Methanol to aromatics: Isolated zinc phosphate groups on HZSM-5 zeolite enhance BTX selectivity and catalytic stability

Article abstract of DOI:10.1039/c9ra09657d

HZSM-5 zeolite combined with unique zinc and phosphorus species, yields excellent selectivity (～85%) to BTX (benzene, toluene, xylenes) in aromatic products. It was found that both zinc and phosphorus species were highly distributed in the pores of the zeolite channel network to form isolated zinc phosphate groups, which directly bond to the surface of zeolite, leading to a strong Lewis acidic center and an optimized surface acidity distribution favorable for BTX formation and the hydrothermal stability of the catalyst.

Full text of DOI:10.1039/c9ra09657d

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Methanol to aromatics: isolated zinc phosphate
groups on HZSM-5 zeolite enhance BTX selectivity
Cite this: RSC Adv., 2020, 10, 5961
and catalytic stability†
a
Jian Qiao,
Jianqiang Wang,a Anatoly I. Frenkel,b Jiawei Teng,a Xiqiang Chen,a
a
a
a
a
a
*
Jingxian Xiao, Tiezhu Zhang, Zhendong Wang, Zhiqing Yuan and Weimin Yang
HZSM-5 zeolite combined with unique zinc and phosphorus species, yields excellent selectivity (ꢀ85%) to
BTX (benzene, toluene, xylenes) in aromatic products. It was found that both zinc and phosphorus species
were highly distributed in the pores of the zeolite channel network to form isolated zinc phosphate groups,
which directly bond to the surface of zeolite, leading to a strong Lewis acidic center and an optimized
surface acidity distribution favorable for BTX formation and the hydrothermal stability of the catalyst.
Received 19th November 2019
Accepted 18th December 2019
DOI: 10.1039/c9ra09657d
rsc.li/rsc-advances
The conversion of methanol to aromatics (MTA) has been Nevertheless, it is generally accepted that phosphorus inhibits
turning into an increasingly important alternative source of the dealumination of zeolites, which, in turn, increases their
light aromatics including benzene (B), toluene (T), xylenes (X), hydrothermal stability and catalytic selectivity. Our hypothesis
with the development of coal-, shale gas-, or biomass-derived is that the combination of P-modied HZSM-5 catalyst and Zn/
methanol via the syngas route.1–4 By modifying HZSM-5 with HZSM-5 catalyst could enable both the enhancement of
zinc ions,1–14 light aromatics could be selectively produced. hydrothermal stability and aromatics selectivity of MTA cata-
However, it has many drawbacks by exchanging zinc ions lysts. Several groups have reported that adding phosphorus to
directly to HZSM-5, such as low selectivity, loss of zinc15 and the Zn/HZSM-5 promotes the activity and stability of the catalysts in
coke formation on the surface of the catalyst. The catalytic light alkane aromatization by decreasing the loss of zinc,29–31 as
dehydration of methanol produces large amounts of water well as in aromatization of uid catalytic cracking (FCC) frac-
vapor during MTA and the subsequent regeneration process at tions.31–34 There are only a handful of ZnPHZ catalyst types used
temperatures higher than the reaction temperature,16–18 call for in MTA reaction,35,36 and most of such reports have been
catalysts with high hydrothermal stability.19 Therefore, the focused on the reaction studies, not the catalyst structure. In the
preservation of good activity and selectivity in the severe reac- remainder of the Introduction, we will present our motivation
tion and regeneration conditions of MTA processes requires the of synthesizing a hybrid ZnPHZ catalyst.
catalysts to maintain good thermal and hydrothermal stabili-
Controlling acid properties is among the key factors for
ties, moreover, understanding the nature of, and controlling achieving selective transformation using HZSM-5 zeolites37,38 In
over stability of both zinc ions and HZSM-5 framework of the MTA reaction, the Lewis nature of hydroxyl zinc ions can
catalyst are among the main challenges in the eld.
Modication of HZSM-5 by phosphorus (referred to as P/ the cracking process to C3 fragments by decreasing stronger
accelerate the recombination of surface hydrogen,39,40 suppress
¨
HZSM-5) is extensively used to improve the hydrothermal Bronsted acid and enhance cyclization and dehydrogenation of
¼
¼
stability of HZSM-5 zeolite18,20–24 in converting methanol to light C6 –C9 oligomer intermediates to aromatics.40 Hersh et al.41
olens,25 alkylation and/or disproportionation of aromatics.26,27 reported reaction of NH3 with the zinc ions of zinc phosphate
However, the sole use of P-modied HZSM-5 catalyst is rarely glasses to form coordinately bound NH3, showing the Lewis
reported in MTA reaction,7,28 to the best of our knowledge. As nature of the insoluble zinc phosphate. Yuan et al.42 reported
shown in this work, the BTX formation over P/HZSM-5 catalyst the possibility to synthesize hydroxyl zinc phosphate materials
is limited in comparison with Zn/HZSM-5 catalyst. by reacting phosphate groups with zinc nitrate. Hence, in order
to reduce zinc loss in exchanged bridging hydroxyl groups
during MTA process, an incorporation of stabilized phosphorus
aState Key Laboratory of Green Chemical Engineering and Industrial Catalysis, Sinopec
groups seems to be an attractive alternative. By anchoring the
Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China.
phosphorus group on P/HZSM-5 catalyst with the zinc species to
E-mail: yangwm.sshy@sinopec.com; Fax: +86-21-68462283; Tel: +86-21-68462197
form the insoluble zinc phosphate groups, it may inherit the
hydrothermal stability from P-modied catalysts. In addition,
the formation of insoluble zinc phosphate groups might
bDepartment of Materials Science and Chemical Engineering, Stony Brook University,
Stony Brook, NY 11794, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09657d
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contribute its Lewis nature for altering the acidity distribution, volume (Vmicro) of the parent HZ, PHZ and ZnPHZ samples. The
causing better aromatization properties in MTA reaction, as well results indicated that the BET surface area and microporous
as stabilizing the hydroxyl zinc ions by the formation of insol- surface area were both slightly reduced by impregnation of
uble zinc phosphate groups.
