Oligomerization of Light Olefins Catalyzed by Br?nsted-Acidic Metal-Organic Framework-808

Article abstract of DOI:10.1021/jacs.9b03867

Sulfated metal-organic framework-808 (S-MOF-808) exhibits strong Br?nsted-acidic character which makes it a potential candidate for the heterogeneous acid catalysis. Here, we report the isomerization and oligomerization reactions of light olefins (C3-C6) over S-MOF-808 at relatively low temperatures and ambient pressure. Different products (dimers, isomers, and heavier oligomers) were obtained for different olefins, and effective C-C coupling was observed between isobutene and isopentene. Among the substrates investigated, facile oligomerization occurred very specifically for the structures with an α-double bond and two substituents at the second carbon atom of the main carbon chain. The possible oligomerization mechanism of light olefins was discussed based on the reactivity and selectivity trends. Moreover, the deactivation and regeneration of S-MOF-808 were investigated. The catalyst deactivates via two mechanisms which predominance depends on the substrate and reaction conditions. Above 110 °C, a loss of acidic sites was observed due to water desorption, and the deactivated catalyst could be regenerated by a simple treatment with water vapor. For C5 substrates and unsaturated ethers, the oligomers with increased molecular weight caused deactivation via blocking of the active sites, which could not be readily reversed. These findings offer the first systematic report on carbocation-mediated olefin coupling within MOFs in which the Br?nsted acidity is associated with the secondary building units of the MOF itself and is not related to any guest substance hosted within its pore system.

Full text of DOI:10.1021/jacs.9b03867

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Article
Oligomerization of light olefins catalyzed by
Brønsted-acidic Metal-Organic Framework-808
Ping Liu, Evgeniy Redekop, Xiang Gao, Wen-Chi Liu, Unni Olsbye, and Gabor A. Somorjai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03867 • Publication Date (Web): 02 Jul 2019
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Oligomerization of light olefins catalyzed by Brønsted-acidic Metal-
Organic Framework-808
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Ping Liu,†,‡,*,# Evgeniy Redekop,‖,# Xiang Gao,†, ,# Wen-Chi Liu, †, Unni Olsbye,‖ and Gabor A.
⊥
,
Somorjai †,
*
†
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering,
‡
Changzhou University, Changzhou, Jiangsu 213164, China
‖
Centre for Materials Science and Nanotechnology (SMN), Department of Chemistry, University of Oslo, P.O. Box
1033, Blindern, N-0315 Oslo, Norway
⊥
Department of Chemistry, Kavli Energy NanoSciences Institute at Berkeley, and Berkeley Global Science Institute,
University of California-Berkeley, Berkeley, California 94720, United States
KEYWORDS: Light olefins, Isobutene, Oligomerization, Dimerization, Metal-Organic Framework-808
ABSTRACT: Sulfated Metal-Organic Framework-808 (S-MOF-808) exhibits strong Brønsted-acidic character which
makes it a potential candidate for the heterogeneous acid catalysis. Here, we report the isomerization and oligomeriza-
tion reactions of light olefins (C3-C6) over S-MOF-808 at relatively low temperatures and ambient pressure. Different
products (dimers, isomers and heavier oligomers) were obtained for different olefins, and effective C-C coupling was ob-
served between isobutene and isopentene. Among the substrates investigated, facile oligomerization occurred very specif-
ically for the structures with an α-double bond and two substituents at the second carbon atom of the main carbon chain.
The possible oligomerization mechanism of light olefins was discussed based on the reactivity and selectivity trends.
Moreover, the deactivation and regeneration of S-MOF-808 were investigated. The catalyst deactivates via two mecha-
nisms which predominance depends on the substrate and reaction conditions. Above 110 °C, a loss of acidic sites was ob-
served due to water desorption, and the deactivated catalyst could be regenerated by a simple treatment with water vapor.
For C5 substrates and unsaturated ethers, the oligomers with increased molecular weight caused deactivation via blocking
of the active sites, which could not be readily reversed. These findings offer the first systematic report on carbocation-
mediated olefin coupling within MOFs in which the Brønsted acidity is associated with the secondary building units of
the MOF itself and is not related to any guest substance hosted within its pore system.
Introduction
zation13,14, cyclization15, biomass transformation16,17, ben-
zylation18 and alkylation19.
