Assembly line synthesis of isoprene from formaldehyde and isobutene over SiO2-supported MoP catalysts with active deposited carbon

Article abstract of DOI:10.1039/c7ra05078j

Isoprene is a very important monomer for synthetic rubber. Its synthesis in the presence of MoP catalysts via the vapour phase reaction of isobutene with formaldehyde has been studied. The catalysis by various catalysts was characterized by TG analysis, the low-temperature adsorption of nitrogen, XRD, element analysis, TPD, the FT-IR of adsorbed pyridine, XPS and MAS NMR. A chemical process was proposed and confirmed. Isoprene was synthesized in an "assembly line" process through different active sites at the surface of the MoP catalysts, where the active sites were derived from P and Mo species and deposited carbonaceous species. In the induction period, carbon species are preferentially deposited on P species, leading to a decrease in the active Mo and P species (active sites 1), accompanied with a burst in active carbonaceous species (active sites 2). This soundly describes the burst in catalyst capacity in the induction period, which then decreases over time on stream at the expense of the active carbonaceous species. Accordingly, through the prior formulation of sites 1 with sites 2 in a catalyst, the time-consuming induction period can be nearly eliminated. We also observed that a good ratio of these two active sites can efficiently retard the catalyst deactivation. This study clarifies the roles of acid sites and active species in MoP catalysts in the synthesis of isoprene and shows that their optimal ratio can help reduce the length of the induction period and extend the lifetime of the catalysts.

Full text of DOI:10.1039/c7ra05078j

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Assembly line synthesis of isoprene from
formaldehyde and isobutene over SiO2-supported
Cite this: RSC Adv., 2017, 7, 37392
MoP catalysts with active deposited carbon†
ab
a
a
a
Yunqi Lia and Chenxi Bai
*
Yanlong Qi,
Long Cui,
Quanquan Dai,
Isoprene is a very important monomer for synthetic rubber. Its synthesis in the presence of MoP catalysts via
the vapour phase reaction of isobutene with formaldehyde has been studied. The catalysis by various
catalysts was characterized by TG analysis, the low-temperature adsorption of nitrogen, XRD, element
analysis, TPD, the FT-IR of adsorbed pyridine, XPS and MAS NMR. A chemical process was proposed and
confirmed. Isoprene was synthesized in an “assembly line” process through different active sites at the
surface of the MoP catalysts, where the active sites were derived from P and Mo species and deposited
carbonaceous species. In the induction period, carbon species are preferentially deposited on P species,
leading to a decrease in the active Mo and P species (active sites 1), accompanied with a burst in active
carbonaceous species (active sites 2). This soundly describes the burst in catalyst capacity in the
induction period, which then decreases over time on stream at the expense of the active carbonaceous
species. Accordingly, through the prior formulation of sites 1 with sites 2 in a catalyst, the time-
consuming induction period can be nearly eliminated. We also observed that a good ratio of these two
active sites can efficiently retard the catalyst deactivation. This study clarifies the roles of acid sites and
active species in MoP catalysts in the synthesis of isoprene and shows that their optimal ratio can help
reduce the length of the induction period and extend the lifetime of the catalysts.
Received 5th May 2017
Accepted 11th July 2017
DOI: 10.1039/c7ra05078j
rsc.li/rsc-advances
condensation of IB and formaldehyde to produce 4,4-
dimethyldioxane-1,3 (DMD) in the liquid phase, and (2) the
cracking of DMD into IP over solid catalysts (Scheme 1a).18 The
1 Introduction
The decline in catalytic capacity is a problem of great concern
for various reactions over heterogeneous catalysts and is mainly
due to carbon deposition.1–7 Since the concept of catalysis by
carbonaceous deposits was rst proposed,8 the deleterious
deposited carbon was found to act as efficient active sites in
various reactions.9–15 These processes signicantly involve an
induction period forming new active species. Therefore, those
features require a greater understanding of the chemical
processes involving coke deposition, where understanding the
process in and aer the induction period is particularly crucial
for revealing the catalyst behaviour.
two-step process has various drawbacks,19–21 and consequently
a new promising route is urgently needed. Developing a one-
step selective process is a very attractive pathway in this
regard on account of its simplied procedures, and due to the
possibility of easily manufacturing high purity isoprene
(Scheme 1a). This process is based on heterogeneous catalysts,
include zeolites,22,23 phosphates,20,24 sulfates,25 oxide catalysts,26
heteropolyacids,19,21,27 and silver catalysts.28 These reactions are
associated with high initial yields of isoprene (60–70%), but also
involve a rather low selectivity and a short catalyst life time due
to its rapid deactivation. Therefore, it is of great signicance to
gain an in-depth insight into the variation of catalysts (especial
for active sites) with carbon deposition. Furthermore, the
catalytic capacity declines not only because of the variation of
catalysts or active sites, but also due to the variation in chemical
process bridging the catalyst deactivation and the active sites
(catalysts) variation.
