Dehydra-decyclization of 2-methyltetrahydrofuran to pentadienes on boron-containing zeolites

Article abstract of DOI:10.1039/d0gc00136h

1,3-Pentadiene (piperylene) is an important monomer in the manufacturing of adhesives, plastics, and resins. It can be derived from biomass by the tandem ring-opening and dehydration (dehydra-decyclization) of 2-methyltetrahydrofuran (2-MTHF), but competing reaction pathways and the formation of another isomer (1,4-pentadiene) have limited piperylene yields to <60percent. In this report, using detailed kinetic measurements of 2-MTHF dehydra-decyclization on zeolites with disparate acidities (boro-, and alumino-silicates) and micropore environments (MFI, MWW, and BEA), weakly acidic borosilicates were shown to exhibit ca. 10-30percent higher selectivity to dienes at about five-to-sixty times lower proton-normalized rates than aluminosilicates (453-573 K). Dehydra-decyclization site time yields (STYs) were invariant for aluminosilicates within the investigated frameworks, indicating the absence of pore-confinement influence. However, individual site-normalized reaction rates varied by almost an order of magnitude on borosilicates in the order MWW > MFI > BEA at a given temperature (523 K), indicating the non-identical nature of active sites in these weak solid acids. The diene distribution remained far from equilibrium and was tuned towards the desirable conjugated diene (1,3-pentadiene) by facile isomerization of 1,4-pentadiene. This tuning capability was facilitated by high bed residence times, as well as the smaller micropore sizes among the zeolite frameworks considered. The suppression of competing pathways, and promotion of 1,4-pentadiene isomerization events lead to a hitherto unreported ～86percent 1,3-pentadiene yield and an overall ～89percent combined linear C5 dienes' yield at near quantitative (～98percent) 2-MTHF conversion on the borosilicate B-MWW, without a significant reduction in diene selectivities for at least 80 hours time-on-stream under low space velocity (0.85 g reactant per g cat. per h) and high temperature (658 K) conditions. Finally, starting with iso-conversion levels (ca. 21-26percent) and using total turnover numbers (TONs) accrued over the entire catalyst lifetime as the stability criterion, borosilicates were demonstrated to be significantly more stable than aluminosilicates under reaction conditions (～3-6× higher TONs).

Full text of DOI:10.1039/d0gc00136h

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Dehydra-decyclization of 2-methyltetrahydrofuran
to pentadienes on boron-containing zeolites†
Cite this: DOI: 10.1039/d0gc00136h
a
b
c
a,f
d,e
Gaurav Kumar, Dongxia Liu,
Dandan Xu, Limin Ren, Michael Tsapatsis* and
a
Paul J. Dauenhauer
*
1
,3-Pentadiene (piperylene) is an important monomer in the manufacturing of adhesives, plastics, and
resins. It can be derived from biomass by the tandem ring-opening and dehydration (dehydra-decycliza-
tion) of 2-methyltetrahydrofuran (2-MTHF), but competing reaction pathways and the formation of
another isomer (1,4-pentadiene) have limited piperylene yields to <60%. In this report, using detailed
kinetic measurements of 2-MTHF dehydra-decyclization on zeolites with disparate acidities (boro-, and
alumino-silicates) and micropore environments (MFI, MWW, and BEA), weakly acidic borosilicates were
shown to exhibit ca. 10–30% higher selectivity to dienes at about five-to-sixty times lower proton-nor-
malized rates than aluminosilicates (453–573 K). Dehydra-decyclization site time yields (STYs) were invar-
iant for aluminosilicates within the investigated frameworks, indicating the absence of pore-confinement
influence. However, individual site-normalized reaction rates varied by almost an order of magnitude on
borosilicates in the order MWW > MFI > BEA at a given temperature (523 K), indicating the non-identical
nature of active sites in these weak solid acids. The diene distribution remained far from equilibrium and
was tuned towards the desirable conjugated diene (1,3-pentadiene) by facile isomerization of 1,4-penta-
diene. This tuning capability was facilitated by high bed residence times, as well as the smaller micropore
sizes among the zeolite frameworks considered. The suppression of competing pathways, and promotion
of 1,4-pentadiene isomerization events lead to a hitherto unreported ∼86% 1,3-pentadiene yield and an
overall ∼89% combined linear C5 dienes’ yield at near quantitative (∼98%) 2-MTHF conversion on the
borosilicate B-MWW, without a significant reduction in diene selectivities for at least 80 hours time-on-
stream under low space velocity (0.85 g reactant per g cat. per h) and high temperature (658 K) con-
ditions. Finally, starting with iso-conversion levels (ca. 21–26%) and using total turnover numbers (TONs)
accrued over the entire catalyst lifetime as the stability criterion, borosilicates were demonstrated to be
significantly more stable than aluminosilicates under reaction conditions (∼3–6× higher TONs).
Received 11th January 2020,
Accepted 26th March 2020
DOI: 10.1039/d0gc00136h
rsc.li/greenchem
1
Introduction
Conjugated C4–C5 dienes, currently manufactured as a bypro-
duct of the cracking of naphtha and gas oil fraction of crude
1
,2
oil, are vital bulk chemicals for the elastomer industry. Due
to a recent surge in the supply of shale-gas ethane, refineries
are finding ways to decouple the production of ethylene and
C4–C5 fractions including isoprene, butadiene, and piperylene
a
Department of Chemical Engineering and Materials Science, University of
Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, USA.
E-mail: hauer@umn.edu
b
Department of Chemical and Biomolecular Engineering, University of Maryland,
College Park, MD, 20742, USA
(
1,3-pentadiene), accentuating the need to produce these
c
Department of Chemical Engineering, University of Massachusetts Amherst, 685 N.
chemicals from alternative on-purpose routes possibly utiliz-
Pleasant Street, Amherst, MA 01003, USA
d
3
–6
ing renewable feedstocks such as lignocellulosic biomass.
Department of Chemical and Biomolecular Engineering & Institute for
While isoprene is understandably the most valuable C5 diene
isomer, 1,3-pentadiene has recently attracted attention as a
high potential feedstock for fine chemical synthesis using
NanoBioTechnology, Johns Hopkins University, 3400 N. Charles Street, Baltimore,
MD 21218, USA
e
Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road,
Laurel, MD 20723, USA. E-mail: tsapatsis@jhu.edu
7–10
transition metal catalysis.
f
Zhang Dayu School of Chemistry, Dalian University of Technology, No. 2 Linggong
We previously reported a thermochemical route to 1,3-buta-
diene, 1,3-pentadiene, and isoprene, from the vapor-phase
dehydra-decyclization of biomass-derived saturated five-mem-
Road, Dalian, 116024, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc00136h
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3
6,37
bered cyclic ethers tetrahydrofuran (THF), 2-methyl- nium transition state ion pair formed upon ring opening,
tetrahydrofuran (2-MTHF), and 3-methyltetrahydrofuran and is extensively studied for hydrodeoxygenation to penta-
1
1,12
(
3-MTHF), respectively.
vity to corresponding dienes on phosphorus-containing all- catalysts.
silica zeolites. However, the nature of active sites and the sur- surrogate cyclic ether to probe weakly acidic materials in more
These studies reported high selecti- nols, and consequently pentane, on metal-phosphide
3
8–41
Due to its high reactivity, 2-MTHF can act as a
3
7
rounding silica framework in these materials remains detail for dehydra-decyclization.
1
3,14
nebulous.