Metal-Organic Frameworks (MOFs) are emerging as a
promising class of heterogeneous catalysts due to their
unique physical and chemical properties including high
surface area, adjustable pore structure, tunable element
composition and the potential for surface modification.1-6
Importantly, their well-defined molecular structure and
chemical environment may contribute to a more detailed
mechanistic understanding of catalytic reactions, thereby
enabling the development of "bespoke" catalysts.7-12 In
recent years, acidic MOFs have attracted significant re-
search interest because of their potential application in a
large class of acid-catalyzed reactions, including isomeri-
The sulfated MOF-808 obtained by treating Zr-based
MOF-808 with sulfuric acid was reported to have strong
acidity.15 Subsequently, the strong Brønsted acid site in
the sulfated MOF-808 was identified as a water molecule
co-adsorbed in an equatorial plane of a zirconia cluster
with a bidentate sulfate group that is chelated to a single
zirconium atom. The adsorbed water participates in hy-
drogen bonding with the sulfate, giving rise to a labile
proton.20 Preliminary screening of catalytic properties
showed that the sulfated MOF-808 was active and selec-
tive in the dimerization of isobutene.
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Oligomerization of light olefins (C3–C6) to heavier, and ii) chelating mode to a single zirconium atom follow-
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more valuable products (gasoline, diesel, jet fuel, or syn-
thetic lube oils) has been commercially used for over 80
years.21-24 Numerous studies demonstrated that different
olefins reacted according to different oligomerization
mechanisms, depending on the type of catalyst (acidity,
structure) and reaction conditions (temperature, pres-
sure).25-30 Solid acids catalyze the oligomerization reac-
tions via a carbocation-mediated mechanism which has
been extensively characterized in zeolites28,31, phosphoric
acid32, sulfated polymers23 and heteropolyacids33. However,
the majority of research on applications of MOFs for the
oligomerization of light olefins has so far been focused on
ethylene dimerization on metal centers and not Brønsted
sites.34-37 Moreover, cross-coupling of different olefins can
provide an efficient route for C–C bond formation, how-
ever, it has rarely been reported.
ing activation by heating under dynamic vacuum (Figure
1c). 20 The Brønsted acidity of S-MOF-808 originates from
the water molecules adjacent to chelating sulfate ligands.
As-synthesized S-MOF-808 has nearly identical crystal
structure as that of pristine MOF-808, as confirmed by
PXRD, N2 adsorption, SEM and TEM (Figures S2-6).
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The purpose of this study was to investigate the reac-
tion of different olefins (C3-C6) over sulfated MOF-808 in
order to fulfill a significant gap in the literature concern-
ing carbocation chemistry in MOFs and to explore the
potential of this catalyst for technologically-relevant ole-
fin transformations. Unsaturated (vinyl) ethers were also
employed as reactants to better understand the reaction
mechanism, since alkoxy groups can contribute signifi-
cantly to carbocation stabilization. The results demon-
strated that olefins (isobutene, 2-methyl-1-butene, and 2-
methoxypropene) with very similar structure resulted in
drastically different products distributions (dimers vs.
isomers vs. heavier oligomers). Additionally it was
demonstrated that sulfated MOF-808 can effectively cata-
lyze the coupling of isobutene and isopentene. The deac-
tivation and regeneration of sulfated MOF-808 were also
studied, and the deactivated catalyst in the isobutene re-
action could be completely regenerated by water vapor
treatment.
Figure 1. The structure of (a) pristine MOF-808, (b) sulfated
S-MOF-808, and (c) schematic representation of the two
modes of sulfate ligands coordinated to zirconia clusters.
A series of catalysts including pristine MOF-808, Cl-
MOF-808 (PXRD in Figure S7), S-MOF-808, and S-
MOF808-Hf (PXRD in Figure S8) were compared in the
dimerization of isobutene in order to unambiguously as-
sign the oligomerization activity to sulfated metal-oxide
clusters within MOF frameworks. Cl-MOF-808 was syn-
thesized by submerging pristine MOF-808 in HCl aque-
ous solution instead of H2SO4, and S-MOF-808-Hf was
obtained similarly to S-MOF-808-Zr, but with Hf atoms
taking place of Zr atoms. Preliminary reaction tests (Table
S1) showed that MOF-808 and Cl-MOF-808 were almost
inactive in the dimerization of isobutene, while S-MOF-
808 and S-MOF-808-Hf gave much better conversion and
selectivity to dimers at 101.325kPa, 100 °C. These data es-
tablish that olefin activation occurs on the active sites
associated with sulfate-substituted metal-oxide clusters,
with the nature of the metal playing only a secondary role
in the active site formation. Moreover, the conversion
increased with the increase of sulfate groups (Figure S9).
It is assumed that the amount of sulfate groups represents
the number of active sites. Based on the similar activities
of S-MOF-808 with 2.1 and 2.5 SO4 per Zr secondary
building unit (SBU), the remaining experiments focused
on the reaction studies of S-MOF-808 with 2.1 SO4 per Zr
SBU which consumed less sulfuric acid.