Herein, this study focuses on the condensation of isobutene
(IB) with formaldehyde leading to isoprene (IP), which is
a strategically important monomer for fabricating synthetic
rubber, elastomers and resins.16 The earlier route for IP
synthesis was based on
a
two-step reaction:17 (1) the
Krzywicki et al.24 reported using different concentrations of
H3PO4 on Al2O3 to prepare catalysts with various acidic strength
sites. The study suggested that the formation of IP was observed
on strong acidic sites over catalysts with H0 # ꢀ12.7, but the
catalysts with exclusively strong acidic sites were less active than
those having acid sites of various strengths. This inferred that
aKey Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, # 5625, Renmin Road, Changchun, 130022, P. R.
China. E-mail: baicx@ciac.ac.cn
bUniversity of the Chinese Academy of Sciences, # 19, Yuquan Road, Beijing 100049, P.
R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra05078j
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proposed without consideration of the inuence of the cata-
lysts, and indeed, how the original sites and forming sites
(carbon) work is still unknown, in which also the inuence of
catalysts variation on the active sites and the chemical route is
still unresolved.
To gain deeper insight into the synthesis of IP, its non-
ignorable inuence on the catalysts (especially the active sites)
variation on the chemical reaction pathway, which essentially
leads to activity decline, was investigated. In this study, MoP
catalysts were applied for the synthesis of IP. Their catalytic
capacity was found to reach a steady state and then decline in
a short time, which was helpful to focus the study on the cata-
lysts (active sites) variation with time on stream (TOS). It was
rst reported that the one-step synthesis of IP from IB and
formaldehyde over MoP catalysts involves almost simultaneous
processes (namely DMD forming and DMD cracking) in the
primary route, where these two processes are catalysed by the
cooperation between different sites (marked as forming sites
and cracking sites), almost like in an “assembly line” type
process. The need to increase the cracking sites at the begin-
ning accounts for the need for an induction period, which could
almost be eliminated by introducing cracking sites into fresh
catalysts. The nature of the sites has been studied. In addition,
the imbalance between the forming and cracking sites is the key
reason for the catalyst activity declining.
Scheme 1 Synthesis of isoprene from IB and formaldehyde: (a) one-
step and two-step processes, (b) reaction pathways of the one-step
process.
the reaction pathway involved different active sites on the
catalysts surface. The reaction pathways over 12- and 10-ring
zeolites has been suggested, as seen in Scheme 1b.29 Dumitriu
et al.22,23 pointed out the crucial role of the acid sites on the
zeolites strength in the selective synthesis of IP and reported
2 Experimental
¨
that moderate Bronsted acid sites resulted in high efficiency
and excellent selectivity. He suggested that DMD might be an
intermediate during Prins condensation, where it is formed and
easily cracked over catalysts with strong and medium acid sites
at high temperature. Dang et al.25 suggested that IP production
was not only related to the acid sites but also to the base sites. In
addition, it was reported that strong acid sites led to side
reactions and carbon deposition, with the latter resulting in
catalyst deactivation.
Much research has concluded that strong acid sites lead to
side reactions, while moderate sites are in favour of IP
synthesis; moreover, they attributed catalytic capacity declining
to carbon deposition or the loss of active sites.30 Unfortunately,
it was concluded that the declining activity can be attributed to
the catalyst variation, but these conclusions were made without
any consideration of the chemical processes, and the effect of
deposited carbon, which has been conrmed at “work sites”,
has been neglected.
Ivanova et al.9 reported that the yield of IP increased rapidly
with carbon deposition in the induction period and suggested
that carbon played a key role in the selective synthesis of IP. By
gas chromatography-mass spectrometry (GC-MS) analysis, it
was demonstrated that deposited carbon with unsaturated
branched surface species could alone catalyse IB and formal-
dehyde into IP and promote selectivity. Moreover, the main by-
products were investigated, it was found that carbon monoxide
formation proceeded via formaldehyde decomposition over
Lewis acid sites, DMD was observed during the steady-state
period, and the main reaction network involving methyl-
butenols was also proposed.21 However, this network was
2.1 Materials and method
The catalysts were prepared by initial wetness impregnation.
