Synergistic effects of tungsten and phosphorus on catalytic crackingof butene to propene over HZSM-5

Nianhua Xue, Lei Nie, Dongmei Fang, Xuefeng Guo, Jianyi Shen, Weiping Ding *, Yi Chen

Lab of Mesoscopic Chemistry, Department of Chemistry, Nanjing University, Hankou Road 22, Nanjing 210093, China

Applied Catalysis A: General 352 (2009) 87–94

A R T I C L E I N F O

Article history:

Received 11 April 2008

Received in revised form 27 September 2008

Accepted 29 September 2008

Available online 11 October 2008

Keywords:

HZSM-5

Tungsten

1-Butene cracking

D2/OH exchange

Microcalorimetry

A B S T R A C T

Synergistic catalysis effects among tungsten, phosphorus and HZSM-5 on 1-butene cracking to propene

and ethene have been demonstrated by catalytic tests. The tungsten–phosphorus-modified HZSM-5 (W–

P/HZSM-5) catalyst, with very low density of acid sites, offers a fairly high conversion rate of butene and

selectivity to propene. The status of doped tungsten is characterized by using techniques of D2/OH

exchange, NH3 adsorption microcalorimetry, FT-IR spectroscopy, Raman spectroscopy, N2 adsorption

and X-ray photoelectron spectroscopy. The tungsten would be monotungstate that interacted with

phosphorus before steam treatment and partly congregated to polytungstate species during steaming

process at high temperature. The enhanced performance of the catalyst for 1-butene cracking to propene

and ethene can be correlated to the synergistic effect between the doped tungsten and phosphorous on

the reaction network of the cracking process. The W–P/HZSM-5 is a promising catalyst for the 1-butene

cracking to propene and ethene.

� 2008 Elsevier B.V. All rights reserved.

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Applied Catalysis A: General

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1. Introduction

Propene is one of the important raw materials for theproduction of a series of petrochemicals, such as polypropylene,acrylonitrile, propylene oxide, cumene, phenol, and isopropylicalcohol [1]. Due to the fast growing demand of propenederivatives, a number of studies have been performed to enhancepropene production. Catalytic cracking of mixed C4 olefins orother less valuable feedstock appears to be an attractivealternative to produce the required propene. It has been reportedthat 1-butene reactions on acid catalysts included isomerizationand dimerization, in which octenes were produced as inter-mediates for cracking products, e.g., propene, ethene andpentenes [2–6]. Strong acidic catalysts, e.g., acidic zeolites, areneeded for this process. The butenes on strong acid sites are alsoinclined to undergo further oligomerization to by-products suchas aromatic products and coke. An appropriate amount of steam inthe feedstock is effective to remove the coke deposits [7] but isalso responsible for the dealumination of zeolites. Phosphorus-modified HZSM-5 has shown attractive hydrothermal stability [8–14] and our recent studies have revealed the possible nature of thestability caused by phosphorus doping during the steam treat-ment at high temperature [13].

* Corresponding author. Tel.: +86 25 83595077; fax: +86 25 83686251.

E-mail address: dingwp@nju.edu.cn (W. Ding).

0926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.09.029

Among heterogeneous acid catalysts under investigation, theheteropoly acids (HPAs) are particularly strong Bronsted acids. TheH3PW12O40 (HPW) is unlikely to be formed in zeolite channels byintroduction of tungsten to phosphorus-modified HZSM-5 (P/HZSM-5), however, the interaction between tungsten and phos-phorus species should be expected by doping tungsten species to P/HZSM-5. Furthermore, it has been documented that supportedtungsten oxides are good catalysts for isomerization of hydro-carbons, e.g., 1-butene, n-decane and C7+ paraffins [15–19].Considering these properties of tungsten and the anticipatedinteraction between phosphorus and tungsten, we conclude thatdoping tungsten to P/HZSM-5 would further promote its catalyticperformance for 1-butene cracking. We report here the significanteffect of tungsten doping on the catalytic performance of P/HZSM-5for 1-butene cracking to propene. Several characterization techni-ques such as D2/OH exchange, NH3 adsorption microcalorimetry, N2

adsorption, FT-IR spectroscopy, Raman spectroscopy, and XPSmeasurements were used to investigate the tungsten status inthe catalyst and the mechanism for such catalytic improvements.