Results and discussion
On the origin of catalytic activity
S-MOF-808 was prepared according to the established
procedure15 by first synthesizing pristine MOF-808 (Fig-
ure 1a), Zr6O5(OH)3(BTC)2(HCOO)5(OH2)2, and then ex-
changing a sulfate ligand onto zirconia clusters (see Sup-
porting information, Section S1). In MOF-808, each 6-
connected (6-c) zirconia cluster is linked to six 1,3,5-
benzenetricarboxylate (BTC) linkers to form a 3D porous
framework which contains two different types of pores.
The smaller tetrahedral pores are isolated from each other,
while the larger adamantane pores are interconnected to
form continuous channels. Sulfate substitution occurs at
the larger continuous channels in MOF backbones to af-
ford
S-MOF-808,
[Zr6O4(OH)5.6(C9H3O6)2(HCOO)0.18(SO4)2.1](H2O)2 (Figure
1b). X-ray diffraction analysis of S-MOF-808 showed that
sulfate ligands coordinate to zirconia clusters in a biden-
tate fashion with two crystallographically different sites: i)
bridging mode to two zirconium atoms when in solution
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Table 1 Reactions of different olefins over S-MOF-808 catalysts
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2
3
4
a
STYiso
[mmol/gcat/h]
Soligom (%)
Trimers
/
STYoligom
[mmol/gcat/h] (X, %)
Entry
Reactant
Dimers
/
Tetramers
/
(X, %)
5
6
1
/
<0.05 (<0.1)
7
8
9
2
/
6.28 (15.7)
0.05 (0.13)
2.69 (4.48)
72.5
/
23.7
48.9
3.57
3.6
51.0
/
3
1.03 (2.57)
45.3 (75.5)
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4
8.93
5
7.4 (12.3)
0.58 (0.97)
35.6
9.59
/
6
7
/
<0.10 (<1)
0.85 (1.40)
100
/
/
/
/
12.9 (21.6)
33.9
Reaction conditions: 4 mmol/h reactant (gas) (entry 1-3), ca. 6 mmol/h reactant (liquid) (entry 4-7), 53 mmol/h He, 0.1 g catalyst,
101.325 kPa, 100 °C, Time on stream (TOS) 5h. a STYiso and STYoligom: Site-time-yield of isomerization and oligomerization, respec-
tively; Isomer (double-bond isomerization): 2-butene (entry 3), 2-methyl-2-butene (entry 4), 2-methyl-1-butene (entry 5), 3-
methyl-2-pentene (entry 7).
ful to point out the key structural features by which vari-
ous substrates can be distinguished. Propene, isobutene,
Reactions of light C3-C6 olefins
and cyclopentene are not capable of forming isomers. The
The reactions of different olefins (C3-C6) were investi-
methyl shift and the ring-opening steps that would be
gated over S-MOF-808 under ambient pressure flow con-
required to isomerize isobutene and cyclopentene, re-
ditions. As shown in Table 1, different activities and prod-
spectively, are highly activated and, thus, precluded in
uct distributions were observed for different light olefins.
this temperature range. The most stable carbocations that
Almost no reaction occurred for propene. Isobutene and
can be formed from propene, linear butene, and cyclo-
cyclopentene mainly produced dimers, albeit with drasti-
pentene by protonation and double-bond shift are sec-
cally different activities, while other olefins (1-butene, 2-
ondary, while the rest of the reactants can form more sta-
methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-
ble tertiary cations. Finally, only isobutene and 2-methyl-
pentene) preferred isomerization.
1-butene contain a structural motif in which an α-double
Over S-MOF-808 with Brønsted acidic site, the C-C
bond has two substituents in the second position.
coupling of olefins most likely proceeds as cationic
Electron-donating substituents typically facilitate the
polymerization, i.e. via the formation of a carbocation by
rate of cationic oligomerization because they stabilize the
olefin protonation and the subsequent attack of the se-
reaction intermediates–carbenium ions.38,39 Therefore, a
cond olefin molecule by this carbocation, followed by the
reasonable starting hypothesis to understand the reactivi-
product deprotonation and desorption.38 The C-C cou-
ty of S-MOF-808 towards different substrates is that elec-
pling pathway competes with the monomolecular isomer-
tron-donating substituents and carbocation stabilization
ization pathway, where the carbocation deprotonates to
determine which molecules are more likely to form oli-
release a more thermodynamically-stable isomer of the
gomers. For propene, the ability of one methyl group to
original olefin. Bimolecular isomerization via dimeriza-
donate electrons is insufficient to generate a stable sec-
tion and cracking is unlikely at such low temperatures.