The supporting spherical SiO2 (60–100 meshes, Sinopharm
Chemical Reagent Co., Ltd, China) was rst calcined at 450 C
ꢁ
for 5 h and then dried at 110 ꢁC in an oven before use. In
a typical procedure 0.01 mol of ammonium molybdate tetra-
hydrate ((NH4)6Mo7O24$4H2O, Tianjin chemical Reagent Corp.,
China) was added into a ask with deionized water at 55 ꢁC,
kept stirring until the formation of homogeneous phase, and
then 0.15 mol of phosphoric acid (H3PO4, Beijing Chemical
Corp., China) was added and stirred for 30 min to obtain the
precursor solution. This was followed by the addition of 20 g of
supporting SiO2, with the solutions then aged overnight at
ambient temperature. The samples were dried at 110 ꢁC for 3 h,
calcined in ꢀa1ow of dry air to 450 ꢁC with a temperature ramp of
4.5 ꢁC min and then kept at this temperature for 5 h. The
mole ratio of H3PO4/(NH4)6Mo7O24$4H2O was 5, 10, 15, and 25,
with xing the MoO3 load at 20 wt% in theory. The prepared
catalysts were denoted as MoP X (A, B, C, D), in which X cor-
responded to the mole ratio of H3PO4 and (NH4)6Mo7O24$4H2O
(5, 10, 15, 25, respectively).
2.2 Characterization
TG experiments were performed on a TA SDT Q 600 instrument.
Temperature-programmed oxidation was carried out in a ow of
air in the temperature range of 25–800 ꢁC with a heating rate of
10 ꢁC minꢀ1. The BET surface area, pore volume, and pore sizes
were measured on a Quantachrome Autosorb-iQ at liquid
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nitrogen temperature. Inductively coupled plasma (ICP) was ratio was kept at 7, and the weight hourly space velocity (WHSV)
conducted to determine the chemical composition of the of the feed was 10 g (g h)ꢀ1. The products were analysed by
samples.
online gas chromatography. N2 (5 cm3, 99.999%) was used as
X-ray powder diffraction (XRD) patterns were obtained on the internal standard for precise mass carbon balance tests. The
a Bruker D8 Advance diffractometer using Ni-ltered CuKa formaldehyde concentration was determined by titration with
radiation at 40 kV and 20 mA (l ¼ 0.154 nm). X-ray photoelectron Na2SO3. Finally, the coke catalysts were collected and denoted
spectroscopy (XPS, ESCALAB 250, Thermo) was used to examine as MoP X-y, where y was TOM (min). The products from 4 min to
the fresh catalysts and coke catalysts.31 P and13 C MAS NMR 9 min were collected by a cold trap under liquid nitrogen and
spectra (Bruker Avance-400 spectrometer) were employed to then placed at room temperature for 30 min to remove any
characterize the fresh catalysts and the coke-deposited catalysts. gaseous products, and the liquid products were then analysed.
Temperature-programmed desorption of ammonia (NH3-
2.3.2 Synthesis and cracking of dioxane-1,3. a-Methylstyr-
TPD) experiments were performed on a TP-5080 (Tianjin Xian- ene and formaldehyde were used as the probe molecules to
quan) with a TCD detector in the temperature range of 25– synthesis dioxane-1,3. This condensation was performed at
700 ꢁC in a ow of dry He (30 ml minꢀ1). In a typical experiment, 95 ꢁC for 3 h in a ask equipped with a condenser. Keeping the
the samples were heated to 400 ꢁC at a rate of 12 ꢁC minꢀ1 and mole ratio of formaldehyde/a-methylstyrene as 4.5, the reaction
held there for 30 min in a ow of He, then cooled to room products were extracted using ethyl acetate and n-heptane (V/V
temperature and saturated with NH3 for 30 min. Subsequently, ¼ 1/5) in solution and determined by GC with dodecane as the
ꢀ1
ꢁ
ꢁ
the samples were heated to 100 C with a rate of 10 C min
internal standard. MoP C-y were employed as synthesis catalysts
and kept there for 30 min to physically remove the adsorbed and cracking catalysts. The dioxane-1,3 was cracked in the xed
NH3, while NH3-TPꢀD1 was carried out from 100 C to 700 C at bed at 300 ꢁC, and was then fed by a syringe pump (Harvard
ꢁ
ꢁ
ꢁ
a rate of 10 C min
.