Cho et al. reported the inability of these sites to
In this study, we consider aluminum- and boron-containing
catalyze 2-propylamine Hoffman elimination, and to protonate zeolites in three frameworks (MWW, MFI, and BEA) to evince
1
4
pyridine. Most recently, Gorte and co-workers have reported the role of heteroatom identity and confining environments in
the onset temperature of H–D exchange of toluene on phos- the dehydra-decyclization of 2-MTHF-to-1,3-pentadiene. Using
phorus self-pillared pentasil (P-SPP) to be ≥100 K higher than apparent kinetic measurements, B-sites are shown to exhibit
aluminosilicates, highlighting the weakly acidic nature of higher diene selectivities, albeit at significantly lower rates
1
5
these materials. It is also important to note that these phos- than aluminosilicates (as normalized by total Brønsted acid
phorous active sites are distinctly different from the well- site counts). Moreover, borosilicates are shown to have ca. 3–6
studied Brønsted acidic sites in silicoaluminophosphates times more total turnovers than aluminosilicates under inves-
1
6–18
(commonly known as SAPO zeotypes);
the P-OH function- tigated reaction conditions (T = 573 K, p2-MTHF = 25–26 Torr,
alities on aluminum-free all-silica support in P-zeosils are sig- initial 2-MTHF conversion in the range 21–26%). The diene
nificantly weaker. Based on these observations, the role of acid distribution remains far from equilibrium, and higher bed
site strength on dehydra-decyclization was considered on solid residence times predictably lead to increments in 1,3-penta-
Brønsted acids with relatively well-understood active sites diene (1,3-PD) formation rates, consistent with its favorable
weaker than bridged hydroxyls in aluminosilicates.
thermodynamic conformation compared to 1,4-pentadiene
The tuning of Brønsted acidic site strength has been (1,4-PD). The diene distribution is also framework-dependent;
implemented in zeolites by incorporating different trivalent 10-MR channels in MFI and MWW show higher preference to
1
9–22
atoms in the framework.
Iglesia and co-workers have 1,3-PD over 1,4-PD than 12-MR channels in BEA. These find-
reported deprotonation energy (DPE) determined from peri- ings are then utilized to achieve a stable ∼86% 1,3-PD yield on
odic density functional theory calculations as a measure of the boron-containing MWW framework.
2
3
acidic strength of microporous solid acids. Furthermore, the
3
+
3+
same group has shown the effect of heteroatoms like Al , B ,
3
+
and Ga in MFI framework on DPE values, indicating that the
acid site strength among heteroatom-substituted zeolites
2
Materials and methods
2
4
2.1 Material synthesis and characterization
increases as H-Al-MFI > H-Ga-MFI ≫ H-B-MFI. Notably, boro-
−
1
silicates exhibit ∼80–100 kJ mol higher DPE values than alu- The following were purchased and used without any further
2
4,25
minosilicates, making them weakly acidic.
It is therefore treatment: 2-methyltetrahydrofuran (2-MTHF, ≥98% with BHT
unsurprising that purely borosilicate zeolites are rarely used as stabilizer, TCI Chemicals), pyridine (99.8%, Sigma Aldrich),
for acid catalysis and have been traditionally used either as tert-butylamine (≥98%, Sigma Aldrich), 1,4-pentadiene (≥98%,
2
6–28
precursor to the synthesis of Lewis acid catalysts,
or more TCI chemicals), 1,3-pentadiene (cis- and trans-mixture, ≥98%,
recently to modify the textural or acidic properties of TCI Chemicals), 4-penten-1-ol (≥99%, Sigma Aldrich),
2
9–32
aluminosilicates.
Among other examples, Chen et al. have 4-penten-2-ol (≥99%, Sigma Aldrich), 3-penten-1-ol (≥95%,
reported that incorporation of the appropriate amount of Alfa Chemistry), 2-penten-1-ol (≥95%, trans-, Sigma Aldrich)
boron concentrates the aluminum more selectively in the sinu- 2,6-di-tert-butyl pyridine (DTBP, ≥97%, Sigma Aldrich), silicon-
2
9
soidal channels of MWW; others have been able to tune dioxide (quartz chips, 4–20 mesh, Sigma Aldrich), tetrapropyl-
aluminum siting by competitive boron incorporation in other ammonium hydroxide solution (TPAOH, 40 wt% in water,
3
2,33
34
frameworks including MFI
and BEA.
Furthermore, Sigma Aldrich), NaOH (Macron Chemicals), fumed silica (Cab-
Gounder and co-workers have recently shown that boron incor- o-sil M5, scintillation grade, Acros Organics), ammonium
poration can be used to regulate crystallite sizes in MFI inde- nitrate (≥98%, Sigma Aldrich), boric acid (≥97%, Macron
3
0
pendent of total aluminum content.
2 4 7
Chemicals), anhydrous borax (Na B O , ≥99%, Sigma Aldrich),
We have recently highlighted the mechanisms and path- tetrapropylammonium hydroxide (TPAOH 40 wt% in water,
3
5
ways for THF dehydra-decyclization on ZSM-5. These cyclic Alfa Aesar), piperidine (99%, Sigma Aldrich), tetraethyl-
ethers undergo rate-limiting ring-opening on Brønsted acid ammonium hydroxide solution (TEAOH, 40 wt% in water,
sites to an alkoxide, which rearranges and dehydrates to form Sigma Aldrich), hexamethyleneimine (≥99%, Sigma Aldrich),
unsaturated alcohols as intermediates, followed by their de- tetraethyl orthosilicate (TEOS, ≥99%, Sigma Aldrich), cetyltri-
hydration to form dienes. The key competing pathway is retro- methylammonium chloride (CTAB, Sigma-Aldrich, 99%
Prins condensation, which results from the fragmentation of purity), sodium aluminate (MP Biomedicals).
+
the adsorbed C
n
ether/alkoxide to Cn−1 olefin and formal-
NH
4
form of ZSM-5 (CBV8014, Si/Al = 40), and Al-BEA
12.5) were purchased from Zeolyst
towards C–O bond rupture due to a kinetically relevant carbe- International. MCM-22 (Si/Al = 24) was synthesized using hexa-
dehyde. 2-MTHF is more reactive than THF and 3-MTHF (CP814C, Si/Al
=
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methyleneimine as the structure-directing agent using the inate temperature gradients across the bed. Reactant partial
4
2
same procedure reported by Corma and co-workers, (detailed pressures were varied using a combination of carrier gas flow-
procedure is included in section S1.1 in the ESI†). MCM-36 rates (10–150 sccm) by a mass flow controller (Brooks
was prepared by swelling and pillaring of MCM-22 as reported Instruments 5850E), and volumetric flowrate of the reactant
4
3
by Maheshwari et al. (detailed procedure is included in using a syringe pump (Cole Parmer 74905 series). All transfer
section S1.1 in the ESI†). Boron-containing zeolites in three lines were maintained at temperatures ≥400 K to avoid con-
2
6
44,45
24
different frameworks MWW, BEA,
and MFI were syn- densation of any species. Online analysis of the reactor
thesized modifying existing hydrothermal synthesis pro- effluent was performed using a gas chromatograph (Agilent
cedures, and detailed steps are included in section S1.1 in the 6890) equipped with a quantitative carbon detector (QCD,
4
6
ESI.† All catalysts were calcined in a boat placed within a 1″ Polyarc™) and a flame ionization detector (FID). Separation
quartz tube under air flow at 823 K using a ramp rate of 2 K was performed using an HP-PLOT column (Agilent,
Q
−
1
min for 10 hours prior to any catalytic testing. We employed 19091P-QO4). All carbon mass balances closed to within
aluminosilicates with relatively low Si/Al (in the range of 12.5 ±10%. Unless otherwise specified, error bars represent 95%
to 40) to keep them comparable to bulk boron loadings in the confidence intervals on independent replicate measurements
borosilicate analogues.
on fresh catalyst beds from the same batch. As noted earlier,
Powder X-ray diffraction (XRD) patterns were collected on a retro-Prins condensation of 2-MTHF produces butenes and for-
Bruker AXS D5005 diffractometer using Cu Kα radiation (= maldehyde in equimolar ratio, and the amount of formal-
1
.5418 Å) with a step size of 0.02° and a step time of four dehyde was indirectly calculated by the quantification of
seconds. Inductively Coupled Plasma Optical Emission butenes.