ondary carbenium ion, making propene oligomerization
The primary factors that govern Gibbs free energies of
problematic. In comparison, two methyl groups in isobu-
various processes and, therefore, control the outcome of
tene increase the electron density on the double bond and
the competing reactions are: (1) charge stabilization in the
stabilize tertiary carbocation, facilitating the C-C coupling
carbocation, (2) steric considerations during microporous
reactivity for isobutene. The most stable carbocation that
diffusion and adsorption of olefins, and (3) local structur-
the next olefin - 1-butene - can form is secondary, and, in
al factors affecting the transition state of the C-C coupling
line with the carbocation stability hypothesis, it forms
reaction. Before relating the observed reactivi-
nearly no oligomers. Surprisingly, it also exhibits very low
ty/selectivity trends to the reaction mechanism, it is use-
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isomerization rate despite a strong thermodynamic driv- Temporal Analysis of Products (TAP) study
Page 4 of 10
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ing force for the 1-butene to 2-butene transformation.
Steady-state data at atmospheric flow conditions
were complemented by pulse-response TAP experiments
at the low pressure regime, which provide unique insight
into the mode of interaction between a probe molecule
and a catalyst in the limit of zero coverage.42 In a typical
TAP experiment (see Supporting information, Section S2),
the exit-flow rate curves of different gases directly reflect
the differences in their residence times within the catalyst
and, therefore, TAP data can be used to evaluate the rela-
tive strength of interactions between olefins and S-MOF-
808 in the absence of inter-molecular interactions and the
effects of pore crowding. Figure 2A depicts a set of isobu-
tene responses over an inert material, non-acidic pristine
MOF-808, and sulfated S-MOF-808. We note that the
parent MOF interacts slightly with the olefin, resulting in
a noticeable delay with respect to the inert standard. This
weak interaction is attributed to electrostatic forces be-
tween the probe molecule and the framework rather than
diffusional hindrance, since isobutene molecules are
much smaller than the pore diameter. S-MOF-808 delays
the olefin to a much greater extent due to the presence of
acidic sites. When comparing the responses of different
olefins in Figure 2B, their residence times correlate posi-
tively with the total length of the carbon chain, but are
nearly insensitive to the isomer structure. These data de-
finitively rule out the effects of adsorption strength
and/or diffusional limitations on the relative activities of
isobutene and 1-butene, strongly suggesting that it is the
increased charge stabilization and the unique steric ar-
rangement of the oligomerization transition state that are
implicated in the superior activity of isobutene in the C-C
coupling.
The next two substrates in the table – 2-methyl-1-
butene and 2-methyl-2-butene – can form a stable tertiary
carbocation from which a more thermodynamically stable
2-methyl-2-butene isomer emerges after desorption40,
consistently with the thermodynamic equilibrium distri-
bution of pentene reported by Mäurer et al.41. The isomer-
ization rates for these olefins fall in line with thermody-
namic expectations (Table 1, entries 4 and 5). It would be
expected by the analogy with isobutene, which also pro-
duces a tertiary carbocation, that 2-methyl-butenes would
additionally produce a significant amount of oligomers, as
exemplified for 2-methyl-1-butene in Scheme S1. As seen
from Table 1, 2-methyl-1-butene does oligomerize five
times faster than 2-methyl-2-butene. However, both
structural isomers of pentene exhibited relatively low oli-
gomerization activity. For cyclopentene reaction, the con-
version to oligomers was very low (<1%) although the se-
lectivity to dimers reached 100%. Finally, 3-methyl-1-
pentene mostly isomerized according to the stability of
the carbocation and reacted to some degree into a dimer.
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Taken together, the aforementioned observations sug-
gest that the propensity of olefins for C-C coupling is gov-
erned by a combination of two factors – the presence of a
terminal double bond in the α-position and the ability to
produce a stable tertiary carbocation upon protonation.
Isobutene offers both features and, uniquely among other
olefins, does not engage in a side isomerization reaction
into β-olefin that could lower the yield of oligomers. 2-
methyl-1-butene also satisfies these criteria, but it can also
isomerize by shifting the double bond into the β-position
and, therefore, mostly yields the isomer. Several peculiari-
ties in Table 1 remain unexplained within this general
trend, e.g. low isomerization activity of 1-butene.
B
A
Additionally, comparison with other solid acid catalysts
(Table S2) in the isobutene oligomerization reveals that
although the activity of S-MOF-808 (under its applicable
reaction temperature (Figures 4) and weight hourly space
velocity (WHSV) (Figure S10) conditions) could not be
favorably compared to non-MOF catalysts, it provides one
of the highest dimer selectivities reported. The kinetic
curves of the commercial resin - Amberlyst-15 and S-
MOF-808 showed that Amberlyst-15 had a higher activity
than S-MOF-808 at the same reaction conditions (Figure
S11). However, Amberlyst-15 had more acid capacity than
S-MOF-808 (4.70 vs. 1.54 mmol/g) (Table S3). Therefore, a
higher TOF value of isobutene conversion was obtained
over S-MOF-808. Moreover, S-MOF-808 exhibited more
than two-fold TOF value of dimers formation than Am-
berlyst-15 due to its much higher selectivity to dimers
(Table S3). High selectivity and unique preference for
isobutene motivates further research into carbocation-
mediated reactions in MOFs which will provide a valuable
extension of their catalytic repertoire.