2. Experimental

2.1. Materials

The series of P/HZSM-5 catalysts were prepared as describedpreviously [13]. HZSM-5 (Si/Al = 35, Shanghai Research Institute ofPetrochemical Technology) was impregnated with an aqueous

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–9488

solution of (NH4)2HPO4 under stirring and then was evaporatedand calcined in air. The phosphorus contents were designed as 1, 2and 3 wt.%. The obtained samples were designated as 1P-Z, 2P-Zand 3P-Z, respectively.

Samples with 5.9 wt.% tungsten were obtained from HZSM-5,1P-Z, 2P-Z and 3P-Z by impregnation with an aqueous solution ofammonium metatungstate ((NH4)6H2W12O40, Aldrich, 99.99%).The mixture was stirred for 2 h and the excess water was removedby evaporation at 373 K for 12 h. The obtained samples werecalcined at 773 K for 3 h in air. The number of tungsten atoms wasequal to that of phosphorus in 1P-Z sample. The tungsten-modifiedP/HZSM-5 materials were designated as 1P-W-Z, 2P-W-Z and 3P-W-Z, respectively.

All the above samples were also hydrothermally treated at1073 K for 4 h in 100% autovapored water (0.1 g min�1, 0.1 MPa).The steamed samples were identified by a suffix of ‘‘-S’’ in theirname.

2.2. Characterization

The density of hydroxyls in catalysts was determined by D2/OHexchange. Prior to the exchange reaction, about 400 mg of catalystwas activated at 723 K for 1 h in 20% O2/Ar (flow rate = 100 -mL min�1). Then the catalyst was cooled in Ar (80 mL min�1) toroom temperature. Deuterium exchanged with protons present inthe samples was measured by increasing the temperature to1073 K at 10 K min�1. The signal of HD molecules was monitoredusing a mass spectrometer (Inficon Transpector 2). Argon was usedas internal standard in order to calculate the formation of HDduring the exchange between D2 and hydroxyls in samples.

To determine the surface acidity of the samples, the adsorptionheats of ammonia were measured at 423 K using Setaram C-80calorimeter. The calorimeter was connected to a volumetricsystem equipped with a Baratron capacitance manometer forthe pressure measurement and gas handling. Samples werepretreated at 673 K in 500 Torr O2 for 1.5 h. Then the sampleswere evacuated at the same temperature for 1.5 h. When thethermal equilibrium was reached at 423 K, microcalorimetric datawere collected with probe molecules of NH3 dosed sequentially.

The surface areas and pore volumes of the catalysts wereobtained by means of the nitrogen adsorption measurement at77 K on a Micrometrics ASAP 2020 instrument. Samples weretreated at 623 K under vacuum before N2 adsorption. The totalsurface area was calculated using the classical BET method. Themicropore areas and pore volumes were evaluated with the t-plotmethod. The XRD patterns of the samples were recorded in the 2uregion of 5–608 using a diffractometer (Philips X’Pro) with Cu Karadiation. Relative crystallinity data were obtained by normalizingthe peak height of the strongest peak in the 2u range 7–8.58 to 100%with respect to HZSM-5.

X-ray photoelectron spectra were recorded on an AXIS Ultraspectrophotometer using Al Ka radiation (1486.71 eV) at 15 kVand 15 mA with a constant pass energy mode (40 eV). The sampleswere pressed into small disks, and each was evacuated in theprechamber of the spectrometer at 3 � 10�7 Pa at 673 K for 1 h.The regions of W 4f, Al 2p, P 2p, O 1 s and Si 2p were recorded. Allbinding energies were referenced to the C 1s peak at 284.8 eV. Themeasured intensity ratios of the components in samples wereobtained from the area of the corresponding peaks after back-ground subtraction. The overlapped peaks were deconvolutedusing least square fitting with Gaussian–Lorentzian curves.