phosphoric acid to the HZ sample and further reduced by the
Herein, we report preparation of Zn-P/HZSM-5 catalyst with following introduction of zinc species to PHZ sample mean-
isolated zinc phosphate groups, verication of its catalyst while, the external surface area was gradually increased, which
performance and characterization of the local structure and gave the further explanation that both phosphorus acid and
chemical state of Zn and its nearest neighbors that shed light on zinc ions were impregnated into the pore channels.47
the nature of the Zn-P/HZSM-5 complex. The formation of zinc
MAS NMR spectra of HZ (no. 1), PHZ (no. 2) ZnPHZ (no. 3),
phosphate groups by adding zinc ions to P/HZSM-5 catalyst is reference material Zn3(PO4)2, (no. 4, denoted as r-Zn3(PO4)2)
studied by NMR. Detection of possible incorporation of zinc and PZnHZ (no. 5) samples were shown in Fig. 1 as the captions
into phosphate groups and characterization of its nearest described respectively. Fig. 1a shown the 29Si spectra of HZ, PHZ
coordination sphere are accomplished by using XANES and and ZnPHZ samples, in which there were four characteristic
EXAFS techniques in combination with FTIR measurements of peaks, had been deconvoluted and identied as Q4 [Si(OSi)4]
the change of bridging hydroxyl groups. The Lewis nature of as- species (ꢁ112.3 ppm), inequivalent Q4 [Si(OSi)4] (ꢁ115.9
prepared zinc phosphate groups on Zn-P /HZSM-5 catalyst are ppm),24,48 Q4 [Si(OSi)3(OAl)] (ꢁ105.9 ppm) and Q3 [Si(OSi)3(OH)]
identied by NH3-TPD (temperature programmed desorption of (ꢁ100.0 ppm).24,47,48 Based on Loewenstein's rule,49 the frame-
ammonia) and Py-FTIR (FTIR spectroscopy using pyridine as work Si/Al ratio was calculated and list in Table 1.50 It suggested
a probe molecule) measurements. Finally, the catalyst perfor- that H3PO4 impregnation to HZ sample followed by a subse-
mance of ZnPHZ (short for Zn-P /HZSM-5) in MTA reaction is quent thermal treatment at 550 ꢂC led to sort of dealumination
interpreted by correlating catalyst structure and its acidic of the framework, whereas the addition of zinc species did not
¨
properties (the distribution of Bronsted acid and Lewis acid), as affect the zeolite framework.
well as the comparative research with the reference catalysts
Fig. 1b shown the 27Al MAS NMR spectra of HZ, PHZ and
including the parent HZSM-5, P/HZSM-5, Zn/HZSM-5 and P-Zn/ ZnPHZ samples. Typically, the spectra of the HZ sample consist
HZSM-5, denoted as HZ, PHZ, ZnHZ and PZnHZ respectively as of tetrahedrally coordinated framework sites (ꢀ54 ppm, Altet-f),
mentioned above.
distorted tetrahedral coordinated framework sites (30–40 ppm,
51
The structural and crystalline characteristics of the samples Altet-dis
were studied by XRD (ESI, Fig. S1a†). All of the samples were (Altet-nf),52 octahedrally coordinated non-framework aluminum
well crystallized as the MFI structure, as deduced from the XRD species (ꢁ20 to 8 ppm, Aloct
)
or tetrahedrally coordinated non-framework sites
20
)
or framework defect sites (ꢁ20–
spectra. Table 1 shown the relative crystallinity of the samples 8 ppm, Al-(OSi)3(H2O)3, Aloct-f).53 For PHZ sample, the band at
as compared to the parent HZ sample. The relative crystallinity ꢁ8 ppm could be observed due to the association of the octa-
decreased to a certain extent for sample impregnated with hedral Al sites with phosphorus (Aloct-o-p).22,47,54 The broad band
orthophosphoric acid, and this might be due to the deal- appeared at the range of 45 to 15 ppm in comparison to the HZ
umination of the zeolite sample, surface modication and sample. It suggested the increase of Altet-dis and Altet-nf sites,
thermal treatment. Aer the addition of zinc to PHZ sample, the which was correlated with the bonding of Al with phosphorus
crystallinity of HZ for ZnPHZ sample decreased signicantly, species. For ZnPHZ sample, the bands at Altet-dis, Altet-nf, and
but the HZ support still kept the origin structure. Synchrotron Aloct-o-p sites were decreased, and a sharp band appeared at
based XRD (ESI, Fig. S1b†) and scanning TEM (ESI, Fig. S2†) ꢀ38 ppm. Obviously, all these phenomena suggested that the
were used to further conrm the absence of zinc oxides or zinc local environments of both tetrahedrally coordinated Al phos-
phosphate particles formed on the catalyst surface, indicating phoric groups and octahedral Al phosphoric groups redeploy
that the zinc species was highly dispersed, amorphous or singly aer the addition of zinc species, moreover, the octahedral Al
bonds to the catalyst surface.
phosphoric groups combined with zinc ions to form a new
Surface area and micropore analysis datum were listed in structure.
