Journal of Molecular Catalysis A:Chemical 336 (2011) 34–41
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Journal of Molecular Catalysis A:
Chemical
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m o l c a t
a
Influences of alkaline treatment on the structure and catalytic performances of ZSM-5/ZSM-11zeolites with alumina as binder
Xiujie Li,Chuanfu Wang,Shenglin Liu,Wenjie Xin,Yuzhong Wang,Sujuan Xie,Longya Xu ∗
State Key Laboratory of Catalysis,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,457Zhongshan Road,Dalian 116023,China
a r t i c l e i n f o
Article history:
Received 15July 2010Received in revised form 23November 2010
Accepted 12December 2010Available online 8 January 2011Keywords:
Alkaline treatment Alumina binder ZSM-5/ZSM-11
1-Hexene aromatization
a b s t r a c t
Influences of alkaline treatment on the structural properties and catalytic performances of ZSM-5/ZSM-11composite zeolites with alumina as binder at different preparation steps were studied in the present investigation.Temperature-programmed desorption of ammonium (NH 3-TPD)and pyridine infrared (Py-IR)spectra revealed that alkaline treatment sequences changed both the distribution and amount of the acidities in the ZSM-5/ZSM-11-Al 2O 3samples.The mesopores created by alkaline treatment were found beneficial for the diffusion of aromatic molecules,as determined by the xylene-uptake experiments using a tapered element oscillating microbalance (TEOM).In 1-hexene isomerization and aromatization reactions,the sample after extrusion followed by alkaline treatment exhibited excellent aromatiza-tion activity and stability compared with other samples undergoing different treatment sequences.The enhanced catalytic performance could be attributed to the redistribution of acid sites and introduction of more mesopores.
© 2011 Elsevier B.V. All rights reserved.
1.Introduction
Creation of mesopores in micropore zeolite particle to increase the accessibility of reactant molecules to internal surface has grad-ually become an interesting topic in recent years [1–5].A variety of methods have been reported for the introduction of meso-pores in micropore materials among which alkaline treatment attracts more attention due to its simplicity and reproducibility [6–11].Thus far,most of the research focused on MFI zeolite,espe-cially on ZSM-5for its promising fuel upgrading performances [9,12–16].After alkaline treatment,ZSM-5zeolite showed higher catalytic activity and better stability in olefin to aromatics reactions as a result of the enhanced acid site accessibility in hierarchical porous structure [14–16].The aluminum atom in ZSM-5frame-work proved to be important in controlling the silicon extraction process.Meanwhile,Si/Al ratio was also a key factor for the intro-duction of mesopore without affecting micropore structure and acidity [1,17,18].The optimal framework Si/Al molar ratio for desil-ication was found to range from 25to 50[17,18].However,whether the extra-framework aluminum species [19],especially the exis-tence of alumina binder,will affect the de-silicon process is still unknown.
In order to improve the mechanical strength of zeolite for indus-trial applications,addition of binder is necessary during the catalyst
∗Corresponding author.Tel.:+8641184379279;fax:+8641184693292.E-mail addresses:lyxu@dicp.ac ,xiujieli@dicp.ac (L.Xu).preparation process [20].Usually,binders like SiO 2or Al 2O 3are considered catalytically inert for the reaction.However,during the alkaline treatment process,alkaline solution may interact with both binder and zeolites.And enhanced hardness was observed on extru-dates after alkaline treatment as reported by Groen et al.[21].As far as the catalytic performance is concerned,few reports could be found about the optimal alkali-treatment condition for bindered zeolite samples.The best preparation sequence for ,before or after extruder and containing templates or not,is very important for their industrial application.So far,little information is available in the literature due to technical arts or trade secrets.Here,more efforts were put on selecting an optimal treatment condition for zeolite sample with the existence of binder.