Apparatus) with 0.017 ml minꢀ1 accompanied with N2
Qualitative and quantitative analyses were conducted with (99.999%, 90 ml minꢀ1). The cracking products were collected
GC-MS (Agilent 5975 MSD with Agilent SE-54) and GC (Ther- by 10 ml dichloromethane in a ask unꢁder an ice bath for
moFisher Trace 1300 with Agilent VF-5 ms) equipped with a FID, 30 min. The catalysts were heated to 300 C for 2 h under N2
respectively. Mainly liquid products were determined by1
NMR and13 C NMR (AV-400 M).
H
prior to the cracking reaction. The product was examined by GC.
3 Results and discussion
2.3 Catalyst evaluation
3.1 Synthesis of IP over MoP catalysts
2.3.1 Synthesis of isoprene. The condensation of formal- The main characteristics of the MoP X (A, B, C, D) catalysts are
dehyde with isobutene was studied in a xed bed reactor system listed in Table 1. The XRD pattern (Fig. 1) indicated that all the
operating under atmospheric pressure, which was equipped catalysts showed up as typical amorphous materials, which may
with an online GC (Kechuang 9800) with a TCD detector. In be due to the good dispersion of Mo and P individual oxides
a typical experiment, 1.5 g of catalyst was loaded into the reactor (MoOx and POy). In addition, NH3-TPD was conducted to
and incubated in situ using nitrogen at 300 ꢁC for 2 h. Then, the determine the acidity of the MoP catalysts, with the results
temperature was decreased to the reaction temperature. presented in Fig. 3a, showing just one peak, which was slightly
Formalin containing 37 wt% of formaldehyde without further wider and shied to high temperature with an increasing P
purication was fed by syringe pump (Harvard Apparatus), and content. This implied that the MoP catalysts presented a similar
isobutene was liqueed and fed by a syringe pump (Harvard distribution of acidity, but the acidic strength was stronger with
Apparatus) with pressure. The isobutene/formaldehyde molar increasing the P content. On the other hand, those peaks,
Table 1 MoP catalysts characteristics
Surface area
m2 gꢀ1
TG
%
Acidity (TPD NH3)
Elemental analysis
Acidity (FTIR Py)
Catalysts
Mo/P
Moa wt%
Pb wt%
mmol gꢀ1
Bc mmol gꢀ1
Ld mmol gꢀ1
SiO2
MoP A
MoP B
MoP D
243.23
151.00
132.78
29.35
84.94
75.97
70.50
75.80
87.67
58.37
—
—
—
—
2.92
22.06
7.74
11.60
7.82
23.53
—
—
—
—
—
—
—
51.19
16.25
—
1.83
—
—
—
—
—
12.00
19.77
—
6.21
—
1.32
0.70
0.28
0.49
0.55
—
0.53
—
0.54
19.03
18.90
21.05
21.58
21.83
—
22.65
—
19.48
9.54
17.71
46.61
28.98
26.35
—
28.32
—
23.64
1111.08
1138.19
630.65
1228.72
302.48
261.90
222.89
198.43
164.36
MoP C
MoP C-10
MoP C-20
MoP C-40
MoP C-100
MoP C-240
0.85
6.38
a
d
Calculated as MoO3. b Calculated as PO4. c B: Bronsted acid sites. L: Lewis acid sites.
¨
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Fig. 1 XRD patterns of the MoP X catalysts.
Fig. 2 Yields of isoprene from formaldehyde and isobutene versus
time on stream after induction period (300 ꢁC, 1.5 g of catalyst, iC4H8/
CH2O ¼ 7, WHSV ¼ 10 g (g h)ꢀ1).
assigned to the amount of acid sites, showed the order MoP C >
MoP B > MoP A > MoP D (see the details listed in Table 1). The P
¨
species can provide Bronsted sites with the terminal P–OH group,
presented in the order MoP C-240 > MoP C-10 > MoP C-40 > MoP
C-100 > MoP C-20 > MoP C. The content of carbon deposits
increased and reached to the rst peak over MoP C-10, then
increased with TOS. This was similar with the surface C in XPS,
where: MoP C-240 > MoP C-10 > MoP C-100 > MoP C-20 > MoP C-
40 > MoP C. This was attributed to the carbon deposition con-
sisting of alkoxides and ethers in the early stage (10 min on
stream), which were then changed into olen alkenes.9
XPS was conducted to study the changing of the elements on
the catalyst surface (Fig. 3b). Compared with Mo, P changed
noticeably with C in a reverse tend, which indicated P played
a key role for the growth of the coke. This will be discussed in
the following part. The catalyst surface was determined by BET,
and the Mo/P species conrmed by ICP, which was found to
have changed slightly during the reaction time within 240 min
(Table 1). The XRD patterns indicated the MoP catalysts were
amorphous materials and were well dispersed on the SiO2
support (Fig. S1†).