Table 1, including the BET surface area (SBET), microporous
Fig. 1c showed the 31P MAS NMR spectra of PHZ, ZnPHZ,
surface area (Smicro), external surface (Sext) and microporous PZnHZ samples and r-Zn3(PO4)2. The spectrum of PHZ sample
Table 1 Physicochemical characteristics of catalysts
Catalysts
SBET, m2 gꢁ1
Smicroa, m2 gꢁ1
Sexta, m2 gꢁ1
Vmicro, mL gꢁ1
Cry.b (%)
Xc, %
Xd, %
HZ
PHZ
ZnPHZ
361
357
349
226
210
199
135
147
150
0.104
0.096
0.091
100
91
65
44.2
51.5
50.1
46.4
53.4
51.1
a
t-Plot method. b Cry. represents crystallinity based on XRD spectra, estimated by comparing the total XRD peak area of the zeolite sample in range
of 2q from 22ꢂ to 25ꢂ with that of the parent HZ having the strongest diffraction intensity. c X represents the Si/Al ratio based on NMR data by using
d
dm2002 soware.50 X represents the Si/Al ratio based on ICP-AES measurement.
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Fig. 3 (a) FTIR spectra measured at 40 ꢂC after dehydration at 450 ꢂC
for 1 h and (b) Py-FTIR spectra measured at 200 ꢂC, 300 ꢂC and 400 ꢂC
after desorption at the corresponding temperatures of catalysts for: (1)
HZ, (2) PHZ, (3) ZnPHZ.
Fig. 1 29Si (a), 27Al (b), 31P (c) MAS NMR spectra of catalysts: (1) HZ, (2)
PHZ, (3) ZnPHZ, (4) r-Zn3(PO4)2 and (5) PZnHZ.
be seen from P-perspective in 31P NMR spectrum of PZnHZ
sample.
(Fig. 1c(2)) featured two intensive peaks at ꢁ39 ppm ((SiO)x-
Al(OP)4ꢁx species24 with branching P sites in condensed poly-
phosphate,55
polyphosphoric
species,20),
ꢁ33.5
ppm
XANES spectra43–45 related to Zn K-edge for ZnPHZ, PZnHZ,
and ZnHZ samples and reference materials r-Zn3(PO4)2, and r-
ZnO (short for zinc oxide) were shown in Fig. 2a. Absorption
edge energies (as calculated for the rst inection point of the
edge rise) were 9663.7 eV for all samples except for the r-
Zn3(PO4)2 for which the edge energy was 9663.5 eV. The XANES
spectra of ZnPHZ showed the similar pattern to the r-Zn3(PO4)2
but quite different from r-ZnO. XANES data (Fig. 2a inset)
demonstrated the similarity between Zn mirco-environments in
ZnPHZ sample and r-Zn3(PO4)2, whereas the whiteline shape of
PZnHZ catalyst was close to that in P-free sample of ZnHZ
sample. It should be noted that all the samples were measured
at room temperature aer in situ water removal at 430 ꢂC to
eliminate the effect of water adsorption on XANES spectra. We
thus concluded that the similarity of the whiteline shape
between ZnPHZ and r-Zn3(PO4)2 accounts for the similar
structures of phosphante groups that had different
concentrations.
(Al(H2O)5(PO4)55–58) and a broad peak with a maximum at about
ꢁ17 ppm from 10 to ꢁ40 ppm (the specic Al–O–P structures,
which could be short-chain pyrophosphoric acid or pyrophos-
phates bonding to non-framework Al59 or to framework Al56).
The spectrum of ZnPHZ (Fig. 1c(3)) showed a signicant
decrease for Al–O–P structures and a slight reduction for
(SiO)xAl(OP)4ꢁx species and Al(H2O)5(PO4). Comparison of its
spectrum with that of r-Zn3(PO4)2, as shown in Fig. 1c(4), indi-
cated that the transformation of short-chain polyphosphoric
species (10–ꢁ40 ppm) on PHZ sample to zinc phosphate groups
(3 ppm) on ZnPHZ occurred. The 31P NMR spectrum of PZnHZ
in Fig. 1c(5) featured a sharp contrast in the bands distribution
from 20 to ꢁ80 ppm, which clearly indicated that only a small
amount of zinc phosphate groups showed up, which was totally
different in the structure or conguration of zinc and phos-
phorus species from the ZnPHZ sample. The spectra of PZnHZ
sample show that P did see Zn but not too much of it, which
meant that zinc ions could nd the phosphoric groups to form
a new structure – zinc phosphate groups – during the prepara-
tion of ZnPHZ sample, whereas such structure was nowhere to
Fourier transforms (FT)-EXAFS data was shown in Fig. 2b.
The peak positions were lower than real-space distances
between Zn atoms and its nearest neighbors due to the photo-
electron phase shi, not corrected for in Fig. 2b. The peak
˚
around 1.60 A for r-Zn3(PO4)2 in the FT magnitude (shown by an
arrow in Fig. 2b), which was attributed to the nearest oxygen
neighbors with the bond Zn–O–P,60 corresponded to a Zn–O
˚
˚
bond length of 2.04 A. The second main peak around 3.05 A was
Fig.
2 (a) Derivative of normalized XANES region of absorption
coefficient of ZnPHZ, PZnHZ, ZnHZ catalysts and r-Zn3(PO4)2 and r-
ZnO (inset: normalized XANES region of absorption coefficients), and
(b) Fourier transform magnitudes of corresponding k2-weighted
EXAFS spectra.
Fig. 4 NH3-TPD profiles of catalysts: (a) HZ, (b) PHZ, (c) ZnPHZ.