Raman spectra were recorded with a Renishaw Invia Systemequipped with a confocal microscope. A 514.5-nm exciting linewas focused using a 50� objective lens. The laser power at thesample was 20 mW. The IR spectra for the catalysts were recorded

on a FT-IR spectrometer (Bruker Vector 22) with a resolution of4 cm�1 at room temperature.

2.3. Catalytic test

The catalytic performance of the catalysts was tested atatmospheric pressure in a U-type quartz tube with 6 mm innerdiameter for 1-butene (purity > 99%) cracking. The catalystpowders were pressed, crushed and sieved to 0.25–0.84 mmparticles. The catalysts were heated to 803 K and kept at thattemperature for 1 h in 100 mL min�1 air flow. Then the air wasswitched to pure nitrogen (100 mL min�1) for about 10 min. Foreach catalytic test, the nitrogen flow was switched to a gas flowmixed by 1-butene and nitrogen (N2/1-butene = 3, WHSV = 13 h�1)at 803 K. The products were analyzed on line by a gaschromatograph equipped with a PLOT Al2O3/Na2SO4 capillarycolumn and a flame ionization detector (FID).

TG analysis of used catalysts was carried out on a STA 449C-Thermal Star instrument to monitor the amount of coke depositionin the catalytic process. The catalyst loading was about 10 mg andthe gas flow was 20 mL min�1. The catalyst was heated from roomtemperature to 1073 K in air stream at a heating rate of 10 K min�1.

3. Results and discussion

3.1. D2/OH exchange

The H/D isotopic exchange technique has been developed andapplied to determine the solid surface hydrogen exchanged withD2 [13,20–23]. Depositing tungsten to HZSM-5 results in thereduction of hydroxyl density in the zeolite, as shown in Fig. 1a, butit hardly affects the exchange peak temperature. The results implythe partial exchange between the tungsten and HZSM-5 zeolitichydroxyls [21].

Fig. 1b displays the HD evolution profiles for the impregnated P/HZSM-5 catalysts in the temperature range from room tempera-ture to 1073 K. Values in parentheses represent the density ofhydroxyls in the sample and the corresponding peak temperature.When modified with different phosphorus contents, P/HZSM-5catalysts present diverse peak temperatures and densities ofhydroxyls. The density of OH groups decreases with increasingamounts of phosphorus because the interactions of phosphateroots with zeolitic OH groups eliminate the zeolitic OH groups. Thecondensation among the phosphate roots will also decrease thenumber of OH groups. The peak temperatures for 1P-Z, 2P-Z and3P-Z were found at 812, 830 and 838 K, respectively. The variedpeak temperatures reflect the decreased acidity with increasedphosphorus loadings.

As ammonium metatungstate was added to P/HZSM-5, changesin the HD evolution profiles were observed, as presented in Fig. 1c.The peak temperature (795 K) for 1P-W-Z was lower than that of1P-Z (812 K), reflecting the changes of status or environment of OHgroups of P/HZSM-5 during the tungsten modification. The densityof the hydroxyls in 1P-W-Z is further reduced from 285 (1P-Z) to175 mmol g�1. This indicates that tungsten oxides also interactwith induced phosphorus species in addition to the bridgedhydroxyls in HZSM-5. Comparing 2P-Z and 3P-Z, we found that thepeaks of 2P-W-Z and 3P-W-Z move to lower temperatures and wealso found the decreases in hydroxyls densities. It can be concludedthat the tungsten interacting with phosphorus might lead to a littleamount of new type hydroxyls that would affect the peaktemperature of the D2/OH exchange.

Adding an appropriate amount of steam in the feedstock is anadvisable method to prevent or reduce the coke formation in theacid catalysts during hydrocarbon reactions [7]. Thus the hydro-

Fig. 1. D2/OH exchange profiles of (a) HZSM-5 and tungsten-modified HZSM-5, (b)

P/HZSM-5 catalysts calcined at 843 K in air, (c) tungsten-doped P/HZSM-5 calcined

at 773 K in air and steam treatment was not performed, (d) tungsten-doped P/

HZSM-5 treated in steam 1073 K for 4 h. Values in parentheses represent the

quantities of hydroxyls (mmol g�1) measured by the D2/OH exchange and the

corresponding peak temperatures (K).