To further identify the change in the catalysts with TOS, NH3-
TPD was employed to determine the amount and strength of the
acid sites of the MoP C catalysts. The proles in Fig. 4 show one
peak in the temperature range of 100–700 ꢁC for the fresh MoP
(MoP A, MoP B, MoP C and MoP D), and two peaks for those
with coke deposition (labelled as peak 1 and peak 2). It was
evident that the peak in the temperature range 100–400 ꢁC (peak
1) decreased with TOS as following: MoP C > MoP C-10 > MoP C-
20 > MoP C-40 > MoP C-100 > MoP C-240, while the peak in the
¨
and hence the increasing acid sites can be attributed to Bronsted
sites. The surface area and chemical composition were also
determined. These same fresh MoP catalysts were employed for
IP synthesis from IB and formaldehyde (Table 2). MoP C gave rise
to the best yield of IP. With the P content changing, it showed the
order MoP D < MoP A < MoP B < MoP C, which was in line with
the amount of acid sites. Therefore, the amount of acid sites had
a signicant inuence on the reaction. Moreover, the Bronsted
sites also played a key role in this reaction, which was in accor-
dance with most reports for Prins reactions.
MoP C catalyst was taken to study the catalysts behaviour
with TOS. The catalytic capacity over the MoP C-y, where y is the
different time on stream, catalysts is presented in Fig. 2 (aer
the induction period) and Fig. 8 (during the induction period: 5,
10, 20 min on stream). These present the feature of a volcano
curve for the catalytic capacity, which improves in the induction
period and then gradually declines with TOS, which is similar
behaviour to other heterogeneous catalysts. To elucidate this,
the MoP C-y catalysts were analysed by TG, ICP, BET, XRD, XPS,
TPD, FT-IR of adsorbed pyridine and 13C MAS NMR.
¨
3.2 Study of the catalyst variation with TOS
The variation of MoP C with TOS was rst investigated by TG
(Fig. 3a), as summarized in Table 1. The loss weight due to coke
Table 2 Synthesis of IP from IB and formaldehyde over MoP catalysts a
Formaldehyde (%)
Isobutene (%)
Catalystsb Conversion Selectivity Conversion Selectivity Yieldc
MoP A
MoP B
MoP C
MoP D
MoO3
85.36
96.33
92.68
41.42
93.44
94.68
28.13
27.04
48.30
49.97
30.02
37.82
8.63
9.34
9.55
8.55
9.03
8.90
50.87
58.36
96.13
50.70
65.00
84.22
24.01
26.05
44.76
20.70
28.05
35.81
H3PO4
a
The reaction was carried at 300 ꢁC, catalyst mass was 1.5 g, iC4H8/
CH2O ¼ 7, WHSV ¼ 10 g (g h)ꢀ1
.
All the catalysts were supported on
b
Fig. 3 MoP C-y catalysts: (a) TG, and (b) surface element variation with
TOS.
c
SiO2. Yield to isoprene from formaldehyde.
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Table 3 Synthesis of MPD from a-methylstyrene and formaldehyde
over the MoP C-y catalysts a
Catalysts
Conversionb (%)
Selectivityc (%)
Yieldb (%)
MoP C
97.93
90.09
92.65
74.54
62.05
45.00
97.58
90.31
60.87
58.38
62.14
71.56
95.56
81.36
56.40
43.52
38.56
32.20
MoP C-10
MoP C-20
MoP C-40
MoP C-100
MoP C-240
a
The reaction was carried at 95 ꢁC for
3
h, formaldehyde/a-
b
methylstyrene ¼ 4.5, 1.0 g of catalyst. Conversion or selectivity from
c
a-methylstyrene. Yield of MPD.
The nature of the acid sites was studied via FT-IR spectros-
copy of the adsorbed pyridine, as shown in Fig. 5. The main
bands observed over all the samples were assigned as follows:
- 1490 cmꢀ1 was assigned to the H-bonded pyridine;
- 1540 cmꢀ1 was assigned to pyridine protonated on
¨
Bronsted sites;
- 1451 cmꢀ1 was assigned to pyridine adsorbed on Lewis acid
sites.
¨
The amount of Bronsted sites decreased in the order MoP C >
MoP C-10 > MoP C-40 > MoP C-240 as the Lewis acid sites decreased
slightly, with those acid sites attributed to P–OH and Mon+. This
trend was in line with content of P and Mo on the catalyst surface
(Fig. 3b). Certainly, it can infer that the total amount of acid sites
Fig. 4 TPD profiles of (a) MoP X and (b) MoP C-y with TOS.