Propene
IB
1B
2M1B
2M2B
Diffusion
Ads. in MOF-808
Ads. in S-MOF-808
0
1
2
3
4
5
0
1
2
3
4
5
t/s
t/s
Figure 2. TAP exit-flow rate transients at 100 °C. (A) Isobu-
tene over inert material representing bed-scale diffusion,
non-acidic MOF-808, and acidic S-MOF-808; (B) Propene,
isobutene (IB), 1-butene (1B), 2-methyl-1-butene (2M1B), and
2-methyl-2-butene (2M2B) over S-MOF-808. All curves were
height-normalized and scaled in time to match the diffusion
rate of inert standard (neon) in order to facilitate data com-
parison.
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Table 2 Co-reactions of two different olefins over S-MOF-808 catalysts
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2
3
4
5
6
7
8
9
Selectivity a/%
C10
Entry
Reactants
Conversion/%
Isomers
93.5
C8
C9
C12
C13
5.3
C15-16
33.6
9.1(IB)
1
+
31.0
12.1
3.5
13.4
82.5(2M1B)
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40
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42
43
44
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46
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53
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59
60
7.8(IB)
+
2
3
81.9
92.7
41.7
15.2
2.1
1.9
22.6
37.8
7.4
1.3b
9.6
11.3(2M2B)
10.1(IB)
43.8
/
13.9
+
16.5(3M1P)
Reaction conditions: 4 mmol/h isobutene (IB) (gas), 2.7 mmol/h liquid reactant (2-methyl-1-butene (2M1B), 2-methyl-2-butene
(2M2B) or 3-methyl-1-pentene (3M1P)), 53 mmol/h He, 0.1 g catalyst, 101.325 kPa, 100 °C, TOS 5h. a Selectivity to isomer: calculat-
ed based on the converted 2-methyl-1-butene, 2-methyl-2-butene and 3-methyl-1-pentene, respectively. Isomer: 2-methyl-2-
butene (entry 1), 2-methyl-1-butene (entry 2), 2-methyl-2-pentene (entry 3). Selectivity to C8-C16: calculated based on both of
the converted reactants subtracting the isomers. b The selectivity to C14.
Cross-coupling reactions
effect of electron-donating substituents, in this case –
ether oxygen, on the oligomerization reaction rate. As
shown in Table 3, high initial activities (69.2 and 99.5%)
were obtained for both ethers at TOS of 10 min. However
after 40 min on stream, their conversions declined quick-
ly to 3.2 and 12.4%, respectively. The products distribution
showed that less than 30% selectivities to dimers were
obtained, while most of the products were heavier oligo-
mers (degree of polymerization: n=3-5 for 2-
methoxypropene; n=3-8 for ethyl vinyl ether). These heav-
ier oligomers hardly desorb from the surface of the cata-
lyst in accordance with their high boiling points, which
can be confirmed by changes in the color of the catalyst
before and after use. As shown in Figure S12, the color of
the catalyst changed from white to yellow (for 2-
methoxypropene) or dark brown (for ethyl vinyl ether).
The deactivation of the catalyst was attributed to the
deposition of heavier oligomers.
Table 2 shows the co-reactions of isobutene and an-
other olefin including 2-methyl-1-butene, 2-methyl-2-
butene and 3-methyl-1-pentene. Isobutene was selected as
the fixed reactant because of its superior activity and se-
lectivity to dimers. C-C coupling reactions between isobu-
tene and another olefin were observed for all of the three
entries. As shown in entries 1 and 2, a considerable
amount of co-reaction products (C9 and C13) were ob-
tained, indicating efficient C-C coupling between differ-
ent olefins over S-MOF-808. It is worth noting that the
isomerization selectivity of 2-methyl-2-butene (entry 2) in
co-reaction case was lower than that in single reaction
case (entry 5 in Table 1) (81.9% vs. 92.7%) at close conver-
sion levels. It suggests that the carbenium ions formed
from isobutene is most likely to be the initiator of the co-
reaction between isobutene and 2-methyl-2-butene. The
possible co-reaction path is depicted in Scheme 1.