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associated with the bond of Zn-P with the actual bond length of since they were obscured by the bands of Si–OH, Al–OH. It should
3.28 A. In order to quantitatively analyze the local structure of be noted that the bands at 3610 cmꢁ1 were specic to the bridged
˚
zinc species, data analysis of EXAFS spectra (ESI, Fig. S6†) was hydroxyl groups. The peak height of the band at 3610 cmꢁ1
performed by using IFEFFIT package.61–63 Theoretical photo- decreased when phosphorus was impregnated to HZ sample,
¨
electron scattering amplitudes and phase shis of Zn–O and Zn- suggesting that part of the bridging hydroxyl groups (Bronsted
P contributions were generated by FEFF6 soware64 using acid site) were eliminated by reacting with orthophosphoric acid.
crystal structure of r-Zn3(PO4)2. The passive electron reduction On the contrary, the bridging hydroxyl groups kept unchanged or
factor was xed to 1.0, as found from the t to the r-ZnO. The even stronger when zinc was further impregnated to PHZ sample,
results obtained for the ZnPHZ system demonstrated that each indicating zinc ions were not exchanged with the bridging
Zn ion in the nearest coordination shell was surrounded on hydroxyl groups, properly evidenced that zinc ions reacted with
˚
average by 3.5 ꢃ 0.6 A of O atoms with a coordination distance phosphorus groups to form zinc phosphate groups.
2
˚
1.95 ꢃ 0.02 A and meant squared bond length disorder s ¼
Py-FTIR spectra of HZ, PHZ, and ZnPHZ samples aer pyri-
2
˚
0.011 ꢃ 0.005 A . Relatively large uncertainties prevented us dine desorption at different temperatures were shown in
from accurately measuring the number of Zn-P species in the Fig. 3b. The characteristic bands67,70 were clearly observed at
second shell because of the correlation of the coordination ꢀ1450 cmꢁ1 for Lewis acid sites and at 1540 cmꢁ1 for Bronsted
¨
numbers and bond length disorder in the tness. Alternative acid sites. For both the different conditions of samples at the
models were compared, in which we added the Zn–Zn contri- same temperature and the same catalyst at the different
¨
bution (calculated theoretically using r-ZnO structure) as desorption temperature, the bands for Bronsted acid sites
a possible second neighbor to Zn instead, or in addition to, Zn- shied to lower wavenumbers, whereas the bands for Lewis acid
P, but no reasonable results were obtained. Notably, we found sites to higher wavenumbers. It was demonstrated that both
¨
that a model where Zn–Zn was the only contribution to the Lewis acid sites and Bronsted acid sites had different strength
second coordination shell, as in the local structure of r-ZnO, of acid sites existed on the zeolite surface. Quantication of
¨
weak, medium, and strong acid sites for Bronsted and Lewis
was not consistent with the experimental spectra.
For PZnHZ sample, each Zn ion in the nearest coordination acid sites was performed according to the amount of pyridine
˚
shell was surrounded on average by 3.9 ꢃ 0.5 A of O atoms with desorption at different temperature by the same way as every-
2
2
a coordination distance 1.96 ꢃ 0.01 A and s ¼ 0.008 ꢃ 0.002 A ; where.2,35 To allow semi-quantitative comparisons of the peak
˚
˚
for ZnHZ sample, each Zn ion in the nearest coordination shell intensities, all Py-FTIR spectra were normalized using the area
˚
was surrounded on average by 4.1 ꢃ 0.6 A of O atoms with of the overtone lattice vibration bands of the zeolites at
2
2
a coordination distance 2.01 ꢃ 0.01 A and s ¼ 0.008 ꢃ 0.002 A . 1990 cmꢁ1 and 1873 cmꢁ1, as conducted in the literatures.2,36,71
The main summary of the above results was that the Zn–O bond Fig. 4 showed the acidities of different samples measured by
length changed signicantly from PZnHZ to ZnHZ sample, NH3-TPD method.15,40,72–76 It was found that for HZ, PHZ and
whereas the coordination number was similar for the two ZnPHZ samples: (1) the relative total acidity amount was 100%,
samples. Such coordination number of ca. 4.0 of Zn–O was in 99.4% and 78.3%; (2) the weak acid amount was 100%, 128.1%
agreement with the reported structure of ZnHZ catalyst measured and 118.8%; and (3) strong acid amount was 100%, 69.6%, and
aer water removal,65 which suggested Zn species were in 35.4% respectively (ESI, details showed in Fig. S3†).
˚
˚
a similar coordination environments in the rst shell for both
The effect of the reaction time on methanol conversion over
PZnHZ and ZnHZ samples. The shorter average Zn–O bond ZnPHZ sample was shown in Fig. 5a. It could be seen that the
length in the former catalyst was consistent with the contribution catalyst exhibited a stable performance during the reaction
+
of phosphate groups to the nearest environment of zinc ions, procedure. All of the yields of C1–C5 and benzene (C6A)
because the bond length of Zn–O in Zn–O–P groups in Hopeite
˚
structure varied from 1.90 to 2.05 A (ESI, Fig. S7†), hence, had
a fraction of shorter bonds compared to the ZnHZ system.
In brief, the EXAFS tting results, taking together with other
evidences from XANES, NMR and catalytic tests, supported the
idea that both zinc species and phosphate groups existed as the
form of zinc phosphate groups on ZnPHZ sample, which quite
differentiated from those on ZnHZ and PZnHZ samples. The
absence of Zn–Zn bond further suggested that the presence of
these zinc phosphate groups was isolated.
Fig. 3a displayed FTIR spectra of the HZ, PHZ, and ZnPHZ
samples. It could be clearly seen there were four hydroxyl bands,
assigned as amorphous silica hydroxyl groups (3740 cmꢁ1),66
terminal framework silanols (Si–OH, 3720 cmꢁ1),67 extra-
Fig. 5 (a) Product distribution (P. D.) of MTA reaction for catalyst
ZnPHZ; (b) yield of BTX (YBTX), yield of aromatics (YA), selectivity to
BTX (SBTX) of catalyst ZnPHZ in comparison with reference catalyst
framework aluminum species or partially hydrolyzed aluminum
species (3660–3690 cmꢁ1),68 and bridging hydroxyl groups Si(OH)
Al (3610 cmꢁ1).69 Both bands for P–OH groups41 at 3666 cmꢁ1 and
ZnHZ. Reaction conditions: 0.1 MPa, 430 ꢂC, 3.0 g catalyst, WHSV ¼
bands for Zn–OH groups41 at 3620–3674 cmꢁ1 were not discerned 2.0 hꢁ1
.
