZSMꢀ5. Mi and coꢀworkers observed
formation of aromatics during dodecane cracking at 673 K over HꢀZSMꢀ5.31 The amount of
carbon present in the undesired aromatics and cracking products corresponded to approximately
10% of the total carbon detected in all the products. The formation of aromatics was not
observed over γꢀAl2O3 and the niobiumꢀbased acids. Experiments performed over Nb2O5 and
NbOPO4 showed pentadiene formation initially but these catalysts deactivated completely within
4 to 6 h time on stream. Both these niobium acids are white powders but upon removal from the
reactor, the spent catalysts were black in color suggesting deactivation due to coke deposition.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Pentadiene formation turnover frequencies (TOF) were obtained by normalizing specific
rates with the Brønsted acid site densities. The Brønsted acid site (BAS) and Lewis acid site
(LAS) densities for these catalysts were taken from literature except that for SiO2/Al2O3 and Hꢀ
ZSMꢀ5 which were calculated using NH3ꢀTPD and IPAꢀTPD.32 A primarily Lewis solid acid, γꢀ
Al2O3, was also active for the formation of pentadiene, however, the rate was lower as compared
to SiO2/Al2O3. The initial rates of production of pentadiene over SiO2/Al2O3 and γꢀAl2O3 were
496 and 155 µmol gꢀ1 minꢀ1, respectively. All the catalysts underwent deactivation as the reaction
progressed. The pentadiene TOF over SiO2/Al2O3 decreased from 0.97 minꢀ1 to 0.45 minꢀ1 over
19
ACS Paragon Plus Environment

ACS Catalysis
Page 20 of 43
1
2
3
4
5
6
7
8
9
10 h. The initial pentadiene TOF values calculated at time zero were found to be similar for all
the solid acids mentioned in Table 1. The niobiumꢀbased solid acids are not included in the table
as rapid deactivation was observed within a few hours of the start of the reaction. The first order
deactivation constants were calculated by plotting the semiꢀlog plot of pentadiene formation
versus time on stream. SiO2/Al2O3 had the lowest deactivation constant.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Pentadiene formation per Brønsted acid site and first order deactivation constants over
various solid acids for 2ꢀMTHF ring opening
BAS
LAS
Pentadiene TOF kd x 103
Catalyst
SiO2:Al2O3
(µmol gꢀ1) (µmol gꢀ1)
(minꢀ1)
(minꢀ1)
SiO2/Al2O3
HꢀZSMꢀ5
HꢀBEA
10.7
23.0
25.0
20.0
537
652
269
331
145
48
0.92
1.4
0.93
0.85
0.85
2.0
1.8
2.2
162
71
HꢀMOR
We compared the product selectivities over various solid acids at similar levels of
conversion (~35%) in Figure 7. The highest pentadiene selectivity of ~70% was observed over
SiO2/Al2O3, HꢀZSMꢀ5, and AlꢀSnꢀBEA. The other products include butene, pentanal, pentꢀenꢀ
ols, pentꢀenꢀals, and pentene. The predominantly Lewis acid catalyst, γꢀAl2O3, had the highest
butene and pentꢀenꢀol selectivities. The Lewis acid site density for γꢀAl2O3 was determined to be
20
ACS Paragon Plus Environment

Page 21 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
210 µmol/g and no IPA decomposition to propylene was observed, indicating the absence of
Brønsted acid sites. The reactors were loaded with equal amounts (0.05 g) of catalyst for
SiO2/Al2O3 and γꢀAl2O3. Butene selectivities over SiO2/Al2O3 and γꢀAl2O3 are 17.5% and 21.7%,
respectively. The conversion of allylic alcohol to an alkene and aldehyde (reverse Prins
reaction), i.e. conversion of pentꢀenꢀol to butene and formaldehyde in this scenario, is catalyzed
by Lewis acid sites. Since γꢀAl2O3 possesses a higher number of Lewis acid sites, a higher
selectivity to butene is observed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
80
Pentadiene
Butene
Pentanal
70
60
50
40
30
20
10
0
Pentꢀenꢀols
Pentꢀenꢀals
Pentene
SiO2/Al2O3 HꢀZSMꢀ5
HꢀBEA
γꢀAl2O3 AlꢀSnꢀBEA
Figure 7. Product selectivities for 2ꢀMTHF conversion over different catalysts at 623 K. The
selectivities were compared at ~35% conversion.
Ring opening of various heterocyclics
Cyclic ethers such as tetrahydrofurans and tetrahydropyrans are other heterocyclics
formed during hydrodeoxygenation of C6 polyols.33,34 The ring opening of tetrahydrofuran,
21
ACS Paragon Plus Environment