The high initial oligomerization activities of vinyl
ethers over S-MOF-808 reflect the overall electron-
donating properties of their ether functionalities. Com-
pared to isobutene and 1-butene, both 2-methoxypropene
and ethyl vinyl ether have a similar structure, but addi-
tionally contain an oxygen atom in their molecules. The
induction effect would reduce the electron density on the
C-C double bond due to the greater electronegativity of
oxygen. However, the p-π conjugation effect between C-C
double bond and p orbital of oxygen atom effectively in-
crease the electron density of C-C double bond. For vinyl
ether, the p-π conjugation effect is dominant relative to
Scheme 1. The possible co-reaction path of isobutene
and 2-methyl-2-butene over S-MOF-808
The effect of electron-donating substituents
Next, 2-methoxypropene and ethyl vinyl ether were
employed as reactants to further substantiate the positive
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induction effect.38,39 Therefore, oxygen substituent shows
Page 6 of 10
and NMR results (Figures S18, S19) verified that the accu-
1
2
3
4
5
6
7
8
the strong ability of electron-donation, which is condu-
cive to the formation of stable carbenium ion for cationic
oligomerization. Additionally, vinyl ether has a resonance
structure that makes the carbenium ion more stable by
dispersing the positive charge (Scheme S2). In compari-
son, lower conversion of 2-methoxypropene than that of
ethyl vinyl ether might be influenced by the molecular
size. The oligomers of 2-methoxypropene have more
branched chains, which may significantly impede their
diffusion out of the pores of MOF-808.
mulated heavier oligomers remained in the S-MOF-808
pores. It is worth noting that we could not quantify all
products of the reaction of 2-methyl-1-butene because the
sum of the selectivities to the products obtained with GC
did not reach 100% (Table 1). We attribute this discrepan-
cy of the mass balance to the heavier oligomers trapped
within the catalyst framework and in the pipes connect-
ing reactor and GC due to their higher boiling point.
Similar behavior was observed in stability tests of other
reactants.
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100
Table 3 Reactions of vinyl ethers over S-MOF-808 cat-
alysts
90
2-methyl-1-butene
Conversion/%
Selectivity/%
at 10 min
80
70
60
at different TOS
Reactant
Heavier
oligomers
10 min
69.2
40 min
3.2
Dimers
27.9
72.1 a
50
Isobutene
10
0
99.5
12.4
21.4
78.6 b
Reaction conditions: 0.1 g catalyst, 6 mmol/h reactant, 53
mmol/h He, 101.325 kPa, 100 °C. Degree of polymerization of
0
5
10
15
20
25
Time on stream/h
a
b
heavier oligomers (n): 3-5, 3-8, measured by mass spec-
troscopy.
Figure 3. The conversion of isobutene and 2-methyl-1-butene
over S-MOF-808 as a function of TOS. Reaction conditions: 6
mmol/h isobutene, 6 mmol/h 2-methyl-1-butene, 53 mmol/h
He, 0.1 g catalyst, 101.325 kPa , 100 °C.
Stability study
The stability behavior for the reactions of isobutene
and 2-methyl-1-butene over S-MOF-808 is illustrated in
Figure 3. The conversion of isobutene gradually increased
in the first 5 h, and then remained stable at around 10%
up to 22 h. The initial increase of conversion is related to
the activation of the catalyst, that is, the transformation
of adsorbed water to Brønsted acid sites. Unlike in the
isobutene reaction, fast deactivation was observed in the
reaction of 2-methyl-1-butene where the conversion
dropped by approximately 45% within 12 h from its start-
ing value of 88.4%. The PXRD and N2 adsorption results
showed that the spent catalysts maintained their crystal-
linity and pore structure (Figures S13-16). However, the
spent catalyst after 2-methyl-1-butene reaction had less
pore volume than the spent catalyst after isobutene reac-
tion (Figures S15-16). The TG analysis of spent catalysts
(Figure S17) showed that the weight loss from the spent
catalyst after isobutene reaction was similar to that from
the fresh catalyst, suggesting that no additional heavier
oligomers or carbon were deposited in the pores of S-
MOF-808. The elemental analysis of the spent S-MOF-
808 after isobutene reaction (Table S4) showed that the S
content in the catalyst did not decrease obviously after 22
h of reaction, indicating that the sulfate groups was stable
enough under this reaction conditions. The spent catalyst
after 2-methyl-1-butene reaction had 32.4% weight sur-
plus with respect to the fresh catalyst up to 250 °C. The IR
100
80
60
40
20
0
12
11
10
9
Dimers
8
Trimers
7
6
Tetramers
5
80
90
100 110 120
80
90
100 110 120
Reaction temperature/oC
Reaction temperature/oC
Figure 4. The conversion and selectivity of isobutene reaction
over S-MOF-808 as a function of reaction temperature. Reac-
tion conditions: 0.1 g catalyst, 6 mmol/h isobutene, 53
mmol/h He, 101.325 kPa.