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hydrocarbons slightly decreased with time on stream (TOS),
while the yields of toluene initially increased and then steadily
Discussion
+
remained. The C9+ aromatics (C9 A) yield continuously increased
The IZPG (short for isolated zinc phosphate groups), as we
mentioned above, were perfectly evidenced by XANES and
EXAFS methods, as well as FTIR measurements. From the
procedure of catalyst preparation method,47 the local mirco-
environment of could be further deduced from the principal
phosphorus groups which were conrmed to bond with zinc
ions. As from the results of 27Al NMR spectra of ZnPHZ catalyst
(ESI, Fig. S4†), the descent of Altet-dis, Altet-nf, and Aloct-o-p and an
emerging sharp band at ca. 38 ppm suggested that the trans-
formation of both tetrahedrally Al phosphoric groups and
octahedrally Al phosphoric groups to zinc phosphoric groups
took place. Meanwhile, from the 31P MAS NMR spectra(ESI,
Fig. S5†), different amounts of zinc addition also demonstrates
that (SiO)xAl(OP)4ꢁx species (ꢁ39 ppm) gradually decreased, the
nonreactive Al(H2O)5(PO4) species (ꢁ33.5 ppm) remained
unchanged, whereas the amount of zinc phosphate groups (3
ppm) increased step by step and the principal peak centred on
ꢁ17 ppm was related to the specic Al–O–P structures, which
could be assigned to short-chain pyrophosphoric acid or pyro-
phosphates bonding to non-framework Al59 or to framework Al55
and revealed systematic increase occurred at the bands of all
distorted tetrahedral Al sites and a systematic decrease in
octahedral Al sites. On the other hand, only a slight uctuation
as prolong the reaction time. These phenomena implied that BTX
+
and C9 aromatics might be produced through a common
¼
¼
intermediate, such as (C6 –C9 ) oligomer intermediates. The
yield of BTX (YBTX) was stable, and the selectivity to BTX shown
only a slightly decrease due to the yield of total aromatics (YA)
slightly increased. For HZ, PHZ and ZnHZ catalyst, all showed the
similar fuction in the effect of the reaction time on methanol
conversion (ESI, Fig. S8†). Fig. 5b compared the performances of
ZnHZ and ZnPHZ catalysts under the same reaction conditions.
It clearly showed that ZnPHZ catalyst owned better selectivity to
BTX.
Table 2 and Fig. S8† displayed the catalytic reactivity of
ZnPHZ catalyst in the conversion of methanol to light
aromatics, including the HZ, PHZ, ZnHZ, and PZnHZ catalysts.
Methanol was completely converted on all catalysts under the
same reaction conditions. The differences in the reaction
performance could be attributed to changes in the nature of the
catalytic sites present: (1) the parent HZ catalyst exhibited only
25.61% yield of BTX and 75.58% selectivity to BTX; By addition
of Zn to PHZ, the ZnPHZ catalyst showed 50.59% yield of BTX
(32.85% for PHZ) and 84.73% selectivity to BTX (81.89% for
PHZ); (2) ZnPHZ catalyst showed different product distribution
+
(featured as lower C1–C5 light hydrocarbons and signicantly
¨
appeared at the peak of tetrahedral Al sites (Bronsted acid sites)
+
lower C9 aromatics) from the ZnHZ catalyst; (3) toluene and
as the increase of zinc amount from 0 to 3.0% to PHZ catalyst. It
was a compelling proof that preferential reaction between
phosphorus groups and zinc ions took place by adding zinc ions
to PHZ catalyst.
xylene were the main products in the MTA reaction for all the
catalysts, among which ZnPHZ catalyst had the maximal values
of 22.39% and 23.40% for YT and YX, respectively; (4) ZnPHZ
catalyst exhibited the superior performance to the reference
catalysts, ZnHZ and PZnHZ samples. W. D. Xiao82 et al. reported
a MTA catalyst, HZSM-5 zeolite catalyst (SiO2/Al2O3 ¼ 30)
modied by 1.0 wt% ZnO and 2.0 wt% P, by using of the special
titanium made U-shaped reactor, in the reaction condition of
temperature range from 380 to 480 ꢂC, pressure:15–50 kPa,
This unique structure of IZPG was also conrmed in compar-
ison with the zinc environment of ZnHZ and PZnHZ samples, from
the EXAFS data in r-space (ESI, Fig. S5†), the zinc mirco-
environment of ZnHZ and PZnHZ samples, exhibited a similar
uxion, which meant the coordination environments for both
catalysts was similar, but quite different from ZnPHZ sample. It was
also expected that the lower intensity of the Zn–O peak suggested
a lower coordination numbers for Zn-P bonds in the ZnPHZ system
compared to the PZnHZ and ZnHZ samples, as well as r-Zn3(PO4)2,
consistent with the conclusion made by examination of the XANES
data (vide supra) that zinc phosphate groups were generated on
ZnPHZ sample during the procedure of adding zinc ions to PHZ
obtained the best aromatic product: YA ¼ 59.61%, with YB
¼
8.50%, YT ¼ 27.70%, YX ¼ 20.18%, which exhibited nearly the
same yield of aromatics, slightly different to the PZnHZ catalyst
in the BTX distribution, owing to the different content of Zn and
P. Meanwhile, this MTA catalyst, similar to ZnPHZ catalyst,
should contain the isolated zinc phosphate groups.