In this contribution,influences of alkaline treatment sequence on structure and the ensuing catalytic performance of catalyst in an olefin aromatization reaction were studied.ZSM-5/ZSM-11composite zeolite was chosen as a model catalyst since it had been successfully industrialized in alkylation reaction of benzene with the dilute ethylene in FCC off-gas [22,23].And it also showed good catalytic performances in the 1-hexene aromatization reaction [24].Alumina was selected as binder with a weight percent of 20%.Four catalysts with different alkaline treatment sequences were prepared and e
val-uated over 1-hexene isomerization and aromatization reaction.NH 3-TPD and Py-IR spectra were applied to detect the acid-ity information before and after alkaline treatments.Pore structure information and the mass transfer factors were fur-ther characterized by N 2adsorption and m -xylene adsorption
1381-1169/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.lcata.2010.12.007
碱处理对以铝作粘结剂的ＺＳＭ－５／ＺＳＭ－１１沸石的结构和催化性能的影响
碱处理改变ＺＳＭ－５／ＺＳＭ－１１－Ａｌ２Ｏ３的酸性位分布和酸量。
碱处理形成的介孔有利于芳烃分子的扩散，采用微量天平实验二甲苯吸收得到验证。１－己烯异构化和芳构化反应中，挤条成型的样品经碱处理后表现出优异的芳构化活性和稳定性。
碱处理对酸性位的再分布并形成更多的介孔，使催化剂具有更优异的催化性能。
X.Li et al./Journal of Molecular Catalysis A:Chemical 336 (2011) 34–4135
as-ZSM-5/ZSM-11
alumina binder
NH 4+ exhange
NH 4-ZSM-5/ZSM-11-Al 2O 3
calcination
S 2
+AT-NaZSM-5/ZSM-11
AT-NaZSM-5/ZSM-11-Al 2O 3
S 3
NH + exhange AT-as-ZSM-5/ZSM-11
AT-as-ZSM-5/ZSM-11-Al 2O 3
S 4
NH + exhange Scheme 1.Preparation methods of ZSM-5/ZSM-11-Al 2O 3samples with different alkaline treatment conditions.
experiments using tapered element oscillating microbalance (TEOM).
2.Experimental
2.1.Catalyst preparation and evaluation
NaZSM-5/ZSM-11composite zeolite (Si/Al molar ratio =25)was obtained from Fushun Petrochemical Corporation of SINOPEC,China.Detailed synthesis conditions were described in Ref.[25].Sample 1was prepared by extruding a mixture of alumina and Na-ZSM-5/ZSM-11zeolite into strips of 2mm diameter.Sample 2was obtained by leaching Sample 1with 0.2M NaOH solution at 80◦C for 2h (solution/zeolite ratio of 5cm 3/g).Samples 3and 4were NaZSM-5/ZSM-11zeolite without and with organic template,sub-jected to alkaline treatment and extrusion process,respectively.All the samples in this study were exchanged with 0.5M NH 4NO 3solution at 80◦C for 2h and then calcined at 520◦C to make the zeolite to transform from Na-form to H-form.Detailed preparation method was described in Scheme 1.Four samples of the above in H-form were defined as S 1,S 2,S 3and S 4,respectively.
The 1-hexene aromatization and isomerization reaction was performed in a continuous flow reactor.Typically,3.5g catalyst was loaded in the middle part of fixed-bed stainless reactor with 320mm in length and 12mm in diameter.The reaction was per-formed at 350◦C,0.5MPa with a weight hourly space velocity (WHSV)of 2h −1.Before the reaction,the catalyst was pretreated at 500◦C for 2h in nitrogen flow to remove the water.Hexene feed (97%1-hexene,3%2-hexene)was introduced to the reactor when the temperature decreased to 350◦C.The gas and liquid products were analyzed by a Varian-3800gas chromatograph equipped with an FID detector and a 100m PONA capillary column.
2.2.Cyclo-hexane and m-xylene uptake measurements
A Rupprecht and Patashnick TEOM-1500mass analyzer was used to measure the uptakes of cyclo -hexane and m -xylene in the calcined and alkaline-treated samples.50mg zeolite samples were loaded in the TEOM.Prior to the uptake experiments,the samples were heated in 20ml/min Ar flow at a rate of 5◦C/min to 500◦C for 240min and then cooled to 120◦C or 150◦C.As the baseline was stable,adsorption of cyclo -hexane or m -xylene in flowing Ar (total flow rate of 20ml/min)was subsequently measured at 120and 150◦C,respectively.The corresponding mass changes information during the uptake process was recorded upon time.