Temperature dependence of isobutene dimerization
The influence of the reaction temperature on the
dimerization of isobutene over S-MOF-808 is shown in
Figure 4. With increasing reaction temperature from 80
to 120 °C, the conversion rose up to first up to 110 °C and
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then decreased. We attribute this behavior to the stability deactivation curve was nearly the same as that of the
1
2
3
4
5
6
7
8
of active Brønsted-acidic sites. The acidic site originates
from the adsorbed water participating in a hydrogen bond
with sulfate that is chelated to one zirconium atom. At
the higher temperature (>110 °C), the adsorbed water is
removed easily from the surface of MOF-808, resulting in
the decay of the acidic site, thus reducing the catalytic
activity.20 In general, the selectivity to dimers decreased
and the selectivities to trimers and tetramers increased
with increasing reaction temperature. At the lower reac-
tion temperatures (≤90 °C), the selectivity to dimers re-
mained at a high value (>85%), while it decreased at
100 °C due to increased conversion and continued to de-
crease with further increase in temperature.
fresh catalyst (Figure 5c), indicating that the catalyst was
completely regenerated by treating with water vapor.
The proposed mechanism of regeneration of S-MOF-
808 is shown in Figure 6. During the preparation of S-
MOF-808, the bridging mode of sulfate ligands coordinat-
ing to zirconia clusters changed into chelating mode by
heating under dynamic vacuum, and then forms acid site
by hydrogen bonding interaction with terminal water. At
the high temperature (150 °C), the terminal water de-
sorbed and the acid site was removed, resulting in the
deactivated catalyst. When the deactivated catalyst was
placed in humid air, the absorbed water could not re-
form hydrogen bonds with the sulfate at room tempera-
ture because the chelating mode of sulfate was likely to
return to the bridging mode. Therefore, the Brønsted acid
could not be restored by this method. It was worth noting
that the conversion of isobutene had a slight increase
within the first 3 h over the regenerated catalyst in humid
air. At the high reaction temperature of 150 °C, both de-
sorption of water and formation of acid site were occur-
ring at the same time. The increase of activity could be
caused by the formation of acid sites during the reaction
process. However, the desorption of water at this elevated
temperature eventually counteracted the formation of
new acid sites. Therefore, the activity decreased subse-
quently after 3 h. When the deactivated catalyst was re-
generated with water vapor at 100 °C, the water would be
returned to the original state directly, recovering the cata-
lytic activity completely.
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16
c
a
b
14
12
10
8
6
4
2
0
5
10 15 20 25 30 35 40 45 50 55 60
Time on stream/h
Figure 5. The performance of regenerated S-MOF-808 in the
isobutene reaction. (a)Fresh catalyst; (b) deactivated catalyst
regenerated by adsorbing water in humid air at room tem-
perature for one day, (c) deactivated catalyst regenerated by
feeding water vapor (33 mmol/h H2O gas with 30 mmol/h He)
at 100 °C for 50 min. Reaction conditions: 0.1 g catalyst, 6
mmol/h isobutene, 53 mmol/h He, 101.325 kPa, 150 °C.
Catalyst regeneration
The regeneration of the catalyst is the most direct
and effective way to confirm or reject its deactivation
mechanism. According to the previous discussion, the
deactivation of S-MOF-808 in the isobutene reaction was
attributed to the desorption of terminal water from the
clusters at high temperature. Hypothetically, the catalyst
could be regenerated by returning water to its original
position. Based on this idea, we conducted the isobutene
reaction over S-MOF-808 at 150 °C to prepare a deactivat-
ed catalyst. The conversion of isobutene sharply dropped
from 13.6 to 3.1% within 21 h (Figure 5a). First, the deac-
tivated catalyst was placed in humid air to absorb water at
room temperature. The weight of the catalyst increased
by 25% after one day. However, activity did not regener-
ate significantly after this room temperature water ad-
sorption (Figure 5b). Then, we treated the deactivated
catalyst by feeding water vapor at 100 °C. Surprisingly, the
activity of the catalyst was completely restored, and the
Figure 6. Proposed mechanism of regeneration of S-MOF-
808.