Spectrometry (ICP-OES) was used for elemental analysis.
A combination of Mears’ and Weisz-Prater criterion was
Textural information of all synthesized samples was character- used to investigate the external and internal mass transfer
4
7
ized through Ar physisorption in an Autosorb iQ2 porosimetry limitations, respectively (section S2 in the ESI†). Under near
instrument (Quantachrom). Prior to analysis, catalysts were differential conditions (X2-MTHF < 13%), while the rates for BEA
outgassed at 573 K for six hours and subsequently cooled and MFI (with both heteroatoms) were found to be free from
down to room temperature under vacuum. BET specific any diffusional limitations, such a conclusion could not be
surface area measurements were used to represent the total reached for B-MWW and MCM-22, and the reported rates on
surface area of the catalyst materials; total pore volume was these materials likely reflect a complex interplay of reaction
0
determined using a single point measurement at P/P = 0.95. and intra-crystalline diffusion. All aluminosilicates deactivated
SEM was performed on a JEOL JSM-6500F scanning micro- under reaction conditions over time scales of a few hours, and
scope operated at 2.0 kV. All TEM images were obtained with a all reported rates were corrected to a bare catalyst surface by
FEI Tecnai G2 F30 TEM operating at 300 kV using a charge- using a first-order deactivation model and extrapolating
coupled device (CCD) camera. MAS NMR experiments were obtained rates to time zero. Kinetic experiments at different
performed using a Bruker DSX-500 and a Bruker 4.0 mm MAS temperatures were conducted using fresh catalyst beds to mini-
probe. The spectral frequencies were 500.2, 160.5, and mize systematic errors. Alternatively, borosilicates deactivated
1
11
29
9
9.4 MHz for H, B, and Si nuclei, respectively. Samples at much longer time scales than aluminosilicates, and
1
11
were spun at 13 kHz for H and B detections, and 8 kHz for reported rates on these materials were all corrected to a refer-
Si. For B MAS NMR, 0.5 s π/12 pulse was used. The chemi- ence condition of 523 K to account for any small intervening
cal shifts were calibrated to external references of TMS for H deactivation during kinetic measurements. Brønsted acid site
and Si, and BF
2
9
11
1
2
9
11
3
(OEt)
2
for B.
densities of all aluminosilicates were measured using the
Hoffman-elimination of tert-butylamine. The Reactive Gas
Chromatography (RGC) technique was used to quantify all
2
.2 Catalytic experiments
48
All kinetic measurements were performed in the temperature butene isomers resulting from Hoffman elimination of tert-
range of 430–550 K at total pressures of 1.00–1.05 bar in an butylamine on a Brønsted acid site (BAS), assuming each
upflow fixed bed reactor. Catalyst samples were pressed and butene molecule was produced on a unique BAS.
sieved to particle sizes ranging from 250–500 µm and placed
Previous works by Gorte and co-workers shows that alkyl-
between deactivated quartz wool plugs in a 1/4″ quartz U-tube. amines can desorb without undergoing Hoffman elimination
4
9
Void volume in the tube was minimized by loading quartz on the acid sites in borosilicates due to their low reactivity.
chips upstream of the catalyst bed. A 1/16″ K-type thermo- Moreover, the nature of these sites as well as boron coordi-
5
0
couple (Omega) was placed just above the catalyst bed for nation changes with the extent of hydration, and it is likely
1
1
temperature measurements. The furnace temperature was that part of tetrahedral boron in solid state B MAS NMR is
regulated by using a temperature PID controller (Omega CN actually trigonally coordinated under reaction conditions. To
7
800). All catalyst samples were calcined in situ at 823 K in 40 account for these factors, the BAS count for borosilicates was
sccm air (99.997%, Minneapolis Oxygen) using a ramp rate of estimated by in situ pyridine titration experiments during
.0 K min− . The reactor was thereafter cooled to reaction 2-MTHF dehydra-decyclization in the temperature range
1
3
temperature and purged with He (99.995%, Matheson) for at 453–477 K. 2-MTHF dehydra-decyclization was carried out
least 30 minutes prior to introducing the reactant feed to elim- until initial transients subsided, and an instantaneous switch
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ð
t X
was made to the 2-MTHF/pyridine mixture (molar ratio ∼
1
NHþ
TONðtÞ ¼
n ꢁ FC ðt′Þdt′
n
3
10 : 1) at identical volumetric flowrate. Total 2-MTHF con-
0
n
ð
ð3Þ
sumption rate was monitored with titrant uptake and linearly
extrapolated to zero rates to quantify the total number of cata-
lytically relevant acid sites in borosilicates. Experiments with
selective DTBP titrations to deconvolute the catalytic role of
different pore systems in aluminum-containing MWW (namely
MCM-22, and MCM-36) were conducted at 453 K, sufficiently
lower than temperatures corresponding to the onset of DTBP
t X
¼
n ꢁ STYC ðt′Þdt′
n
0
n
TONðtÞ
ftotal‐turoversðtÞ ¼
ð4Þ
TONðtX2
Þ
MTHF ꢂ3%
where F
i
is the molar flow-rate of product i, N
acid site (BAS) density, STY is the site time yield for product i,
H
+
is the Brønsted
5
1,52
desorption on aluminosilicates.
Similar to in situ pyridine
i
titration on borosilicates, 2-MTHF dehydra-decyclization was
carried out for a fixed time (ca. 175 minutes), and an instan-
taneous switch was made to 2-MTHF/DTBP mixture (molar
ratio ∼ 650 : 1) at identical volumetric flowrate. Separation of
compounds for these measurements were performed with a
Restek RTx-5 column.
mcat. is mass loading of the catalyst, and n is the number of
carbon atoms in product i.
i
3
. Results and discussion
Experiments to evaluate the stability of all catalysts were 3.1 Characterization of synthesized materials
performed at 573 K and p2-MTHF ∼ 25 Torr; bed residence
Powder X-ray diffraction patterns (PXRD), scanning and/or
times and carrier gas flowrates were adjusted on different cata-
lysts to achieve an initial conversion of ∼21–26%. To provide a
quantitative description of stability, we used total turnovers
during catalyst lifetime (eqn (3)), which is a measure for the
total moles of 2-MTHF converted to carbonaceous products
per proton over the entire lifetime of catalyst. Total TONs
were calculated until the conversions for all catalysts decreased
to ≤3%. The time scales to achieve this criterion for borosili-
cates was 300–470 h on stream, while aluminosilicates comple-
tely deactivated within 24–75 hours. Furthermore, by calculat-
ing the fraction of total turnovers (eqn (4)), product selectiv-
ities (eqn (2)) were compared as a function of reaction progress
allowing us to compare selectivities on catalysts with non-iden-
tical total turnovers.
transmission electron microscopy images (SEM/TEM), and Ar-
porosimetry measurements of MCM-22/MCM-36/B-MWW,
ZSM-5/B-MFI, Al-BEA/B-BEA are reported in Fig. S1, S2, and S3,
respectively, of the ESI (sections S1.2–1.4†). A detailed discus-
sion of the boron environments and the corresponding results
of B and Si MAS NMR are included in the ESI (section S1.5
and Fig. S4†); these data were found consistent with previous
reports on borosilicate zeolites with three-, and four-co-
ordinated boron (details in section S1.5). Elemental analyses,
textural properties, Brønsted acid site densities, and crystallite
sizes of all catalysts are listed in Table 1.
5
3
11
29
Trace aluminum impurities in borosilicates have frequently
30
lead to misinterpretation of their catalytic properties, and
the catalytic rates with boroaluminosilicate materials have
been shown to scale with aluminum contents as low as
Fi
54,55
∼100 ppm.