ꢁ
¨
(mainly Bronsted sites) will decrease with TOS. Compared with
temperature range 400–700 C (peak 2) increased in the order
¨
MoP C, the Bronsted sites of MoP C-240 remarkably dropped down
low, but the yield of IP just decreased by nearly half, which implied
that the active sites for the condensation of IB with formaldehyde
MoP C < MoP C-10 < MoP C-20 < MoP C-40 < MoP C-100 < MoP
C-240. Peak 1, corresponding to NH3 adsorbed on the acid sites,
could be assigned to the mainly medium strength acid sites and
slightly stronger acid sites (>400 ꢁC). Therefore, from the
beginning of the reaction, the shi to a lower temperature
indicated that the strength of the acid sites changed to weaker
ones. The amount of acid sites determined by NH3-TPD corre-
lated with the mass of the catalysts listed in Table 1. Peak 2 was
attributed to gasication of the coke deposition, it was also
conrmed by treating the catalyst under 530 ꢁC for 30 min prior
to the adsorption of NH3 (Fig. S2†).
¨
were Bronsted sites as well as Lewis sites.
3.3 Study on a chemical pathway of catalysts with TOS
To gain insights into the inuence of the variation of the cata-
lysts on the chemical process, it was rst hypothesized that 4,4-
Fig. 6 GC chromatograms of MPD cracking over MoP C-y catalysts at
300 ꢁC for 30 min, 1.0 g of catalyst, 18 mg minꢀ1 of MPD with 90 ml
Fig. 5 FTIR spectra of pyridine adsorbed of MoP C with TOS at 150 ꢁC. minꢀ1 of N2.
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dimethyldioxane-1,3 (DMD) was the primary intermediate for
the one-step synthesis of IP in this study. DMD was conrmed,9
but cracked easily over 275 C.23 Therefore, the reaction route
ꢁ
was taken as a two-step process involving rst DMD forming
from aldehyde and alkene, then cracking to produce IP fast.
Here, a-methylstyrene and formaldehyde were used as probe
molecules to clarify the response of DMD forming and cracking
to the variation of the catalysts with TOS, which led to the
activity dropping. First, the MoP C-y catalysts were applied for
the condensation of a-methylstyrene and formaldehyde to
synthesize 4-methyl, 4-phenyl-dioxane-1,3 (MPD), as shown in
Table 3. The MPD yield reached 95.56% over MoP C and
decreased in the order MoP C > MoP C-10 > MoP C-20 > MoP C-
40 > MoP C-100 > MoP C-240, based on the variation of peak 1 in
NH3-TPD (Fig. 4b). The signicant decline in yield suggested the
ability of the catalysts to form MPD decreased with TOS. On the
other hand, those catalysts were also employed for MPD
cracking, which involved C–O bond cleavage and C–C bond
cleavage to give 2-phenyl-butadiene-1,3 and a-methylstyrene,
respectively. MoP C-240 showed excellent selectivity for C–O
bond cleavage to produce the desired product 2-phenyl-
butadiene-1,3 (Fig. 6). Also, the selectivity for C–O bond
Scheme 2 Catalysts behaviour with TOS.
cleavage showed the order MoP C-240 > MoP C-100 > MoP C-40 >
MoP C-20 > MoP C-10 > MoP C. Moreover, there were many
signicant by-products for MoP C, and they decreased in the
order MoP C > MoP C-10 > MoP C-20 > MoP C-40 > MoP C-100 >
MoP C-240. This suggested that the catalyst selectivity increased
with TOS, corresponding with the trend of peak 2 in NH3-TPD
(Fig. 4b). The peak at 12.7 min was attributed to MPD, which
increased in the order MoP C < MoP C-10 < MoP C-20 < MoP C-
40 < MoP C-100 < MoP C-240, this meant that the catalyst ability
to crack MPD decreased with TOS (Fig. 6). Therefore, it can be
concluded that MPD was formed and cracked at different active
sites over the MoP catalysts, which were denoted as the forming
sites (sites 1) and cracking sites (sites 2). Considering Fig. 4b
and 5, it can be deduced that sites 1 were due to acid sites, while
sites 2 were due to active carbon deposits, respectively.