Fig. 2. Differential heats of ammonia adsorption at 423 K over tungsten- and/or

phosphorus-modified HZSM-5.

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–94 89

thermal stability of the acidic catalysts is important to estimatetheir performance for hydrocarbon reactions. The phosphorus-modified HZSM-5 exhibits remarkable hydrothermal stability; themechanism has been intensively discussed in previous work [13].

One kind of hydrothermally stable acid sites with a special D2/OHexchange peak at �693 K forms in P/HZSM-5 during the steamprocess. The D2/OH exchange peak at 708 K was observed for 1P-W-Z-S and the OH site would be considered similar to thatobserved in steamed P/HZSM-5 but a little affected by the dopedtungsten species. The D2/OH exchange profiles of 2P-W-Z-S and3P-W-Z-S are also presented in Fig. 1d. For both the samples, theD2/OH exchange peaks move to lower temperatures after steamtreatment compared with those of the untreated samples. Thedistributed environment of OH groups causes the varied D2/OHprofiles.

3.2. NH3 adsorption microcalorimetry

The variations of NH3 adsorption heats versus NH3 coveragewere measured at 423 K on several typical samples. As shown inFig. 2, HZSM-5 shows an initial adsorption heat of ammonia at184 kJ mol�1 and a sharp drop at the surface coverage around400 mmol g�1. The adsorption with the heat above 80 kJ mol�1 canbe assigned to the adsorption of NH3 on Bronsted acid sites inHZSM-5 [24]. Significant effects of phosphorus and tungstendoping on the density and the strength of acid sites are observed.There is a sharp decrease in the total number of the Bronsted acidsites when 1 wt.% phosphorus is introduced into the HZSM-5. Dueto the elimination of zeolitic hydroxyls by phosphorus species, thesaturated coverage of ammonia on 1P-Z was found to be586 mmol g�1, which is much lower than that of HZSM-5(911 mmol g�1). The initial heat of ammonia adsorption on 1P-Zis similar to that on HZSM-5, implying that a part of zeolitichydroxyls is free of phosphorus covering.

Tungsten addition causes very little change in the initial heat ofthe ammonia adsorption, comparing 1P-Z (190 kJ mol�1) with 1P-W-Z (189 kJ mol�1). The relative high differential heat (from160 kJ mol–1 to 80 kJ mol–1) in 1P-W-Z than that in 1P-Z isattributed to Bronsted acid sites being stronger than those in 1P-Z.The stronger acid sites are related to the tungsten that interactswith phosphorus in modified zeolite, as discussed above forD2/OH exchange. Furthermore, the saturated coverage of ammoniaon 1P-W-Z sample was 633 mmol g�1, more than that of 1P-Z(586 mmol g�1), reflecting the contribution of tungsten species.

By comparing the differential curve of ammonia adsorption on1P-W-Z-S with that of 1P-W-Z, one finds that the effect of steamingon the number and strength of acid sites in 1P-W-Z is remarkable.After steaming, the number of Bronsted acid sites in 1P-W-Zdecreases from ca. 140 to 20 mmol g�1. And the saturated coverageof ammonia reduces to 291 from 633 mmol g�1. Such results must

Fig. 3. IR spectra of the typical catalyst samples.

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–9490

be related to the dealumination during the severe steamtreatment. The 1P-W-Z-S sample contains relative weaker acidsites with much less density than those of 1P-W-Z.

3.3. IR and Raman results

The IR spectra of three typical W–P/HZSM-5 samples areshown in Fig. 3. The absorption peaks at 1226, 1100, 803, 547 and453 cm�1 are assigned to the framework vibration of HZSM-5[25]. For the phosphorus containing samples, the nas,P–O

(�1080 cm�1) cannot be distinguished due to the overlap withthe strong absorption of the zeolite framework vibration andthe low content of phosphorus in the samples. The nas,W–O

(�982 cm�1) from tungsten containing samples also cannot be

Fig. 4. Raman spectra of tungsten- and/or phosphorus-modified HZSM-5 before and after

(b) 1P-Z, 1P-W-Z and 1P-W-Z-S; (c) 2P-Z, 2P-W-Z and 2P-W-Z-S; (d) 3P-Z, 3P-W-Z and

identified. When the 1P-W-Z is steamed at 1073 K for 4 h, theabsorptions at 803, 547 and 453 cm�1 that originate from theframework vibrations move to the 808, 550 and 457 cm�1

locations, respectively, indicative of the change of zeoliteframework. Dealumination, more or less, is inevitable upon themodification and steam treatment.