To avoid these artifacts in this work, high-
STYi ¼
ð1Þ
mcat::NH
þ
purity silica source Cab-o-Sil M5 with aluminum content <
1.73 ppm (ICP-MS, Galbraith laboratories) was used instead of
Ludox colloidal silica for all borosilicate synthesis to eliminate
the presence of aluminum, and ICP-OES (Galbraith labora-
tories) showed Si/Al ≥ 11 000 in synthesized borosilicates,
leading to ≤1.5 µmol g− BAS density from trace aluminum,
indicating that the synthesized borosilicates are sufficiently
Selectivity ð%C basisÞ ¼
i
Total carbon present in the producti
ꢀ
100
Total carbon from the reactanct converted to products
1
ð2Þ
Table 1 Structural, textural, and acidic properties of all zeolites
a
BET surface area (m g−1)
b
2
Total pore volume (cm g−1)
c
3
BAS (µmol g−1
+
g
Catalyst
Provenance
Si/T
)
H /T
RSEM (µm)
d
MCM-22
MCM-36
B-MWW
ZSM-5
B-MFI
Al-BEA
B-BEA
This work
This work
This work
Zeolyst
This work
Zeolyst
24.1
32.3
13.2f
40.0
38.3f
12.5
23.8
562
645
506
405
510
439
474
0.28
0.41
0.30
0.26
0.29
0.29
0.28
447.6
233.8
149.6
345.3
49.5
624.6
61.1
0.68d
0.47
0.13
1.8 ± 0.6
6.3 ± 1.2
15.3 ± 0.7
0.44 ± 0.09
0.32 ± 0.11
0.58 ± 0.12
3.7 ± 0.4
e
d
0.81
e
0.11
d
0.46
e
This work
0.09
a
b
c
(
T = Al/B) determined by ICP-OES (Galbraith Laboratories). Determined from Ar adsorption–desorption isotherms. Determined from Ar
d
adsorption–desorption isotherms at P/P = 0.95. Obtained by the quantification of butenes formed from the Hoffman elimination of tert-butyla-
mine. Obtained during in situ pyridine titration during 2-MTHF dehydra-decyclization in the temperature range 453–477 K. From the manufac-
turer. Calculated from at least 50 crystallites from their respective SEM micrographs, with the errors indicating a 95% CI on measurements.
0
e
f
g
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free from aluminum impurities. While deboronation of the ation, we did not observe the recovery of rates even at 477 K for
borosilicates can occur under less severe conditions than alu- over two hours, which indicates that pyridine remains a revers-
minosilicates (section S1.5 in the ESI†), we have not carried ible yet strongly bound titrant for B-sites initially titrated in
out varying extents of deboronation to rigorously study its the temperature range investigated. The irreversible nature of
effect on activity trends in this work.
pyridine adsorption has previously been shown at slightly
The acid site density in borosilicates was measured using lower temperature (433 K) during methanol dehydration on
2
4
in situ pyridine titration during 2-MTHF dehydra-decyclization B-MFI. The results (Fig. 1) confirm the presence of acid sites
in the temperature range 453–477 K. The mole fraction of pyri- which can protonate pyridine in these materials, the quantifi-
−
5
dine was kept low (∼1.9 × 10 ) so the partial pressure of cation of which allows for the calculation of site time yields
-MTHF before and after its introduction could be assumed (STYs) for the dehydra-decyclization pathway. Notably, the
2
the same. The reduction in mass-normalized total 2-MTHF number of protons per heteroatom for boron-containing zeo-
conversion rates were monitored with the uptake of the titrant, lites remains significantly lower than corresponding values for
and the total number of B-sites were calculated by the cumu- aluminosilicates (Table 1), as has been reported by Jones et al.
2
4
lative pyridine uptake to completely suppress the rates (Fig. 1). previously for B-MFI. Unlike aluminosilicates, the site hom-
While the titration for B-MWW was carried out at 453 K, the ogeneity indicated by framework-independent DPE values has
corresponding experiments for B-MFI and B-BEA were carried not been established for borosilicates, and it is possible that a
out at 473 K and 477 K, respectively, due to their significantly majority of boron in the borosilicates is either not associated
lower reactivity than B-MWW (also discussed in section 3.2).
with a proton, or a fraction of protons associated with B lack
Pyridine was found to titrate all the B-sites relevant in the acidic strength to protonate pyridine irreversibly under the
-MTHF dehydra-decyclization on B-MWW, as shown by the reaction conditions. However, it is reasonable to assume that
2
complete suppression of rates (Fig. 1). For B-BEA and B-MFI, the fraction of B sites unable to protonate pyridine (proton
−
1 56
the rates dropped to ∼85–90% of their initial values. Pyridine affinity = 930 kJ mol
)
cannot protonate 2-MTHF (proton
saturation was ascertained by monitoring pyridine break- affinity = 851 kJ mol− ) either. Therefore, the reported STYs
through from the catalyst bed, and multiple gas-sampling on these materials are likely accurate even if this method does
injections with constant pyridine peak areas (with values not titrate all acid sites in borosilicates.
1
57
sufficiently close to 2-MTHF : pyridine molar ratio of ∼300)
As shown in Fig. 1, the mass-normalized 2-MTHF consump-
were observed to ensure pyridine saturation. Furthermore, tion rates for B-MFI were almost twice the corresponding value
upon switching back the feed to 2-MTHF after pyridine satur- for B-BEA, even though total boron content in B-BEA was
∼
1.5× higher than B-MFI. Moreover, B-MFI and B-BEA had
−
1
similar total BAS counts (∼50 and ∼60 µmol g , respectively)
Table 1). Thus, the dehydra-decyclization site time yields for
(
B-MFI were ∼2.4× higher than B-BEA, even at 5 K lower temp-
erature. This observation of higher rates in low-boron content
B-MFI (Si/B 38) vs. high boron-content B-BEA (Si/B 24) points
to the lack of quantitative correlation between bulk boron con-
tents and rates, and underpins the necessity to probe the
acidity of borosilicates strictly under reaction conditions due
to their likely complex speciation behavior and changes in
coordination upon contacting basic molecules including oxy-
genates. We believe that these results indicate that the relative
distribution of active boron species is different in the three
borosilicates investigated; i.e., bulk boron content does not
correlate with active boron species under reaction conditions.
An accurate explanation for this framework-dependent cata-
lytic behavior of borosilicates would involve the estimation of
Brønsted acid site strength of boron sites in these three frame-
works. However, the different possible boron environments
(
Fig. S4 in the ESI†) as well as the presence of crystallographi-
Fig. 1 Total consumption rate of 2-MTHF as a function of pyridine
uptake over B-MWW (■, black), B-MFI ( , blue), B-BEA ( , red); inset
cally distinct T-sites which may differ in their DPE values,
makes the estimation of a single representative DPE value
shows the corresponding data for B-BEA on a magnified scale. Titrant
flow is an equivalent volumetric flowrate of 2-MTHF/pyridine with difficult without also having a priori knowledge of relative dis-
2
-MTHF: pyridine ∼ 310. Inset shows the data for B-BEA on a magnified
scale (reaction conditions: T = 453 K for B-MWW, 473 K for B-MFI, and
77 K for B-BEA; P2MTHF = 10.5 Torr for B-MWW, 4.5 Torr for B-BEA and
tributions of these sites under reaction conditions. The dis-
tinct behavior of boron sites in SOD, and FER frameworks has
been highlighted before by Fois et al., who probed these sites
by first principles calculations to study the non-identical
4
B-MFI; WHSV = 0.60–4.50 g 2-MTHF per g cat. per h; carrier gas (He)
flowrate = 60 sccm for B-MWW, 40 sccm for B-MFI and B-BEA; X2MTHF
≤
5
8
1%).
nature of ammonia adsorption on these materials.