Many studies have suggested that carbon was deposited and
covered in the stronger acid sites, which thus exposed the
medium and weak acid sites, which corresponded to the
selective activity of IP synthesis.26,31 This process leads to the
appearance of an induction period and a decrease in the
number of acid sites, and can even result in deactivation of the
Fig. 8 GC chromatograms of fresh MoP C and a mixture of catalysts at
Fig. 7 (a) NH3-TPD curves, and (b) GC chromatograms of MPD 5 min (a), 10 min (b) and 20 min (c), and elimination of the induction
cracking at 300 ꢁC for 30 min, 1.0 g of catalyst, 18 mg minꢀ1 of MPD period (d). Reaction conditions: 300 ꢁC, 1.5 g of catalyst, iC4H8/CH2O
with 90 ml minꢀ1 of N2.
¼ 7, WHSV ¼ 10 g (g h)ꢀ1
.
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Fig. 9 GC-MS of the products at the beginning 4–9 min (300 ꢁC,
catalyst mass is 1.5 g, iC4H8/CH2O ¼ 7, WHSV ¼ 10 g (g h)ꢀ1).
Fig. 11 XPS of P 2p (a) MoP C, (b) MoP C-10, (c) MoP C-40 and (d) MoP
C-240.
catalysts. In this case, the narrower distribution of medium acid
sites will lead to a higher selectivity. In this work, MoO3/SiO2
had a narrower distribution of acid sites than the MoP C-y
catalysts, thus it will give the best selectivity for MPD cracking.
Nevertheless, it seemed not the case as presented in Fig. 7.
Therefore, the acidity of sites is primary, but its nature is also of
paramount importance. This also further veried that the
deposited carbon is not useless.
Based on all the above discussion, it can be deduced that sites
1 on the catalysts favour the condensation of aldehyde and alkene
and are responsible for forming DMD, while sites 2 favour C–O
bond cleavage for diene-1,3. Thus, IP was “assembly line”
synthesized on sites 1 in cooperation with sites 2 over catalysts. At
the start of the reaction, sites 1 presented an outstanding ability
to form DMD, but sites 2 had not been formed or were in trace
amount only, so the amount of IP was low. When there was
sufficient sites 2, DMD was chiey synthesized and efficiently
cracked into IP, as presented in Scheme 2. Then with increasing
TOS, the decline of IP can be attributed to the decrease in sites 1,
which led to DMD cutting down.
If this deduction were right, it would suggest that: (1)
a catalyst possessing sites 1 and sites 2 can simultaneously give
good performance at the beginning of the reaction, and (2)
a large amount of DMD will appear at the beginning of the
reaction because of the excellent capacity to form DMD but the
poor ability to crack it. To further conrm (1), a mixture of MoP
C and MoP C-240 (1 : 1) were employed for the synthesis of IP, as
shown in Fig. 8. The mixture catalyst showed catalytic behav-
iour, as desired at the beginning of reaction, and the induction
period was nearly eliminated. As for (2), the products from the
synthesis of IP over fresh MoP C at the beginning of 4–9 min
were collected using a cold trap with liquid nitrogen and
determined by GC and GC-MS, as shown in Fig. 9. It was also
conrmed that forming DMD was the primary route for the
synthesis of IP in a one-step process over the MoP catalysts. It
must be pointed out that DMD can be formed and cracked at
both sites 1 and sites 2, which were thus competitive sites for
those two processes. Based on the experimental data, sites 1
were more abundant prior to DMD forming, while sites 2 were
more abundant prior to this for the selectivity of DMD cracking,
thus it is easy to conclude that keeping the balance of sites 1
with sites 2 is key to averting the deactivation of the catalysts.
3.4 Study of the nature of the active sites
To further clarify the nature of the active sites, XPS and MAS
NMR studies were conducted. Fig. 10 shows the XPS proles of
Scheme 3 Lewis acidity of Mn+ converted into Bronsted acidity sites:
¨
Fig. 10 XPS of Mo 3d (a) MoP C, (b) MoP C-10, (c) MoP C-40 and (d) (a) octahedrally coordinated Mo6+ as [MoO6], (b) tetrahedrally coor-
MoP C-240.
dinated Mo6+ as [MoO4].
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Fig. 14 Study of the coke deposit on P and Mo species at 10 min (a)
and 40 min (b) by TPD.