2.3.Characterization
2.3.1.XRD and SEM measurements
X-ray diffraction (XRD)patterns were obtained at room temper-ature on a Rigaku D/Max-RB diffractometer using Cu-K ␣radiation.Powder diffractograms of samples were recorded over a range of 2Âvalues from 5◦to 50◦under the conditions of 40kV and 100mA at a scanning rate of 5◦/min.Scanning electron micrographs (SEM)were taken using a Quanta 200F field-emission microscope.
2.3.2.NH 3-TPD measurements
Temperature-programmed desorption of ammonium (NH 3-TPD)measurements were carried out in a conventional U-shaped stainless-steel micro-reactor (i.d.=4mm)using flowing helium (He)as the carrier gas.The NH 3concentration in the NH 3-TPD pro-cess was monitored by an on line gas chromatograph (Shimadzu GC-8A)equipped with a TCD detector.Typically,140mg sample was pretreated at 600◦C for 1h in the flow of He (25ml/min),cooled to 150◦C and saturated with NH 3gas.Then,the sample was purged with pure He stream for certain time until a stable baseline was obtained.Subsequently NH 3-TPD experiment was carried out in the range of 150–600◦C at a heating rate of 18.8◦C/min.
2.3.3.FT-IR spectra
FT-IR spectra were recorded using Bruker Vertex 70infrared spectrometer in the range of 400–4000cm −1with a resolution of 4cm −1on thin wafers of samples.Prior to pyridine sorption the wafers were degassed under vacuum at 500◦C (ca .10−2Torr)for 1h.Then the samples were cooled to room temperature and IR spectra were recorded as the background.Adsorption of pyridine vapor was achieved at room temperature for 30min.After subsequent evac-uation at 150◦C for 0.5h,IR-pyridine spectra were recorded.The ratio of Br nsted to Lewis acids was obtained by using corrections developed by Emeis for porous aluminosilicates [26].reaction mass
2.3.4.N 2adsorption measurements
Nitrogen adsorption experiments were performed at −196◦C on an ASAP-2000system in the static measurement mode.Samples were outgassed at 350◦C for 10h before the measurements.Specific surface areas were calculated by the BET method,the pore volume was determined by N 2adsorption at a relative pressure of 0.98.
Ｓ１－成型后铵交换
Ｓ２－成型后铵交换再碱处理
36X.Li et al./Journal of Molecular Catalysis A:Chemical 336 (2011) 34–41
10
20
30
40
50
S 4
S 3
S 2
2 theta
S 1
a
22.0
22.5
23.0
23.5
24.0
24.5
25.0
25.5
26.0
b
[133]
[151]
[051]
[303]S 4
S 3
S 2
2 theta
S 1
[501]
Fig.1.XRD patterns of HZSM-5/ZSM-11-Al 2O 3samples with different alkaline treatment conditions:(a)5–50◦and (b)22–26◦.
3.Results and discussions
3.1.Structure and morphology of ZSM-5/ZSM-11-Al 2O 3samples under different alkali-treatment conditions
Fig.1a shows the XRD patterns of untreated ZSM-5/ZSM-11-Al 2O 3and samples with different alkaline treatment processes.S 1exhibited the typical diffraction peaks of ZSM-5/ZSM-11zeo-lite locating at 7.9◦,8.8◦,23.2◦and 23.9◦,respectively [27].To give clearer diffraction peaks information about ZSM-5/ZSM-11,the 2Âdiffraction angles between 22.0◦and 26.0◦were magnified in Fig.1b with corresponding h k l faces labeled.No preferential losses of
peak intensity could be observed on the samples after different alkaline treatment sequence,which confirmed the intact zeolite crystalline structure in all samples irrespective of the treatment conditions.Besides,the diffraction peak assigned to ␥-Al 2O 3could not be observed in the XRD patterns which may be due to the low alumina content in the sample.
In order to get more information about the sample morphology before and after alkaline treatments,SEM images were taken and shown in Fig.2.It could be seen that ZSM-5/ZSM-11zeolite parti-cles agglomerated and the particle size was around 2m.Alumina existed in disordered and irregular state.As compared to HZSM-5/HZSM-11,S 1,S 2,S 3and S 4exhibited irregular shapes due to the existence of alumina binder.No obvious changes were found upon the alkaline treatment,since S 1showed similar shape to S 2,S 3and S 4.Besides,alkaline treatment did not show too much influ-ence on the particle.Grooves and defects could not be observed obviously on the zeolite surface as reported by other researchers [9,16].