ACS Catalysis
Page 22 of 43
1
2
3
4
5
6
7
8
9
tetrahydropyran, and 2,5ꢀdimethyltetrahydrofuran was studied over SiO2/Al2O3. Dehydration of
THP resulted in same set of products as for 2ꢀMTHFꢀ pentadienes, pentenes, pentanal, pentꢀenꢀ
ols, pentꢀenꢀals, and butene. Over SiO2/Al2O3, the ring opening of THF showed formation of
butadiene, butanal, butenes, propene, and formaldehyde. The formation of propene and
formaldehyde via reverse Prins reaction was also observed by Dauenhauer and coꢀworkers in a
recent study involving the dehydraꢀdecyclization of tetrahydrofuran to butadiene over
phosphorous selfꢀpillared pentasil.35 The formation of butꢀenꢀol, however, was not observed. The
products observed during the dehydration of 2,5ꢀDMTHF are hexadienes, 2ꢀhexanone, hexꢀenꢀ
ones, butenes, and acetaldehyde. The hexadiene isomers included 1,5ꢀhexadiene, cis and trans
mixtures of 1,3ꢀhexadienes, 1,4ꢀhexadienes, and 2,4ꢀhexadienes. As in the case of THF, enols
(hexꢀenꢀols in the case of 2,5ꢀDMTHF) were not seen in the product stream. The possible
mechanisms for ring opening of THF and 2,5ꢀDMTHF are shown in Scheme 2. For both THF
and 2,5ꢀDMTHF, the formation of carbonyl compounds (i.e. butanal and 2ꢀhexanone,
respectively) was observed whereas the enol species are not seen. The product selectivities for
dehydration of various heterocyclics are listed in Table S2 in the Supporting Information. We
speculate that the pathway for formation of the carbonyl compound is favored, as carbenium ions
(Scheme 2, species D and species F) with hydroxyl group attached to the carbon bearing the
positive charge are more stable than species C and E. This behavior is in agreement with work
involving CꢀO hydrogenolysis of cyclic ethers by Chia et al. who found that the presence of
hydroxyl substituent alpha to the dehydration center increases the stability of the carbenium
ion.36 The dehydration of THP proceeds in a pathway similar as that for THF. The rates of pentꢀ
enꢀol formation starting from 2ꢀMTHF and THP are 122.2 and 1.2 µmol gꢀ1 minꢀ1, respectively.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
22
ACS Paragon Plus Environment

Page 23 of 43
ACS Catalysis
1
2
3
4
5
6
The lower specific rate for pentꢀenꢀol formation for THP can again be explained based on the
relative stability of the carbenium intermediates.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. Proposed mechanisms for ring opening of tetrahydrofuran (THF) and 2,5ꢀ
dimethyltetrahydrofuran (2,5ꢀDMTHF) over SiO2/Al2O3
23
ACS Paragon Plus Environment

ACS Catalysis
Page 24 of 43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
As discussed earlier, the ring opening of 2ꢀMTHF can proceed via formation of primary
or secondary carbenium ions. However, for 2,5ꢀDMTHF, the dehydration can proceed only via
secondary carbenium ion formation. Dehydration for both THF and THP, which do not have the
methyl branching, can proceed only by primary carbenium ion formation. The rates of ring
opening of these cyclic ethers were compared by performing experiments in the differential
24
ACS Paragon Plus Environment

Page 25 of 43
ACS Catalysis
1
2
3
4
5
6
7
8
9
conversion regime for the same partial pressure of the reactant (Table 2). The flow rates of the
cyclic ether and the inert carrier gas helium were adjusted to maintain the initial partial pressure
at 2.4 kPa. The lowest rate of ring opening was observed for THF. Among the C5 ethers, 2ꢀ
MTHF and THP, the rate of ring opening of the former was ~92% higher. We hypothesize that
the rate of 2ꢀMTHF dehydration is higher than that of THP owing to the higher stability of
secondary carbenium ion as compared to that of the primary carbenium ion. 2,5ꢀDMTHF can
undergo ring opening via two routes to form a secondary carbenium ion and thus showed the
highest rate of dehydration.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Rates of ring opening of various heterocyclics over SiO2/Al2O3 at 623 K and 1 atm
Heterocyclics
Rate of Ring Opening
Conversion (%)
(µmol gꢀ1 minꢀ1)
Structure
Name
2,5ꢀDMTHF
20.2
14.8
7.3
6573.7
2678.6
1397.3
1096.8
2ꢀMTHF
THP
THF
10.1
The C6 diene, transꢀ1,4ꢀhexadiene, is used in the production of ethyleneꢀpropyleneꢀdiene
monomer and is produced via ethylene and 1,3ꢀbutadiene codimerization. Behr and Miao have
shown 40.3% yield to transꢀ1,4ꢀhexadiene by the codimerization of 1,3ꢀbutadiene and ethylene
in water and propylene glycol solvent over RhCl3.37 In our study, a high conversion reaction for
2,5ꢀDMTHF showed a change in conversion from 85.4% to 75.3% over 8.3 h time on stream
25
ACS Paragon Plus Environment