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Although the reactivity of borosilicates is unlikely to orig- has been previously implemented for several chemistries.
inate from aluminum impurities, our data don’t rule out the Some examples where boron-modified zeolites were utilized
possible role of defect silanol nests in catalysis; the aberration for this purpose include: dehydration of 2,3-butanediol to
5
9
in the rates of B-BEA and B-MFI can be rationalized by consid- butanone over boric acid impregnated on ZSM-5; Beckmann
ering silanol density differences in B-BEA and B-MFI. However, rearrangement of cyclohexanone oxime to caprolactam on
6
0,61
62
we find it unlikely for this to result in observed reactivity differ- B-MFI,
and B-ZSM-5; catalytic cracking of MTBE to isobu-
6
3
ences between these two samples (as high as ∼8× in Fig. 2(A)), tene and methanol on B-MFI; intramolecular Prins-cyclisa-
especially when the comparison is not between a defective tion of citronellal on B-TUD-1; isomerization of 1-hexene on
sample and a defect-free sample (both B-MFI and B-BEA were B-MCM-41, dehydrative aromatization of 2,5-dimethylfuran
synthesized under non-HF conditions, and are expected to (DMF) to p-xylene on B-BEA, and dehydration of 2-methyl-
6
4
6
5
2
5
6
6
have defect silanols present).
butanal to isoprene.
The known reaction pathways during the conversion of
-MTHF over Brønsted acid sites are shown in Scheme 1.
Consistent with this scheme, major products observed in our
3
.2 Effect of heteroatom identity in dehydra-decyclization
2
selectivity
The application of borosilicates and boron-modified alumino- reactor effluent included 1,3-,and 1,4-pentadiene (dehydra-
silicates to selectively suppress the production of side reactions decyclization products), butenes and formaldehyde (retro-
Fig. 2 (A) Arrhenius plots of apparent kinetics for the dehydra-decyclization of 2-MTHF, and (B) temperature dependence of dehydra-decyclization
(
DH) to retro-Prins condensation (RP) rate ratios over B-MWW (■, black), MCM-22 (□, black), B-MFI ( , blue), ZSM-5 ( , blue), B-BEA ( , red), and Al-
BEA ( , red); X2MTHF denotes the conversion of 2-MTHF (reaction conditions: p2MTHF = 10.5 Torr, space velocity = 1.10–10.35 mol 2-MTHF per H per
+
min, carrier gas (He) flowrate = 60 sccm, X2MTHF ≤ 13%). Dashed lines in (a) are fits to Arrhenius equation, and dashed lines in (b) are to guide the eye.
+
Brønsted acid site counts (mol H ) for aluminosilicates and borosilicates are quantified using the Hoffman elimination of tert-butylamine, and in situ
pyridine titrations during 2-MTHF dehydra-decyclization, respectively. Error bars in (b) are standard errors originating from uncertainty in the esti-
mation of retro-Prins condensation (RP) rates for borosilicates due to their low activity coupled with low selectivity to this pathway.
11,12,37,87–89
Scheme 1 Known reaction pathways for the conversion of 2-MTHF over solid Brønsted acids.
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Table 2 Measured apparent kinetic parameters of 2-MTHF dehydra- given temperature (523 K). These results indicate that the spe-
decyclization on different zeolites
ciation behavior of active sites in these materials is catalytically
different depending on B siting and/or the micropore environ-
a
E
(
app,DH
STYDH
STYRP
DH/RP
ratio
−
1
(×10−3 s−1
(×10−3 s−1
)
ments around the B-site. Furthermore, the nature of apparent
kinetics for the dehydra-decyclization pathway remains similar
on borosilicates as indicated by similar apparent barriers as on
aluminosilicates (Table 2).
The suppression of the retro-Prins (RP) condensation
pathway on borosilicates is much greater (∼30–50×) than the
suppression of dehydra-decyclization (DH) pathway, leading to
higher diene selectivities (Fig. 2B and Table 2). Consequently,
these materials show ∼6×–30× higher DH/RP rate ratios than
kcal mol
)
)
b
b
ZSM-5
21.5 ± 1.5
19.9 ± 1.4
—
7.11b
1.94b
3.7
5.1
8.7
49.2 ± 10.1
31.9 ± 12.6
24.6 ± 6.4
Al-BEA
MCM-22
B-MWW
B-MFI
5.92
1.17
0.75
b
b
6.50b
b
—
1.65b
0.038b
22.5 ± 2.3
18.2 ± 0.8
0.42
0.013
0.007
c
c
B-BEA
0.17
a
Determined by apparent kinetic measurements under conditions
described in Fig. 2, where the errors are 95% CI on the slope. The
STYs on MWW materials were not under strict kinetic control under
reaction conditions (section S2 in the ESI†), and therefore Eapp,DH is aluminosilicates under near-differential conditions (X2-MTHF
<
b
c
not reported on these materials. Reported at T = 512 K. Reported at
T = 522 K. Errors in the calculations of retro-Prins condensation STYs
for borosilicates are reflected in the standard errors of the corres-
ponding DH/RP rate ratios.
13%) (Fig. 2B). This likely results from the inability of weakly
acidic B-sites to fragment a C–C bond (the rate-determining
3
5
step for retro-Prins condensation ). Furthermore, this behav-
ior is not 2-MTHF conversion-dependent, and borosilicates
show ∼90% selectivity to dienes across all conversions, which
is ca. 10–30% higher than aluminosilicates under similar reac-
Prins condensation products), propene, pentenes, and large tion conditions (Table S2 in ESI†). Furthermore, different
organics (identified as aromatic C6+ fraction). We conducted extents of boron content, albeit in different micropore environ-
apparent kinetic rate measurements in the temperature range ments, do not seem to affect total diene selectivities (which
4
53–573 K to assess the relative rates and selectivities to remain >87% as shown in Table S2†). These results, taken
dehydra-decyclization and retro-Prins condensation, and the together, provide experimental evidence that weakly acidic bor-
results are shown in Fig. 2.
osilicates limit the kinetic branching to competing retro-Prins
The dehydra-decyclization rates were found invariant across pathway in this chemistry.
the three frameworks in aluminosilicates (Table 2), indicating
3
.3 Distribution of diene products
that kinetically relevant ring-opening transition-state (TS) ion-
3
5
pair is likely stabilized to the same extent in MWW, MFI and Besides the production of butenes and formaldehyde in the
BEA. Consequently, the apparent activation energies for the competing retro-Prins reaction pathway, another challenge to
dehydra-decyclization pathway on aluminosilicates (BEA, and selectively produce the conjugated 1,3-pentadiene (1,3-PD) is
MFI) are ∼20 kcal mol (Table 2). These results are consistent the concurrent production of the non-conjugated 1,4-penta-
with Kumbhalkar et al., who have also reported identical diene (1,4-PD) during 2-MTHF dehydra-decyclization. This is
proton-normalized site-time yields for pentadienes’ production illustrated in Scheme 2; 2-MTHF can ring-open from the more
over aluminosilicates having different micropore environments substituted ‘C-2 side’ or the less substituted ‘C-5 side’. Ring
and extra-framework aluminum (ZSM-5 (Si/Al = 11.5), Al-BEA opening from the ‘C-2 side’ is likely more favorable, given the
−1
3
7
(
Si/Al = 12.5), and Al-MOR (Si/Al = 10)). The apparent carbenium character of the kinetically relevant transition state
−
1
dehydra-decyclization barrier of 17.7 kcal mol on amorphous associated with ether ring opening, and leads to the formation
silica alumina in the temperature range 570–660 K is also of primary alkenols, namely, 3-penten-1-ol (E2 type elimination
qualitatively close to the values obtained by us, and small of H ) or 4-penten-1-ol (E2 type elimination of H ). While the
β1
β2
differences are likely caused by changes in apparent kinetics dehydration of 3-penten-1-ol leads to 1,3-pentadiene, 4-penten-
resulting from the differences in surface coverages at high 1-ol dehydration leads to 1,4-pentadiene. Alternatively, the alk-
temperature conditions employed in their study. Notably, oxide formed upon ring-opening from the ‘C-5’ side can only
−
1
these activation barriers are ∼10–12 kcal mol lower than form a secondary alkenol, namely, 4-penten-2-ol. This alkenol
four-carbon THF-dehydra-decyclization on ZSM-5 reported in can further undergo dehydration to form 1,3-pentadiene (E2
3
5
our earlier work under similar reaction conditions, providing type elimination of H ) or 1,4-pentadiene (E2 type elimination
β1
further corroboration of the promoting-effect of a methyl sub- of Hβ2).
stituent on the stabilization of the kinetically relevant tran-
sition state.