Table 2 Reactivity of HZ, PHZ, ZnPHZ and ZnHZ catalysts in MTA
YBTX (wt%)
YA (wt%)
SBTX (%)
Yields (wt%)
+
+
Catalysts
C1 + C2
C3
C4
C5
B
T
X
EB
C9
BTX
A
S
HZ
PHZ
ZnPHZ
ZnHZa
PZnHZa
5.61
5.15
5.08
6.17
7.85
22.47
27.51
15.20
12.16
13.89
24.74
21.09
14.13
12.98
14.09
13.29
6.14
5.89
6.90
11.45
2.60
4.07
4.80
3.27
2.38
9.99
13.01
15.00
23.40
26.20
19.87
0.82
0.64
0.74
0.87
1.32
7.45
6.62
8.37
13.20
13.75
25.61
32.85
50.59
47.71
33.00
33.89
40.12
59.70
61.78
47.13
75.58
81.89
84.73
77.23
70.01
13.78
22.39
18.24
14.23
a
+
+
Data point: TOS@60 min; B ¼ benzene, T ¼ toluene; X ¼ xylene; C5 ¼ alkanes and olens with C5 and plus; C9 ¼ aromatics with C9 and plus; A ¼
total aromatics.
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catalyst. However, upon adding phosphoric acid to ZnHZ sample, a signicant increase was obtained over ZnPHZ catalyst with the
the structure of zinc just slightly changed as evidenced in 31P NMR change of the acidic distribution. As known to all, the acidic
spectra. It should be noted that the amount of zinc phosphate distribution of catalyst played an important role in the catalytic
groups on PZnHZ sample should be very low, if any.
reactivity. The catalyst should have a moderate B/L ratio so as to
From NH3-TPD and Py-FTIR80,81 results, we could further nd obtain the high BTX yield, which preferable contribute the
the IZPG not only altered the acid strength distribution of the formation of BTX and also suppressed the generation of carbon,
zeolite surface but also affected the total acidic amounts. As coke and heavy aromatics.2,36,40 According to the reaction
shown in Fig. 6 and Table S2,† the contributions of Lewis nature mechanism reported,2,6,40 it could be deduced that the cracking
¼
¼
of IZPG could be deduced from the results by the ascent of or methylation of C6 –C9 oligomer intermediates was sup-
Lewis acid distribution. Via the addition of zinc ions to PHZ pressed over ZnPHZ catalyst leading to the decrease of C1–C5
+
sample, the summit of Lewis acid sites was shied to a higher fraction and C9 aromatics, whereas the dehydrogenation
¼
¼
frequency, and indicated that zinc phosphate groups act as reaction of C6 –C8 oligomer intermediates was enhanced over
strong Lewis acid sites. It was reported that the reduction of the ZnPHZ catalyst leading to the high BTX selectivity. It was well
¨
bridging hydroxyl groups (Bronsted acid sites) derived from its accepted that exchanged zinc ions promoted the desorption of
reaction with the supported othorphosphorus acid,77,78 or as H atoms to H2, which greatly res trained hydrogen transfer to
a result of the lattice dealumination by the dislodged aluminum olens but facilitate dehydrogenation of cyclic olens to
species reacting with H3PO4,47 which interpreted the reduction produce aromatics.40 In this case, IZPG might play a similar role
of the strong acid peak in the curve of NH3-TPD in the as ZnHZ catalyst but with more suitable acidity distribution and
temperature range between 300–600 ꢂC (Fig. 4). Meanwhile, the better hydrothermal stability, as well as the MTA catalyst, Zn
impregnation of HZ with phosphoric acid was accompanied by and P co-modied HZSM-5.35,36,82
¨
the creation of Bronsted acid sites with an acid strength lower
As it is known to all, for the MTA process, zinc play the most
popular enroller, employed by HZSM-5 zeolite, which associated
than that of the bridging hydroxyls.22,66
¨
It was thus the acid amount lost by the reduction of Bronsted with the acidic sites of zeolite framework, generated more Lewis
acid sites or the other acid sites could be compensated by acid (ZnOH+, Zn2+-L acid center), make the catalyst more
ammonia uptakes from phosphorus hydroxyl groups, which effective to convert the intermediate olens into aromatics by
accounted for the total acid amount of PHZ sample was close to dehydrogenation, via the expense of the silanol hydroxyl and
the HZ sample, which meant, the phosphoric groups reacted proton acid sites. And the impregnation of phosphorus to ZSM-
with zinc ions and further transformed to zinc phosphate 5 can also adjust the acidity of zeolite and improve its hydro-
groups. The loss of acid amount of ZnPHZ sample should be thermal stability, but excessive introduction of P could lead to
¨
ascribed to the elimination of P–OH groups by the combination the covering of Bronsted acid sites and blockage of pores,
of zinc ions and P–OH groups. Therefore, the total acid amount resulting in MTA activity decreased.
was further reduced. It could be also concluded that the
In the methanol to hydrocarbon (MTH) reaction, both
¨
transformation from phosphate groups to zinc phosphate Bronsted acid sites and Lewis acid sites can regulate the
groups increased the high temperature peak of ammonia conversion of methanol to dimethyl ether and the further
¼
¼
ꢂ
ꢂ
desorption from 444 C to 470 C from Fig. S3 and Table S1.† formation of DME to low-carbon olens (C2 –C3 ), the poly-
¼
Therefore, the strength of IZPG could be identied as strong merization of low-carbon olens to high-carbon olens (C4
–
¼
¨
Lewis acid sites in agreement with the result.