Conclusions
In summary, light olefins (C3-C6) oligomerization was
studied over S-MOF-808 at low temperature (80-120 °C)
and ambient pressure. Different products were achieved
for different olefins at the same reaction conditions. De-
spite similarities of their molecular structures, isobutene,
2-methyl-1-butene and 2-methoxypropene, gave dimers,
isomers and heavier oligomers, respectively. The for-
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Page 8 of 10
(3) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The
Chemistry and Applications of Metal-Organic Frameworks. Sci-
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(MOFs): Routes to Various MOF Topologies, Morphologies, and
Composites. Chem. Rev. 2012, 112, 933-969.
mation of stable carbenium ions and the presence of the
double bond in the α-position are the key for oligomeriza-
tion of olefins. The two methyl groups in isobutene and
the p-π conjugation effect in vinyl ether both promoted
the formation of stable carbenium ions. Additionally, effi-
cient C-C coupling reactions between isobutene and iso-
pentene occurred over S-MOF-808 with a possible carbe-
nium ions initiator formed from isobutene. S-MOF-808
deactivated in isobutene reaction due to the desorption of
water molecules from the catalyst surface at higher tem-
perature (>110 °C), but feeding the water vapor was an
effective method to achieve its complete regeneration.
Overall, the reported results suggest that Brønsted acid
sites in sulfated MOF-808 exhibit surprising oligomeriza-
tion specificity towards α-olefins with double substitution
at the second carbon atom, most likely due to a delicate
balance between adsorption energetics and unique steric
factors in the vicinity of the active site.
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struction of Chemical Warfare Agents Using Metal-Organic
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ASSOCIATED CONTENT
Supporting Information
Details of syntheses and characterization, PXRD patterns,
SEM images, TEM images, N2 gas adsorption isotherms, H/C
NMR spectra, TGA isotherms, IR, elemental analysis and
catalytic tests are available free of charge via the Internet at
http://pubs.acs.org.
(10) Li, X.; Zhang, B.; Fang, Y.; Sun, W.; Qi, Z.; Pei, Y.; Qi, S.;
Yuan, P.; Luan, X.; Goh, T. W.; Huang, W. Metal-Organic-
Framework-Derived Carbons: Applications as Solid-Base Catalyst
and Support for Pd Nanoparticles in Tandem Catalysis. Chem. -
Eur. J. 2017, 23, 4266-4270.
AUTHOR INFORMATION
Corresponding Author
*somorjai@berkeley.edu
*pingliu@cczu.edu.cn
(11) Ji, P.; Sawano, T.; Lin, Z.; Urban, A.; Boures, D.; Lin, W. Ceri-
um-Hydride Secondary Building Units in a Porous Metal-
Organic Framework for Catalytic Hydroboration and Hydro-
phosphination. J. Am. Chem. Soc. 2016, 138, 14860-14863.
(12) Xiao, D. J.; Bloch, E. D.; Mason, J. a; Queen, W. L.; Hudson,
M. R.; Planas, N.; Borycz, J.; Dzubak, A. L.; Verma, P.; Lee, K.;
Bonino, F.; Crocellà, V.; Yano, J.; Bordiga, S.; Truhlar, D. G.;
Gagliardi, L.; Brown, C. M.; Long, J. R. Oxidation of Ethane to
Ethanol by N2O in a Metal-Organic Framework with Coordina-
tively Unsaturated Iron(II) Sites. Nat. Chem. 2014, 6, 590-595.
(13) laerts, L.; guin, E.; Poelman, H.; Thibault-Starzyk, F.;
Jacobs, P. A.; De Vos, D. E. Probing the Lewis Acidity and Cata-
lytic Activity of the Metal-Organic Framework [Cu3(btc)2]
(BTC=Benzene-1,3,5-Tricarboxylate). Chem. - Eur. J. 2006, 12,
7353-7363.
(14) Sabyrov, K.; Jiang J.; Yaghi O. M.; Somorjai G. A. Hydroi-
somerization of n-Hexane using Acidified Metal-Organic
Framework and Platinum Nanoparticles. J. Am. Chem. Soc. 2017,
139, 12382-12385.
(15) Jiang, J.; Gándara, F.; Zhang, Y.-B.; Na, K.; Yaghi, O. M.;
Klemperer, W. G. Superacidity in Sulfated Metal–Organic
Framework-808. J. Am. Chem. Soc. 2014, 136, 12844-12847.
(16) Zhang, Y.; Degirmenci, V.; Li, C.; Hensen, E. J. M. Phospho-
tungstic Acid Encapsulated in Metal–Organic Framework as
Author Contributions
#P. Liu, E. Redekop, and X. Gao equally contributed to this
work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
P.L. acknowledges the financial support by Advanced Cataly-
sis and Green Manufacturing Collaborative Innovation Cen-
ter, Changzhou University for the opportunity to visit Law-
rence Berkeley National Laboratory. U.O. and E.A.R.
acknowledge the support by the Peder Sather Foundation
and the use of NICE - the Norwegian national research infra-
structure – for TAP measurements. The authors thank Dr.
Andrea Lazzarini (UiO) and Mr. Erlend Aunan (UiO) for
their help with IR and NMR measurements of the deactivat-
ed sample, respectively.
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