To assess the validity of Scheme 2, we conducted experi-
ments by feeding pure C5 alkenols, namely, 4-penten-1-ol,
Borosilicates catalyze the dehydra-decyclization pathway at 3-penten-1-ol, 4-penten-2-ol, and 2-penten-1-ol, over ZSM-5 at
STYs which are at-least ∼5× lower than aluminosilicates lower temperature (413 K) than 2-MTHF reaction temperatures,
(Fig. 2A). Furthermore, they exhibit different dehydra-decycli- primarily to maintain differential conditions given the facile
zation STYs depending on framework type unlike aluminosili- dehydration of these alkenols. It was found that the preference
cates. Remarkably, there is approximately an order-of-magni- to produce 1,3-pentadiene from different alkenols follows
tude difference in the dehydra-decyclization STY between the 2-penten-1-ol ≫ 4-penten-2-ol > 3-penten-1-ol ∼ 4-penten-1-ol
most active (B-MWW) and least active (B-BEA) borosilicate at (Fig. S10 in the ESI†). Neither 3-penten-1-ol nor 4-penten-1-ol
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Scheme 2 Proposed pathways for the production of 1,3-pentadiene and 1,4-pentadiene from 2-MTHF over Brønsted acid sites.
was purely selective to 1,3-pentadiene and 1,4-pentadiene, isomerization of the non-conjugated 1,4-PD. They also imply
respectively. Moreover, the most active and selective intermedi- that the spatial gradients in diene concentrations are observa-
ate to 1,3-pentadiene, namely 2-penten-1-ol, does not result ble on the length scales of reactor bed lengths, and a fraction
directly from a simple E2 elimination of the adsorbed alkox- of 1,4-pentadiene formed at small bed length desorbs and
ides indicated in Scheme 2. Previous reports have highlighted reacts further down in the bed to form the thermodynamically
that the migration of a double bond on a carbon chain is a low favored 1,3-PD.
6
7,68
barrier step,
and consistent with this, these results indi-
One would expect the more reactive protons in aluminosili-
cate that both the intermediate primary alkenols as well as the cates to be more consequential in depleting unfavorable 1,4-
produced dienes can inter-convert on a Brønsted acid site, pentadiene concentrations, and it was expected that the 1,3-
which limits mechanistic interpretations about the origin of PD/1,4-PD rate ratio would depend more sensitively on space
diene distribution directly from 2-MTHF.
time (or, equivalently, show higher slopes w.r.t. space time) on
3
.3.1 Tuning selectivity to 1,3-pentadiene. 1,3-PD is aluminosilicates than borosilicates. Interestingly, this was not
3
7,69
thermodynamically more stable than 1,4-PD.
phase thermochemistry data from NIST, it was found that all known to catalyze double bond isomerizations,
Using gas- found to be the case (Fig. 3). While aluminosilicates are
7
0–72
this obser-
diene distributions were far from equilibrium under the reac- vation suggested that even the weakly acidic borosilicates can
tion conditions in this study; selectivity to 1,3-PD was not equi- isomerize 1,4-PD to 1,3-PD. This hypothesis was tested by
librium-limited (section S.4 in ESI†). 1,3-PD was formed as a feeding pure 1,4-pentadiene and 1,3-pentadiene over borosili-
favored diene with both borosilicates and aluminosilicates, cates, and the results are shown in Fig. S9.† 1,3-pentadiene
and there was no obvious correlation between the 1,3-PD/1,4- did not show any conversion to 1,4-pentadiene on all borosili-
PD selectivity ratio and heteroatom identity in the range of cates, consistent with its significantly higher thermodynamic
space times investigated (Fig. 3), which is not surprising given preference than 1,4-PD. Furthermore, the preference towards
this ratio would not only depend on the reactivity of protons, 1,3-pentadiene on borosilicates (B-MWW > B-MFI > B-BEA)
but also on their volume density, as well as the corresponding (Fig. 3) was directly correlated with their reactivity for 1,4-pen-
crystallite sizes. However, the preference towards 1,3-PD tadiene isomerization (Fig. S9†), confirming that the isomeri-
increased with reactor space times on all investigated zeolites zation of 1,4-pentadiene remains facile, even on the weakly
irrespective of the heteroatom identity. For aluminosilicates, acidic borosilicates.
the total diene production rates were higher at low space
Interestingly, 12-MR BEA with both heteroatoms (B and Al)
times, and the increase in 1,3-PD production was only partly consistently exhibited a lower 1,3-PD/1,4-PD ratio than MWW
as a result of secondary enhancements due to 1,4-PD isomeri- and MFI frameworks across two decades of space times
zation. However, on borosilicates, the increments in 1,3-PD (Fig. 3). Furthermore, the preference to 1,3-PD was more pro-
formation rates was concommitant with the decrease in 1,4-PD nounced when these measurements were not in strict kinetic
production such that the overall diene selectivity remained regime (X2-MTHF = 30–60% in Fig. 3). These observations led to
+
nearly unchanged in the space times investigated (0.01–10 H
our hypothesis that differences in 1,3-PD production are likely
min mol− 2-MTHF). These results, taken together, indicate through different extents of diffusion-enhanced isomerization
that the diene ratio can be tuned towards the conjugated 1,3- of 1,4-PD rather than differences in diene kinetic preference
PD by operating under reaction conditions which facilitate the between the investigated frameworks. For example, 10-MR
1
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restricted selectively to a pore system smaller than in MFI
(
intra-layer 10-MR sinusoidal channels).
.3.2 Deconvoluting the pore systems of aluminum-con-
3
taining MWW in catalysis. MWW is formed by calcination of a
precursor layered material, and consists of two independent
pore systems (Fig. 4a). The intra-layer pore network consists of
two-dimensional sinusoidal 10-MR channels (4.1 × 5.1 Å).
Perpendicular to these sinusoidal channels are larger super-
cages (7.1 × 7.1 × 18.1 Å) interconnected by elliptical 10-MR
windows (4.0 × 5.5 Å). The transport between the two pore
systems is restricted by 6-MR constrictions. The external
surface of MWW crystals is terminated by supercages, which
leads to the formation of hourglass shaped surface side-
pockets (7.1 × 7.1 × 9.0 Å). The MWW precursor can be swelled
and pillared prior to calcination to preserve the intra-layer
4
3,73
crystallinity.
Given our previous results indicate the higher
preference of 1,3-PD in medium-pore MFI than large-pore BEA,
the presence of two different pore systems makes MWW a suit-
able topology to probe the effects of micropore environments
in the production of 1,3-pentadiene.