C8 ). Bronsted acid sites can also catalyse the cyclization of
Fig. 6 plotted the specic yield of BTX, yield of total higher olens and the hydrogen transfer of cycloalkanes. In
aromatics and selectivity to BTX in correlation with the corre- order to maximize the performance of certain MTA catalyst with
¨
sponding acidic distribution was plotted by It exhibited that IZPG species, there should be kept counterbalance of Bronsted
acid sites to Lewis acid sites (B to L) ratio nearly 0.2, as Fig. 6 and
Table S2† showed.
Table 3 and Fig. S9† shown the reactivity of the fresh ZnPHZ
catalyst and the regenerated catalysts. Technically speaking,
xed bed reactor is not suitable for MTA process due to the
deactivation rate.4,16,79 In the present research, the catalyst was
operated in the xed bed reactor for 240 min, then the reaction
was terminated to regenerate in situ to simulate the uidized
bed reaction for testing the stability of ZnPHZ catalyst. As
indicated in Table 3, ZnPHZ catalyst exhibited excellent recov-
erable performance during as many as 5 cycles of reaction-
regeneration were carried out. Only a slightly decrease could
be found in the yield of BTX. It was pronounced that such
catalyst has a robust structure with good thermal and hydro-
thermal stabilities. Table 4 summarized the Zn and P contents
for the fresh and regenerated catalysts aer reaction-
regeneration cycles. The initial P and Zn contents in the fresh
Fig. 6 Relationship between yields of BTX, aromatics and BTX selec-
tivity, in MTA reaction, and the distribution of different type of acidic
sites in weak, medium and strong acidic strength for HZ, PHZ, and
ZnPHZ respectively. Reaction conditions see Fig. 5.
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Table 3 Reactivity of fresh ZnPHZ catalyst and regenerated catalysta
YBTX (wt%)
YA (wt%)
SBTX (%)
Yields (wt%)
C1 +
C2
+
+
Run no.
C3
C4
C5
B
T
X
EB
C9
BTX
Aromatics
Selectivity
0
1
2
3
4
5
5.08
5.69
5.97
6.21
6.60
6.77
15.20
14.82
15.02
14.88
15.10
15.09
14.13
13.78
14.18
14.33
14.70
14.87
5.89
5.96
6.37
6.71
6.71
6.98
4.80
4.52
4.26
4.14
4.19
4.11
22.39
21.55
20.66
20.41
20.51
20.36
23.40
23.95
23.83
23.56
23.15
22.82
0.74
0.74
0.75
0.78
0.74
0.78
8.37
8.99
8.98
8.97
8.29
8.22
50.59
50.01
48.74
48.12
47.85
47.30
59.70
59.74
58.47
57.87
56.88
56.30
84.73
83.71
83.37
83.15
84.11
84.01
a
+
+
Data point: TOS@60 min; B ¼ benzene, T ¼ toluene; X ¼ xylene; C5 ¼ alkanes and olens with C5 and plus; C9 ¼ aromatics with C9 and plus; A ¼
total aromatics.
Table 4 Zn and P contents of the fresh and regenerated catalysts
catalyst. ZnHZ catalyst, with the Zn loading of 0.75 wt%, was
Regenerated (wt%) aer each cycle
Components Fresh (wt%) 1a 2a 3a 4a 5a
prepared similar to ZnPHZ catalyst by impregnating HZ with
Zn(NO3)2 solution. The reference catalyst PZnHZ was prepared
by impregnating ZnHZ with orthophosphoric acid. All the
reference catalysts were prepared with the same P and Zn
loading weight as ZnPHZ.
6a
P@ZnPHZ
Zn@ZnPHZ
Zn@ZnHZ
0.97
0.75
0.73
0.96 0.94 0.93 0.93 0.95 0.91
0.76 0.75 0.73 0.70 0.62 0.59
0.53
—
—
—
—
—
a
Regenerated catalysts analyzed using ICP-AES aer each reaction-
regeneration cycle. Prior to each cycle, the used catalyst were calcined
in air at 550 ꢂC for 240 min for coke removal.
Characterization
Surface area and micropore analysis were analyzed by nitrogen
physisorption data (Micromeritics Tristar 3000, conducted in
Sinopec SRIPT). The crystallinity of catalysts was measured by
powder X-ray diffraction (Bruker Advance D8, conducted in
Sinopec SRIPT; Beamline 14A of the National Synchrotron Light
Source at Brookhaven National Laboratory). Scanning Trans-
mission Electron Microscopy is used for conrmation of the
absence of zinc oxide particles and the catalyst crystallinity of
ZnPHZ (JEOL ARM200F, conducted in Hefei, China). NMR
measurements were conducted at Sinopec SRIPT for 29Si, 27Al
and 31P MAS NMR spectra (Varian VNMR 400).
ZnPHZ catalyst were 0.97% and 0.75% respectively. Aer each
reaction-regeneration cycle, the content of P remains almost
unchanged for all the 6 times running. Meanwhile, the nal
zinc content decreased slightly from 0.75% to 0.59% aer 6
times running. For the ZnHZ catalyst, the zinc content denitely
decreased from 0.73% to 0.53% aer rst run. It is also
conrmed that the loss of zinc15,29–31 took place for ZnHZ cata-
lyst, while the ZnPHZ catalyst stabilized the phosphorus and
zinc by the form of isolated zinc phosphate groups and further
extended the ZnPZ catalyst recycle regeneration times above 20
can be seen in Fig. S10.†
XAFS measurements were performed to identify the chem-
ical states of zinc species. XAFS measurements were done at
beamline X19A of the National Synchrotron Light Source at
Brookhaven National Laboratory and Beamline BL2-2 of the
Stanford Synchrotron Radiation Light source. The catalyst
sample was grounded to ne powder and pressed into a pellet.