3.2.Acidity of ZSM-5/ZSM-11-Al 2O 3samples under different alkali-treatment conditions
3.2.1.NH 3-TPD profiles
Alkaline treatment may lead to the extraction of framework silicon in zeolite and the corresponding acid
ity changes.Acidity amount and strength variations of the samples were characterized by NH 3-TPD measurements.As shown in Fig.3,two main desorp-tion peaks could be differentiated on S 1sample locating at 227and 415◦C,respectively (defined as l and h ),which was attributed to the desorption of NH 3molecules from weak and strong acid sites.A shoulder peak at 275◦C could be observed and defined as m .Compared with the NH 3-TPD profiles of ZSM-5/ZSM-11and Al 2O 3shown in Fig.S1,it could be deduced that the desorption peak at higher temperature (h )mainly came from the ZSM-5/ZSM-11zeolite.And the l and m peak contained the contribution of both ZSM-5/ZSM-11zeolite and alumina binder.After alkaline treat-ment the h desorption peak of S 2,S 3and S 4shifted to lower temperature,which suggested the weakening of the strong acid sites for alkaline solution leaching samples.
S 2was derived from the NaOH leaching of S 1,its total line inten-sity became weaker than that of S 1.However,the peak intensity at 275◦C increased at the loss of peak l and h indicating that alkaline treatment changed the acidity distribution.In order to give quan-titative information about the acidity changes,deconverlutions of different profiles were made and the results were listed in Table 1.Alkaline treatment after extrusion led to the loss of the partial acid sites.The total acidity density decreased from 562mol/g in S 1to 523mol/g in S 2.The number of medium acid sites increased at the loss of s
trong acid sites at high temperature.Such acidity changes were different from the previous observation of Groen et al.[17,18].To verify the essence of acidity changes,NH 3-TPD profiles of ZSM-5/ZSM-11before and after alkaline treatment were mea-sured and shown in Fig.S1.Its changes followed the same trend as previous report.Thus,the introduction of alumina binder led to the different acidity changes in S 2.The state and textual properties
Table 1
Acidity distribution of the samples with different alkali-treatment conditions.Sample l (mol/g)a m (mol/g)a h (mol/g)a Total acid sites (mol/g)a B/L (150◦C desorption)b B/L (300◦C desorption)b S 190923805620.91 2.26S 277101345523 2.25 3.83S 389123397609 2.00 3.00S 4
95
145
398
638
1.92
2.89
a Determined by NH 3-TPD profiles.
b
Determined by Py-IR spectra with different desorption temperatures.
X.Li et al./Journal of Molecular Catalysis A:Chemical336 (2011) 34–41
37
Fig.2.SEM images of HZSM-5/ZSM-11-Al2O3samples with different alkaline treatment conditions.
of alumina changed during the treatment process which could be further evidenced in following characterization results.
S3was obtained from ZSM-5/ZSM-11zeolite without templates treated in NaOH solution before extrusion.Compared with S1,S3 showed an obvious increase in l peak intensity.This was in agree-ment with the previous reports about acidity changes in zeolite samples[17,18].
S4was obtained from ZSM-5/ZSM-11zeolite containing tem-plates treated in NaOH solution before extrusion.The total NH3desorption peak area was the biggest among four samples.Com-pared with S1,not only l peak but also h peak intensity increased from380to398mol/g indicating templates played an important role in keeping zeolites structure and acidity.Few reports could be found concerning the alkaline treatment for zeolites containing organic templates,especially for acidity changes[8,28,29].Groen et al.pointed out that beta zeolite with template was inert to NaOH leaching and template-containing regions were protected from the silicon extraction[8].However,this was not the case for ZSM-
38X.Li et al./Journal of Molecular Catalysis A:Chemical 336 (2011) 34–41
Temperature o
C
Fig.3.NH 3-TPD profiles of HZSM-5/ZSM-11-Al 2O 3samples with different alkaline treatment conditions.
5/ZSM-11zeolite.Relative amount of weak and medium acid sites appeared after alkaline treatment without loss of strong acid sites after alkaline treatment.