Many recent reports have demonstrated the different cata-
lytic performance of acid sites located in these distinct confin-
ing environments in MWW for various chemistries like metha-
2
9,74
75
nol-to-hydrocarbons (MTH),
toluene disproportionation,
7
6
77
and methylcyclohexane /n-heptane cracking. While the sites
in the side pockets are on the external surface and can be
titrated with bulky bases that do not interact with intra-layer
sinusoidal channels, the method cannot be used for titrating
acid sites in the supercages due to their inter-connectivity
through ellipsoidal 10-MR channels. On the other hand,
smaller bases like pyridine have been shown to titrate all sites
7
8
in MWW. However, on swelling and pillaring the MWW pre-
cursor, the supercages don’t collapse to form a 3-D structure
after calcination, and this procedure leads to the preservation
of long-range intra-layer crystallinity while creating inter-layer
mesoporosity (Fig. 4B). Upon successful pillaring, a bulky base
like DTBP can titrate not only the external surface side-pocket
sites, but also the sites in the supercages which are now hour-
glass shaped surface side-pockets located in a mesoporous
environment. This allows probing the catalytic role of different
Fig. 3 Measured selectivity ratios of 1,3-pentadiene/1,4-pentadiene as
a function of reactor space-times over B-MWW (■, black), MCM-22 (□,
black), B-MFI ( , blue), ZSM-5 ( , blue), B-BEA ( , red), and Al-BEA ( ,
red) (reaction conditions: T = 503 K, p2-MTHF = 1.5–120 Torr, carrier gas pore systems in determining the diene distribution by sequen-
(
He) flowrate = 60 sccm). The bracketed values are the corresponding
tially restricting the chemistry to happen in the internal acid
sites by DTBP tiration on MCM-22, and subsequently only in
the 10-MR intra-layer sinusoidal channels by DTBP titration on
MCM-36.
conversion levels. Dashed lines are provided to guide the eye.
channels in MFI likely impose more severe transport restric-
Prior reports have estimated that MCM-22 has ∼8% sites in
tions facilitating 1,4-PD isomerization than 12-MR channels in the external side-pockets, which can be titrated using DTBP
7
8,79
BEA, leading to higher 1,3-PD production rates in MFI. during probe reactions like ethanol dehydration.
However,
However, such direct interpretations could not be drawn for we observed a significantly higher (∼38%) decrease in 2-MTHF
MWW materials due to the highly non-distinct pore systems in consumption rates on MCM-22 upon DTBP saturation (Table 3
its topology (discussed in section 3.3.2). Deconvoluting the and Fig. 4C). It has been shown that DTBP can access pore-
7
9
catalytic role of these distinct pore systems would therefore mouth and channel-intersection sites in MWW, and it is
provide insights into the origin of diene distribution on solid reasonable to expect that the accessibility of DTBP to these
acids. To test our hypothesis, we designed (DTBP) titration sites imposes more severe transport restrictions on 2-MTHF
experiments on alumniumn-containing MWW materials to than ethanol, rendering some untitrated acid sites inaccessible
study the change in diene distributioin as the chemistry was to 2-MTHF. Besides, the ratio of external to internal sites in
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Fig. 4 (A) Detailed schematic of MWW framework indicating the location and relative sizes of all pore systems; (B) detailed schematic of MCM-22
and MCM-36 framework topology; (C) total rate of consumption of 2-MTHF; (D) absolute values of 1,3-PD/1,4-PD rate ratio; and (E) normalized rate
ratio of 1,3-PD/1,4-PD (normalized to t = 0) as a function of time-on-stream after the introduction of DTBP titrant for MCM-22 (□), and MCM-36(☆).
Titrant flow is an equivalent volumetric flowrate of 2-MTHF/DTBP with 2-MTHF : DTBP ∼ 650 : 1 (reaction conditions: T = 453 K, P2MTHF = 10.5 Torr,
WHSV = 3.97–5.05 g 2-MTHF per g cat. per h, X2MTHF ≤ 8%).
Table 3 Comparison of 2-MTHF dehydra-decyclization STYs before ation, the normalized 1,3-/1,4-PD rate ratio on MCM-22
and after saturation by 2,6-DTBP as titrant on MCM-22, and MCM-36
increased with the uptake of DTBP and stabilized at ∼2.8
(
Fig. 4E). As noted earlier, this diene distribution still has con-
STYDH (h−
1
)
Rate loss after DTBP saturation (%)
tribution of intra-layer sinusoidal channels and supercages.
Nonetheless, this result indicates that the diene distribution
shifts towards 1,3-PD when the catalytic contribution of exter-
nal acid sites is completely suppressed. Most notably, the nor-
MCM-22
MCM-36
2.9 ± 0.2
5.3 ± 0.4
37.5 ± 1.4
84.7 ± 2.1
a
Reaction conditions: T = 453 K, P2MTHF = 10.5 Torr, X2MTHF < 8%,
+
space velocities in the range of 0.37–1.25 mol 2-MTHF per H per min. malized 1,3-/1,4-PD rate ratio increased significantly upon
b
Rate loss was calculated using the equation: 1 − (residual rate after
DTBP saturation/rate immediately prior to titrant introduction) × 100
DTBP introduction on MCM-36, and stabilized at ∼4. As noted
earlier, the sites in the super-cages are made accessible
through pillaring of MCM-22. Hence, any residual rate after
DTBP saturation on MCM-36 can be directly attributed to the
intra-layer 10-MR sinusoidal channels. Indeed, when the
chemistry was restricted to occur only in these sinusoidal
channels, the diene distribution became highly skewed in
favor of the conjugated 1,3-pentadiene. Once again, we note
that these measurements were not performed in strictly
kinetic regime (section S2 in ESI†), thus supporting our other
findings that impediment in diffusion facilitates 1,4-PD iso-
merization, thus leading to increments in 1,3-PD selectivity in
medium-pore (10-MR) zeolitic channels.
(
%). All error bars represent a 95% CI from three independent
measurements on fresh/recalcined beds.
MWW depends on the particle size, and a higher drop in rates
could also reflect more external specific surface area versus pre-
vious reports. Both the decrease in the rates (Table 3 and
Fig. 4C) and the timescales over which the rate dropped, were
higher on MCM-36 than MCM-22, clearly indicating a higher
number of sites being accessible to DTBP in MCM-36.
As seen in Fig. 4D, the selectivity ratio of 1,3-PD/1,4-PD was
in the range of ∼2–3 prior to DTBP introduction on MCM-22,
while it was ∼1–1.2 for MCM-36, illustrating the role of pore
environments on the diene distribution; mesopore environ-
3.4 Long-term stability of borosilicates
ments in MCM-36 were detrimental to 1,3-PD production, con- Catalytic evaluation of borosilicates or boron-modified alumi-
sistent with our other findings. Upon the introduction of nosilicates has been shown to increase time-on-stream stability
3
1,32,59,80–82
DTBP, the diene distribution shifted towards the conjugated for a variety of chemistries.
However, recent reports
diene on both catalysts. To study the relative changes in the have highlighted the limitations of using time-on-stream (TOS)
diene distribution independent of absolute values, the for- as a metric of catalyst stability, specially when comparing
5
3,83,84
mation rate ratio of 1,3-PD/1,4-PD was normalized by its initial materials with varying reactivity,
as is the case with the
value (at time zero). This also ensured that any product distri- two classes of zeolites in this study. We therefore use total
bution changes due to catalyst deactivation did not affect the turnover numbers per reactive proton (TON) during catalyst
5
3
mechanistic interpretations. Concomitant with DTBP satur- lifetime as the stability criteria (eqn (3)). As noted earlier,
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turnover number at time-on-stream t is the cumulative moles
of 2-MTHF-derived carbon converted to all products per mole
of active site from time-on-stream 0 to time, t. The reactivity
differences between boro-, and aluminosilicates manifest
themselves in the different site-time yields (STYs) in (eqn (3)).
This formalism further allows us to study changes in selectivity
trends as a fraction of total turnovers, which is a more rigorous
5
3
selectivity comparison for catalysts disparate in total TONs.