ACS Catalysis
Page 26 of 43
1
2
3
4
5
6
7
8
9
(Figure 8). Throughout the reaction, a high selectivity to hexadiene isomers, particularly cis and
trans 2,4ꢀhexadiene isomers, was observed. Nearly 75% yield towards hexadienes is observed
under these reactions conditions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
100
2,5ꢀDMTHF Conversion
Hexadienes Yield
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
Time on stream (h)
Figure 8. 2,5ꢀDMTHF conversion and hexadienes yield. Reaction conditions: 0.01 g SiO2/Al2O3,
5 wt% 2,5ꢀDMTHF in dodecane, 60 cm3 minꢀ1 He, T: 623 K; WHSV: 13.3 hꢀ1; P: 1 atm.
CONCLUSIONS
We have shown that cyclic ethers can undergo ring opening to produce dienes in high yields over
solid acid catalysts. Based on space time studies and changes in product distribution with time on
stream, we have proposed a reaction network for 2ꢀMTHF dehydration over SiO2/Al2O3. Ring
opening occurs via both a primary concerted pathway involving hydrideꢀtransfer and dehydration
of a carbenium intermediate, and a secondary pathway passing through the formation of pentanal
and pentꢀenꢀols. The catalyst deactivates as the reaction progresses, but this deactivation is
reversible and the initial activity of the catalyst is fully restored upon calcination in air. A
fractional rate order (0.24) dependence with 2ꢀMTHF is observed due to high surface coverage.
26
ACS Paragon Plus Environment

Page 27 of 43
ACS Catalysis
1
2
The apparent activation energy for 2ꢀMTHF dehydration was calculated to be 74 kJ molꢀ1. Other
solid acids with both Brønsted and Lewis acid sites were active for the dehydration reaction to
dienes with the catalytic activity for dienes formation being proportional to the concentration of
Brønsted acid sites on the catalyst. Amongst the various cyclic ethers, 2,5ꢀDMTHF showed the
highest activity towards dehydration, resulting in the formation of hexadienes with ~75% yield at
623 K.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACKNOWLEDGEMENTS
This work was supported by ExxonMobil Research and Engineering.
SUPPORTING INFORMATION
Reactor schematic, verification of absence of mass transfer limitations, and product selectivities
for dehydration of various heterocyclics
REFERENCES
1.
2.
3.
Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044ꢀ4098.
Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Green Chem. 2010, 12, 1493ꢀ1513.
Pace, V.; Hoyos, P.; Castoldi, L.; de Maria, P. D.; Alcantara, A. R. ChemSusChem 2012,
5, 1369ꢀ1379.
4.
5.
6.
Jessop, P. G. Green Chem. 2011, 13, 1391ꢀ1398.
Elliott, D. C.; Frye, J. G. U.S. Patent 5883266, 1999.
Xing, R.; Qi, W.; Huber, G. W. Energy Environ. Sci. 2011, 4, 2193ꢀ2205.
27
ACS Paragon Plus Environment