The status of tungsten is better understood by the Ramanmeasurement, as shown in Fig. 4. The tungsten-modified HZSM-5possesses Raman bands at �807, �709, and �269 cm�1, which areassigned to the WOX species, corresponding to the crystallite formof WO3 (803, 711, and 271 cm�1, Fig. 4a). The results reveal thatimpregnated ammonium metatungstate on HZSM-5 easily trans-form to polytungstate species after calicination in air. Some ofmolecules may be in the channels of HZSM-5.

In contrast to HZSM-5, for the P/HZSM-5 catalysts, only a smallquantity of crystallite-like WOX species appeared when tungsten isintroduced to these catalysts, as shown in Fig. 4b–d. The weakbands are the characteristic of microcrystallite WOX particles thatdo not completely disperse [26]. Most of the tungsten species maybe monotungstate interacting with phosphorus rather thanpolytungstate species on HZSM-5 surface. After the W–P/HZSM-5 catalysts have been steamed at 1073 K for 4 h, however, thecorresponding intensities of the Raman peaks caused by the WOX

crystallites increase significantly. This implies the transformationof surface monotungstate to polytungstate species. The dispersedtungsten oxide species in P-W-Z samples congregate to largerparticles in steam at high temperature, meaning that theinteractions between tungsten species and phosphorus are notstrong enough against aggregation in steam environment.

3.4. XPS

As shown in Fig. 5a, the tungsten-modified HZSM-5 (W/HZSM-5)sample is characterized by W 4f7/2 and W 4f5/2 peaks, respectively,

steam treatment. (a) Crystalline WO3 and tungsten-modified HZSM-5 (W/HZSM-5);

3P-W-Z-S.

Fig. 5. XPS spectra for the regions of: (a) W 4f, (b) P 2p, (c) Al 2p, and (d) O 1s for W/HZSM-5, 1P-W-Z and 1P-W-Z-S samples heated in the prechamber of the spectrometer at

673 K and vacuum for 1 h.

Table 1Textural properties of tungsten- and/or phosphorus-modified HZSM-5 catalysts.

Sample BET surface

area (m2 g�1)

Micropore

area (m2 g�1)

Pore volume

(cm3 g�1)a

Relative

crystallinity (%)

HZSM-5 341 204 0.22 100

W/HZSM-5 333 193 0.18 99

1P-Z 304 182 0.18 99

1P-W-Z 296 163 0.18 89

1P-W-Z-S 291 100 0.15 80

2P-Z 265 177 0.18 82

2P-W-Z 262 164 0.16 81

2P-W-Z-S 297 116 0.15 78

3P-Z 219 162 0.17 81

3P-W-Z 235 167 0.16 75

3P-W-Z-S 262 63 0.13 70

a Total pore volume obtained by summing the volumes of micropores and

mesopores.

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–94 91

located at ca. 36.3 and 38.4 eV. These values are typical for thepresence of supported tungsten oxide [27]. When tungstenspecies are doped to the phosphorus-modified HZSM-5, e.g., 1P-W-Z, the W 4f signals shift to higher energy. In contrast, theW 4f binding energies of steamed 1P-W-Z (Fig. 5a) shift back tothe values similar to those of W/HZSM-5. Such results suggestthat the weak interactions between the phosphorus andtungsten help the dispersion of tungsten, but the interaction isnot strong enough to hold the tungsten in a high dispersionduring the steam treatment. The tungsten contents at thesurface, as analyzed by XPS, are 0.34% of 1P-W-Z and 0.44% of 1P-W-Z-S, indicate that some tungsten species move from the zeoliticpores to the external surface during the steam treatment. Thephosphorus signals (Fig. 5b) are weak and appear similar amongthe samples. The relative intensity of P 2p goes up after steamtreatment, implying the migration of small amounts ofphosphorus from the channels to the external surface. The contentof P-atom in external surface of 1P-W-Z measured by XPS is 0.48%versus 1.02% of 1P-W-Z-S.