Fig. 5A shows the conversions on all zeolites as function of
time-on-stream (TOS). In all the three investigated frameworks,
borosilicates exhibited ∼8–13× lifetimes, indicating their sig-
nificantly higher stability under reaction conditions. To probe
whether the relatively low Si/Al ratios in aluminosilicates were
conducive to deactivation, we compared the time-on-stream
data for two aluminosilicates with widely different Si/Al ratios
(
(
40, and 140, respectively) starting with iso-conversion levels
∼25%) (Fig. S7†). The detrimental effect of using high alumi-
num content (among aluminosilicates) on catalyst lifetime
(within a factor of ∼1.6×), was found to be much smaller than
the effect of heteroatom identity (i.e. Al/B). Interestingly, for
both alumino-, and borosilicates, the catalyst lifetime
decreased in the order MWW > MFI > BEA. It is likely that C–C
Fig. 6 Selectivity to (left to right and top to bottom) 1,3-pentadiene,
chain elongation steps terminate at shorter chain lengths in 1,4-pentadiene, butenes, and C6+ compounds, as a function of the frac-
tion of total turnovers B-MWW (■, black), MCM-22 (□, black), B-MFI ( ,
blue), ZSM-5 ( , blue), B-BEA ( , red), and Al-BEA ( , red); reaction con-
ditions are same as Fig. 5.
the 10-MR frameworks MFI and MWW. However, these con-
densation reactions leading to coking precursors occur more
readily in the straight 12-MR channels of BEA, and are detri-
mental to total catalyst lifetime. Fig. 5B shows the total TONs
for all zeolites employed in this study. As expected from the
time-on-stream data, borosilicates showed ∼3–6× higher total the selectivity to both 1,3-pentadiene and 1,4-pentadiene
TONs than aluminosilicates starting from similar 2-MTHF con- decreased for all zeolites; this reduction was mirrored by an
versions (Fig. 5B).
increase in C6+ products (identified as alkylated aromatics),
The product selectivities of all major products under reac- which are likely the coking precursors. Based on this obser-
tion conditions were plotted as a function of total turnovers vation, we conclude that linear pentadienes can further
(
eqn (4)) and are shown in Fig. 6. As the catalysts deactivated, undergo condensation and/or cyclization reactions leading to
Fig. 5 (A) 2-MTHF conversion as a function of time-on-stream on B-MWW(■), MCM-22 (□), B-MFI ( , blue), ZSM-5 ( , blue), B-BEA ( , red), and Al-
BEA ( , red); (B) total turnover numbers during catalyst lifetime (primary axis), and initial conversion (secondary axis) for all zeolites (reaction con-
+
ditions: T = 573 K, P2MTHF = 25–26 Torr, space velocity = 7.22–24.3 mol 2-MTHF per H per min, carrier gas (He) flowrate = 10–140 sccm). Error
bars represent the standard errors in the calculation of total turnovers with the trapezoidal rule.
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these larger fractions. The product selectivity towards butenes corresponding diene selectivities were sustained for over
was consistently lower on borosilicates than aluminosilicates 80 hours on-stream. Due to the lower activity of B-MFI and
at all stages of reaction progress, consistent with the inability B-BEA, the yields obtained were limited by 2-MTHF conver-
of borosilicates to fragment C–C bonds. Notably, the selectivity sions, and under similar experimental conditions, the 1,3-PD
to higher C6+ fraction was consistently lower in borosilicates yield was found to be ∼76% and ∼24% on B-MFI, and B-BEA,
(
∼15–22%) than aluminosilicates (25–40%), especially later in respectively.
the reaction progress ( ftotal-turovers(t) > 0.5), when the catalysts
had significantly deactivated. This is likely a result of weak-
3.6 Conclusions
binding of these larger hydrocarbons, enabling desorption The systematic catalytic evaluation of boro-, and aluminosili-
from boron-acid sites but irreversible adsorption and conse- cates in three zeolite topologies (MWW, MFI, and BEA) reveals
quent condensation reactions on the aluminum acid sites, that weakly acidic borosilicates suppress the competing retro-
leading to coke formation. It is therefore likely that the higher Prins pathway to butenes, as well as complex condensation
stability of borosilicates is a direct consequence of reduced C6+ pathways leading to coking precursors in the dehydra-decycli-
fraction formation.
zation of 2-methyltetrahydrofuran, thus leading to ∼90%
selectivity to dienes across different 2-MTHF conversions. The
thermodynamic favorability of 1,3-pentadiene is shown by the
increments in the diene distribution ratio of 1,3-/1,4-penta-
3
.5 Maximizing the yield of 1,3-pentadiene in the dehydra-
decyclization chemistry
Recent reports of renewable catalytic pathways to piperylene diene by increasing reactor space-times and tighter micropore
production have utilized 2-MTHF as a feedstock. Other feeds environments in MFI and MWW. Lastly, total catalyst lifetime
have been less common, but a notable recent example includes in the chemistry is topology-dependent, with 12-MR BEA
a two-step deoxydehydration of xylitol leading to a 1,3-penta- showing lower TONs for both heteroatoms, than MFI and
8
5
diene yield of ∼52%. The hydrogenolysis of 2-MTHF is a fre- MWW. Borosilicates exhibit a remarkable improvement in
quently studied chemistry on metal phosphides, but the pres- total catalyst lifetime in all three frameworks, ranging from
ence of high pressure of H leads to the production of satu- ∼3× in MWW to ∼6× in BEA. These strategies are utilized to
2
rated species, and the overall yield to pentadienes is typically report a stable 1,3-pentadiene yield of ∼86% on B-MWW.
low; Oyama and co-workers have reported ∼50% yield of penta-
dienes during the hydrodeoxygenation of 2-MTHF over WP/
3
9
SiO
2
at 548 K. Other reports have reported negligible pro- Conflicts of interest
38,40,41
duction of pentadienes in this chemistry.
There are no conflicts to declare.
Dehydra-decyclization of 2-MTHF appears to be a promising
chemistry to obtain high yields of 1,3-PD; Norman has recently
reported that a ternary V-Ti-P oxide results in ∼59% 1,3-PD
8
6
Acknowledgements
yield during 2-MTHF dehydra-decyclization. Dumesic and
co-workers have reported 68% combined 1,3 + 1,4-pentadiene
This research was supported by the Minnesota Corn Growers’
Association. G. K. acknowledges the NSF Center for
Sustainable Polymers at the University of Minnesota, a
National Science Foundation supported Center for Chemical
Innovation (CHE-1901635). Parts of this work were carried out
in the Characterization Facility, University of Minnesota,
which receives partial support from NSF through the MRSEC
program. We thank Dr Sojong Hwang (Solid state NMR facility,
Caltech) for all solid state NMR experiments, and Dr Stavros
Caratzoulas, Dr Omar Abdelrahman, Dr Anargyros
Chatzidmitriou, and Dr M. Alexander Ardagh for helpful tech-
nical discussions.
3
7
yield in the same chemistry on amorphous silica/alumina.
However, their reported yield dropped from 68% to 52% over
8 hours on-stream due to catalyst deactivation. Our previous
5
studies on phosphorus-containing zeosils, while selective to
dienes at low conversions, show a moderate dienes’ yield of
1
2
6
0% at quantitative 2-MTHF conversions.
As noted before, borosilicates suppress both the competing
retro-Prins pathway, as well as condensation reactions leading
to C6+ fraction. Furthermore, these catalysts are significantly
more stable under reaction conditions than aluminosilicates.
Therefore, experiments were designed to probe borosilicates at
near complete conversions to maximize the yield of dehydra-
decyclization products in this chemistry, which predictably
required high temperature and low space velocity conditions
due to their instrinsic low reactivity. The obtained overall
diene selectivities on borosilicates were consistently in the
range of 86–92% independent of experimental conditions and
conversion levels. Most notably, under these low space velocity
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