O–H groups through water dissociation. Those unsaturated Mn+
sites can also act as the adsorption species for reagents and
anchoring centres for the aldehyde molecule.41 The C 1s spec-
trum of the coke was also decomposed (Fig. 12), as seen through
curve tting using the binding energies of certain functional
groups, C–C, C–H, C–O and C]O.42,43 Those groups containing
oxygen may originate from alkoxides, ethers, esters or acetals,
as veried by the 13C MAS NMR peak at 129 ppm. The 13C MAS
NMR display the presence of two broad features in the ranges of
12–50 ppm and 115–150 ppm, as presented in Fig. 13. The peak
at 19 ppm can be attributed to olenic carbon atoms. The
olenic carbons are most probably protonated at the acidic sites
under high temperature to give working sites as C+.9,21
Furthermore, the ratio of the two peaks changed slightly with
TOS, which meant the coke components changed little, which
was much different from this reaction over NbP catalysts. In the
induction period, there was lower selectivity due to the insuffi-
cient number of working sites of C+ because of the low carbon
deposition.
Fig. 12 XPS of C 1s (a) MoP C-10, (b) MoP C-40 and (c) MoP C-240.
Mo 3d, while Fig. 11 shows P 2p of MoP C-y catalysts. Decon-
volution of the spectra indicated two doublets at 235.28/
232.08 eV (Mo6+ 3d3/2/3d5/2), which can be assigned to the high
oxidation state of Mo.32–34 The prole of P 2p exhibited a peak
with high binding energy (134.3 eV), which could be attributed
3+
to PO4
.
35 This indicated that Mo and P on the catalyst surface
were both stable and existed in the form of Mo6+ and PO43+ with
TOS. With regard to the species of Mo6+, it mainly existed as
a tetrahedrally coordinated [MoO4] and/or octahedrally coordi-
nated [MoO6].36,37 This suggested that Mo–O single bonds
exhibited more ionic character but C]O showed more covalent,
which demonstrated that Mo–O single bonds can easily be
broken.38 Moreover, the electronic density of the tetrahedrally
coordinated Mo6+ species was localized on the moieties of its
Mo6+ ¼ O2ꢀ (Mo5+–Oꢀ) double bonds, which inferred that Mo]
O double bonds could act as the adsorption centre for nucleo-
philic groups or reagents, such as Odꢀ]Cd+ of formaldehyde
molecules. Furthermore, the Lewis acidity of coordinative
In summary, with TOS, the improving selectivity for MPD
cracking (Fig. 6) with increasing carbon deposition (Fig. 4b), the
decreasing P species and the little change in the Mo species on
the catalysts surface (Fig. 3b) suggested that active carbona-
ceous species resulted in the good selectivity for MPD cracking.
Considering the decreasing MPD forming, and in particular the
excellent MPD yield over fresh MoP C, this implied that the Mo
and P species account for the MPD forming. Therefore, Mo6+
unsaturated Mn+ sites could convert into Bronsted acidity sites
¨
for phosphate catalysts under a process involving water
(Scheme 3),39,40 which was responsible for generating terminal
3+
and PO4 gave sites 1, while active carbonaceous species gave
sites 2. The increase in sites 2 accounted for the improving
selectivity of C–O bond cleavage. In addition, to conrm the key
role of P species for carbon deposition, the synthesis of IP from
Scheme 4 Proposed process of the “assembly line” process for the
synthesis of IP.
Fig. 13 13C MAS NMR of MoP C-y catalysts.
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but a decline in capacity with TOS. This shows we can nearly
eliminate the induction period by introducing cracking sites
into fresh catalysts. It also inferred that the imbalance between
the forming and cracking sites was a key reason for the catalysts
activity to decline. In this work, acid sites, active species and the
work of those sites were studied, and the relationship among
them was established. This information will provide key
insights for the design of an efficient solid catalyst for the
synthesis of IP. More detailed investigations of the mechanism
are under way.
Conflict of interest
Fig. 15 GC chromatograms at 10 min on stream (300 ꢁC, 1.5 g of
catalyst, iC4H8/CH2O ¼ 7, WHSV ¼ 10 g (g h)ꢀ1).
There are no conicts of interest to declare.
the reaction of formaldehyde with IB was conducted over H3PO4
supported on SiO2 and over MoO3 supported on SiO2. TPD
curves of those two catalysts at 10 min (within the induction
period) on stream and at 40 min (aer the induction period) on
stream are shown in Fig. 14. It can be observed that H3PO4
supported on SiO2 was more advantageous prior to carbon
deposition. Moreover, the content of carbon deposition with the
MoP X catalysts at 10 min on stream also conrmed this result
(Fig. S4†). Consequently, carbon deposition led to the total
Acknowledgements
This study was supported by the National Natural Science
Foundation of China [No. 51473156]; Synthetic Natural Rubber
&
Its Monomer Integrated Industrialization Technology-
Monomer Synthesis Technology [KFZD-SW-401].
Notes and references
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According to this process, the higher yield of SiO2 on supported
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