The Al 2p region of the XPS spectra for tungsten- and/or phosphorus-modified HZSM-5 catalysts are shown in Fig. 5c.For W/HZSM-5 sample, the Al 2p region presents the maximumat 75.05 eV, which is mainly related to the framework Alspecies interacting with tungsten additive or proton. The Al 2pbinding energies were observed at 75.35 and 75.55 eV for 1P-W-Z and 1P-W-Z-S. The shifts reveal that phosphorus stronglyinteracts with Al in zeolite framework. Tungsten additive prefersto be adjacent to the phosphorus species in phosphorus-modified HZSM-5. Steam treatment results in the congregationof tungsten species. The XPS spectra of O 1s (Fig. 5d) and Si 2p(not shown) of the samples are almost the same for tungsten-and/or phosphorus-modified zeolite.

3.5. Porous texture and relative crystallinity

Table 1 shows the physic-chemical characterization dataobtained in this study. The surface area and pore volume of thecatalysts decrease with increase of phosphorus loading; theoriginal HZSM-5 has the largest value. With tungsten doping,the surface area and the pore volume decrease further. Thesteam treatment on P-W-Z catalysts resulted in the furtherdecreased pore volumes due to the collapse of microporousdomain. The modification by phosphorous and tungstenhas little effect on the crystal structure of zeolite, as shownin Fig. 6. The relative crystallinities (Table 1) of the P-W-Zcatalysts, however, decrease to �70% of HZSM-5 after steamtreatment.

Fig. 6. XRD patterns of phosphorus- and tungsten–phosphorus-modified ZSM-5

catalysts.

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–9492

3.6. Cracking of 1-butene

Table 2 lists the catalytic performance of the samples for 1-butene cracking at 803 K. Generally, the increase in the phosphorusloading leads to the decrease in the 1-butene conversion, which isrelated to the decrease in the density of OH groups, as proved byD2/OH exchange. The tungsten doped to HZSM-5 causes only

Table 2Catalytic performance of the samples for 1-butene cracking at 803 K, 0.1 MPa and N2/1

Samples Ethene yield (%)a Propane yield (%)a Propene yield (%)a I

HZSM-5 1.9 1.3 2.1 0

W/HZSM-5 7.0 6.0 9.7 3

1P-Z 12.3 4.2 29.1 3

2P-Z 5.9 1.1 32.1 1

3P-Z 0.8 0.1 11.2 0

1P-W-Z 12.6 4.1 29.5 3

2P-W-Z 9.7 1.9 35.8 2

3P-W-Z 3.6 0.4 27.0 1

1P-Z-S 7.9 1.7 35.7 1

2P-Z-S 0.1 0.1 3.2 0

3P-Z-S 0.0 0.0 1.3 0

1P-W-Z-S 5.3 0.9 33.5 1

2P-W-Z-S 3.4 0.0 29.2 0

3P-W-Z-S 0.1 0.0 2.4 0

a All the data were obtained at 40 min on stream.

Fig. 7. (a) List of the conversion rate of 1-butene over a series of catalysts at the same r

function of time on stream (temperature: 803 K; WHSV = 13 h�1; N2/1-butene = 3).

minor influence on HZSM-5 catalytic performance for the 1-butenereaction. The conversion of 1-butene on the tungsten-modifiedHZSM-5 (W/HZSM-5, 95.2%) is about 4% lower than that of presentHZSM-5 (98.7%), corresponding to the slight decrease in the OHdensity by tungsten doping. The catalytic performance of P/HZSM-5 for the current reaction has been discussed previously [13].