All spectra were collected at room temperature aer water
Experimental procedures
Catalyst preparation
ꢂ
removal at 430 C in transmission mode.
The starting material, HZSM-5 with silica to alumina mole ratio
of 100, was purchased from Shanghai Fuxu Molecular Sieves Co
(China). PHZ catalyst, loading with 1.0 wt% phosphorus, was
prepared by wetness impregnation as follows: (a) the impreg-
nation solution was prepared by mixing 31.6 g of aqueous
solution of orthophosphoric acid (10 wt%, H3PO4) and 118.4 g
H2O at room temperature; (b) 100 g of HZSM-5 zeolite was
added to the solution (a) with stirring at room temperature until
dryness for over 5 hours; (c) the impregnated sample was
subsequently dried at 393 K overnight and then calcined at
Hydroxyl surface groups of HZ catalysts were characterized
by Fourier Transform Infrared Spectroscopy (Nicolet 5700,
conducted in Sinopec SRIPT). NH3-TPD (Set up by Sinopec
SRIPT) and Py-FTIR (Nicolet 380, conducted in Sinopec SRIPT)
measurements were conducted to measure the catalyst acidity
amount and the distribution of acidity type. Chemical analysis
of the zeolite samples was performed by Inductively Coupled
Plasma-Atomic Emission Spectrometry (ICP-AES, conducted in
Sinopec SRIPT). The details are given in ESI.†
450 C for 4 h. The ZnPHZ46 (0.75 wt% zinc), was prepared by
ꢂ
Reaction evaluation
adding the 10.0 g P/HZSM-5 catalyst to 15.0 g of Zn(NO3)2
solution (2.30 wt%, Zn(NO3)2$6H2O), followed by the same The HZ, PHZ, and ZnPHZ catalysts, as well as the reference
stirring, drying and calcination procedure as for the PHZ catalyst ZnHZ, were respectively pressed, crushed and sieved to
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obtain particles in the range of 12–20 mesh (1.4–1.7 mm) for
catalytic evaluation in Sinopec SRIPT. Prior to the reaction, the
catalysts were blown out with owing nitrogen at 430 ꢂC for 2 h.
Methanol conversion was performed in a stainless-steel xed
bed reactor (650 mm length and 10 mm i.d.). The reaction
5 Zeolite Catalysts, Ind. Eng. Chem. Res., 2014, 53(39), 14932–
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3 F. Wang, W. Xiao and G. Xiao, Atomic layer deposition of
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5 Y. Ono, H. Adachi and Y. Senoda, Selective conversion of
methanol into aromatic hydrocarbons over zinc-exchanged
ZSM-5 zeolites, J. Chem. Soc., Faraday Trans. 1, 1988, 84,
1091–1099.
6 Y. Xin, P. Qi, X. Duan, H. Lin and Y. Yuan, Enhanced
performance of Zn–Sn/HZSM-5 catalyst for the conversion
of methanol to aromatics, Catal. Lett., 2013, 143, 798–806.
7 H. Jing, F. Yang, Y. Xia, J. Feng, J. Liu and Q. Zong, A study on
the selectivity of methanol aromatization, Pet. Sci. Technol.,
2012, 30(16), 1737–1746.
8 F. Yang, S. Wang, Y. Xia, W. Bi and W. Wang, Study on
Methanol Aromatization over Zeolite Catalyst H/ZSM-5
Modied with Zinc Ions in Different Contents, Pet. Sci.
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conditions were weight hourly space velocity (WHSV) ¼ 2.0 hꢁ1
,
reaction temperature of 430 ꢂC and atmospheric pressure.
Methanol and the products were analyzed by an online gas
chromatograph (Agilent GC-7890) with a ame ionization
detector (FID). The tubing between outlet of the reactor and GC
ꢂ
was kept at 240 C for prohibiting the reaction products from
condensing. The methanol conversion, BTX selectivity and yield
of the products were calculated from the number of C atoms in
the products.
Conclusions
We demonstrated that isolated zinc phosphate groups can be
successfully anchored to HZSM-5 zeolite channel network
prepared by impregnating zinc ions to P-modied HZSM-5
catalyst, which has a good hydrothermal stability and a suit-
able Lewis acidity regulating the acidity distribution. The stable
zinc phosphate groups on ZnPHZ enhance the hydrogen-
transfer pathway to high BTX selectivity and suppress the
methylation of BTX to heavy aromatics. This research may
provide an alternative way to the effective MTA catalyst design
for achieving high BTX selectivity and inhibiting the formation
of heavy aromatics (C9+) with good thermal and hydrothermal
stabilities.
9 Y. Ni, A. Sun, X. Wu, J. Hu, T. Li and G. Li, Aromatization of
Methanol over La/Zn/HZSM-5 Catalysts, Chin. J. Chem. Eng.,
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preparation of nano-sized H [Zn, Al] ZSM-5 zeolite and its
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12 Y. Inoue, K. Nakashiro and Y. Ono, Selective conversion of
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors acknowledge support from China Petrochemical
Technology Company, Ltd. Use of the NSLS was supported by
the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-98CH10886. The
authors acknowledge the facilities support at the NSLS beam-
line X19A and SSRL beamline BL2-2 provided by the Synchro-
tron Catalysis Consortium (U.S. DOE grant no. DE-SC0012335).
The authors acknowledge nancial support from the National
Key R&D Program of China (project number: 2017YFB0702800),
China Petrochemical Corporation (Sinopec Group).
13 H. Ji, Q. Zhang, L. Chen and H. Shan, Study on the reaction
performance of methanol to aromatics over Zn/HZSM-5
catalysts with alumina, J. Fuel Chem. Technol., 2014, 42(11),
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Ag/ZSM-5 Catalyst in the Aromatization of Methanol, Acta
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