3.2.2.Py-IR spectra
In order to get more information about the changes of Br nsted and Lewis acid sites,Py-IR spectra were carried out to determine the Br nsted/Lewis ratio of the samples.Fig.4shows the infrared spectra of adsorbed pyridine on different samples.The character-istic infrared bands near 1540cm −1are attributed to pyridine ions on Br nsted acid sites and those near 1450cm −1are corresponding to pyridine bonded to Lewis acid centers.As shown in Fig.4,both Br nsted and Lewis acid sites were found on all samples.For S 2,the amount of Lewis acid sites was reduced compared with other samples.It could be deduced that Lewis acid sites were preferen-tially consumed during the alkaline treatment process for zeolite extrudate which was different from the case of pure ZSM-5zeolite [16].As alumina was the main source of Lewis acid sites,one ques-tion appeared whether the state of alumina had changed during the alkaline treatment process.One possible explanation was that part of the alumina binder in S 2dissolved in the alkali-solution.This was also evidenced by the XRF elemental analysis results.For S 1,the original Si/Al ratio was 2.37.And the value increased to 2.50in S 2which was contrast to the case of the pure zeolite sample.Thus,it could be deduced that not only silicon in zeolites but also part of alumina binder was dissolved in the NaOH solution.How-ever,no acidity distribution change could be observed over pure alumina sample before and after alkaline treatment as shown in
Fig S1.For S 1,the B/L ratio was 0.99and it increased to 2.3for S 2.Little difference in B/L ratio was found between S 3and S 4,as shown in Table 1.When the evacuation temperature increased to 300◦C,the peak intensity at 1450cm −1decreased indicating that acidity strength of Lewis acid sites was weaker than that of Br nsted acid sites.As listed in Table 1,the B/L ratio of all samples increased upon desorption temperature among which S 2was the highest.
Combined with NH 3-TPD and Py-IR results,it could be deduced that alkaline treatment may change both the amount and distribu-tion of acid sites.For S 2,due to the simultaneous removal of silicon in ZSM-5/ZSM-11and alumina binder during alkaline treatment procedure,the number of weak and strong acid site was reduced.Meanwhile,the ratio of Lewis acid sites decreased.As to S 3,NaOH solution treatment led to the increase of acid sites at low desorption temperature.For S 4,total acidity increased especially for medium acid sites.The existence of organic template may protect the Si extraction in alkali-solution to some extent which prevented the zeolite framework from collapsing and the loss of acidity amount [8].
3.3.Texture and mass transfer properties
3.3.1.N 2adsorption measurements
Alkaline treatment not only changed the samples’acidity distri-bution but also had influences on the pore structure of the samples.N 2adsorption experiments were applied to get the information about surface area and pore distribution.As shown in Fig.5,the N 2adsorption–desorption isotherms for all samples showed a hys-teresis loop ranging from P /P 0=0.4to P /P 0=1.Existence of alumina binder accounted for the hysteresis loop in a micropore ZSM-5/ZSM-11zeolite.It could be observed that the loop was relatively small for S 1but much larger for S 2,which revealed that alkaline treatment process led to the creation of many mesopores.At the same time,the N 2uptake amount of S 2sample was higher than that of S 1.This could be further confirmed by the corresponding analysis results listed in Table 2.Samples after alkaline treatment possessed a little larger surface area compared with S 1.Meanwhile,S 2had the largest mesopore volume up to 0.1534cm 3/g.For S 3and S 4,the mesopore number exhibited decreasing trend.From this point of view,alumina may contribute to the mesopore number during the alkaline treatment process.To verify this supposition,pure alu-mina sample was prepared and further treated with alkali-solution under the same conditions.As shown in Table S1,the total pore volume of pure alumina increased from 0.4937to 0.5026cm 3/g after alkaline treatment.However,the BET surface area of alumina decreased from 225.0to 200.4m 2/g after the alkaline treatment.This meant the pore structure of alumina binder in S 2may change during the alkaline treatment process.However,the main contri-
14001450
1500
1550S 4
S 3
S 2wavenumber (cm -1
)
1545
1454
S 114001450
15001550
wavenumber (cm -1
)
S 1S 2S 3S 4
b
a
1545
1454
Fig.4.Py-IR spectra of HZSM-5/ZSM-11-Al 2O 3samples with different alkaline treatment conditions.(a)Evacuated at 150◦C for 0.5h and (b)evacuated at 300◦C for 0.5h.
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