ACS Catalysis
Page 28 of 43
1
2
3
4
7.
Dumesic, J. A.; Alonso, D. M.; Gürbüz, E. I.; Wettstein, S. G. U.S. Patent 8399688,
5
6
7
8
2013.
8.
Zeitsch, K. J. The Chemistry and Technology of Furfural and its Many by-products.
Elsevier: The Netherlands, 2000.
9. Dong, F.; Zhu, Y. L.; Ding, G. Q.; Cui, J. L.; Li, X. Q.; Li, Y. W. ChemSusChem 2015, 8,
1534ꢀ1537.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10.
Kunkes, E. L.; Simonetti, D. A.; West, R. M.; SerranoꢀRuiz, J. C.; Gartner, C. A.;
Dumesic, J. A. Science 2008, 322, 417ꢀ421.
11.
Blommel, P.; Dally, B.; Lyman, W.; Cortright, R. U.S. Patent 0198760, 2012.
12.
Huber, G. W.; Chheda, J. N.; Barrett, C. J.; Dumesic, J. A. Science 2005, 308, 1446ꢀ
1450.
13.
Behr, A.; Neubert, P. ChemCatChem 2014, 6, 412ꢀ428.
14.
Neubert, P.; Fuchs, S.; Behr, A. Green Chem. 2015, 17, 4045ꢀ4052.
Maes, M.; Alaerts, L.; Vermoortele, F.; Ameloot, R.; Couck, S.; Finsy, V.; Denayer, J. F.
15.
M.; De Vos, D. E. J. Am. Chem. Soc. 2010, 132, 2284ꢀ2292.
16.
17.
18.
Schniepp, L. E.; Geller, H. H. J. Am. Chem. Soc. 1945, 67, 54ꢀ56.
Kundu, S.; Lyons, T. W.; Brookhart, M. ACS Catal. 2013, 3, 1768ꢀ1773.
Kumar, A.; Hackenberg, J. D.; Zhuo, G.; Steffens, A. M.; Mironov, O.; Saxton, R. J.;
Goldman, A. S. J. Mol. Catal. A-Chem. 2017, 426, 368ꢀ375.
19.
20.
Dumesic, J. A.; Chia, M. U.S. Patent 8404890, 2013.
Bui, P.; Cecilia, J. A.; Oyama, S. T.; Takagaki, A.; InfantesꢀMolina, A.; Zhao, H. Y.; Li,
D.; RodriguezꢀCastellon, E.; Lopez, A. J. J. Catal. 2012, 294, 184ꢀ198.
21.
22.
23.
Johns, I. B. U.S. Patent 2458001, 1949.
Norman, D. W. U.S. Patent 8981172, 2015.
Abdelrahman, O. A.; Park, D. S.; Vinter, K. P.; Spanjers, C. S.; Ren, L.; Cho, H. J.;
Vlachos, D. G.; Fan, W.; Tsapatsis, M.; Dauenhauer, P. J. ACS Sustain. Chem. Eng. 2017, 5,
3732ꢀ3736.
24.
Abdelrahman, O. A.; Park, D. S.; Vinter, K. P.; Spanjers, C. S.; Ren, L. M.; Cho, H. J.;
Zhang, K. C.; Fan, W.; Tsapatsis, M.; Dauenhauer, P. J. ACS Catal. 2017, 7, 1428ꢀ1431.
25.
West, R. M.; Braden, D. J.; Dumesic, J. A. J. Catal. 2009, 262, 134ꢀ143.
28
ACS Paragon Plus Environment

Page 29 of 43
ACS Catalysis
1
2
3
4
26.
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,
5
6
7
8
J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li,
X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd,
J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari,
K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.;
Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R.
L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich,
S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian
09, Gaussian, Inc.: Wallingford, CT, USA, 2009.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
27.
28.
Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O. J. Catal. 1996, 158, 279ꢀ287.
Ryabova, R. S.; Osipova, G. F.; Travin, S. O. Russian Chemical Bulletin 1995, 44, 840ꢀ
843.
29.
Bui, P. P.; Oyama, S. T.; Takagaki, A.; Carrow, B. P.; Nozaki, K. ACS Catal. 2016, 6,
4549ꢀ4558.
30.
31.
Iino, A.; Cho, A.; Takagaki, A.; Kikuchi, R.; Oyama, S. T. J. Catal. 2014, 311, 17ꢀ27.
Xian, X. C.; Liu, G. Z.; Zhang, X. W.; Wang, L.; Mi, Z. T. Chemical Engineering
Science 2010, 65, 5588ꢀ5604.
32.
Kubicka, D.; Kumar, N.; MakiꢀArvela, P.; Tiitta, M.; Niemi, V.; Karhu, H.; Sami, T.;
Murzin, D. Y. J. Catal. 2004, 227, 313ꢀ327.
33.
34.
Kim, Y. T.; Dumesic, J. A.; Huber, G. W. J. Catal. 2013, 304, 72ꢀ85.
West, R. M.; Kunkes, E. L.; Simonetti, D. A.; Dumesic, J. A. Catal. Today 2009, 147,
115ꢀ125.
35.
Abdelrahman, O. A.; Park, D. S.; Vinter, K. P.; Spanjers, C. S.; Ren, L.; Cho, H. J.;
Vlachos, D. G.; Fan, W.; Tsapatsis, M.; Dauenhauer, P. J. ACS Sustain. Chem. Eng. 2017.
36.
Chia, M.; PaganꢀTorres, Y. J.; Hibbitts, D.; Tan, Q. H.; Pham, H. N.; Datye, A. K.;
Neurock, M.; Davis, R. J.; Dumesic, J. A. J. Am. Chem. Soc. 2011, 133, 12675ꢀ12689.
37.
Behr, A.; Miao, Q. Green Chem. 2005, 7, 617ꢀ620.
29
ACS Paragon Plus Environment