As tungsten is doped to the P/HZSM-5, the catalytic perfor-mance is much different from W/HZSM-5. It has been noted thatthe 1-butene conversion is basically proportional to the density ofacid sites in catalyst. However, the 2P-W-Z catalyst with lower aciddensity shows the increased conversion compared with 2P-Z. Theenhanced performance is also observed over the 3P-W-Z catalyst.Furthermore, the relative conversion is present over the 1P-Z and1P-W-Z catalysts with lower acid density in the latter one. Here it isnecessary and reasonable to compare the activities of catalysts onthe basis of OH density.

The conversion rates of 1-butene over the catalysts werecalculated relative to OH density and are shown in Fig. 7a. It is clearthat the tungsten-doped P/HZSM-5 shows enhanced catalyticperformance; the highest specific conversion rate is observed onthe steamed 1P-W-Z, i.e., 1P-W-Z-S in Fig. 7a. In comparison withP/HZSM-5, the enhancement in catalytic performance of W–P/HZSM-5 surely reflects the contribution of the doped tungsten. Theconversion rate of 1-butene over W/HZSM-5, however, is just alittle affected by the tungsten doping. Hence, some synergisticeffects between the tungsten and phosphorous on the acid sites ofzeolite can be deduced. The structure similar to the HPW speciesmight be formed in and out of zeolitic channels and is responsiblefor the enhanced conversion rate. The new acid sites for the

-butene = 3.

sobutane yield (%)a n-Butane yield (%)a C5+ yield (%)a C4 = Conv. (%)a

.6 0.5 92.6 98.7

.0 2.3 66.4 95.2

.0 2.6 31.1 82.4

.6 1.6 26.7 69.0

.4 0.7 17.5 30.6

.1 2.6 30.2 82.5

.1 2.0 23.4 75.0

.1 1.2 23.2 56.4

.8 2.3 23.6 73.2

.1 0.5 4.4 8.2

.3 0.0 2.7 4.0

.4 1.9 23.4 66.5

.9 1.6 21.7 57.4

.0 0.4 7.8 10.6

eaction conditions; (b) relative catalytic performance of 1P-Z-S and 1P-W-Z-S as a

Fig. 8. Variation of propene (a), ethene (b) and C5+ (c) selectivities and propene/propane ratio (d) with the conversion of 1-butene obtained on the catalysts mentioned in this

work at 803 K (WHSV = 13 h�1, N2/1-butene = 3, TOS = 40 min) ((&) P/HZSM-5; (*) tungsten-modified P/HZSM-5). The inset embed in (a) is the variation of propene

selectivity with the conversion of 1-butene (at relative high conversion from 70% to 100%).

Table 3Thermogravimetric analysis performance of phosphorus- and tungsten–phos-

phorus-modified ZSM-5 catalysts after 70 min running at 803 K in 1-butene

cracking process (N2/1-butene = 3, WHSV = 13 h�1).

Sample Coke (wt.%)

1P-Z 4.59

1P-W-Z 4.65

2P-Z 4.54

2P-W-Z 4.74

3P-Z 4.07

3P-W-Z 4.22

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–94 93

interaction of P and W are also deduced by D2/OH exchange andNH3 adsorption microcalorimetry. After steam treatment at 1073 Kfor 4 h, the W–P/HZSM-5 catalysts show more distinct enhance-ment in cracking, as shown in Fig. 7a. This result leads to theconclusion that the synergistic effect of P and W is very stable insteam at high temperature. This is promising for developing thehydrothermally stable catalysts for cracking.

Fig. 7b shows the specific activity of 1-butene cracking over 1P-Z-S sample and 1P-W-Z-S on stream with varied reaction time. Itcan be seen that the specific activity of 1P-W-Z-S is much higherthan that of 1P-Z-S for 1-butene cracking on the basis of OH groups.A similar deactivity is shown but the higher activity even with lowacid density in W–P/HZSM-5 catalyst is quite attractive.

It has been learned, from the present research, that the propeneselectivity varied linearly with the conversion of 1-butene, regard-less of the additives in HZSM-5, as shown in the inset of Fig. 8a. Thismeans the propene selectivity is only related to the density ofactive sites in HZSM-5. Interestingly, from Fig. 8a, the tungsten-doped P/HZSM-5 shows higher propene selectivity than P/HZSM-5at the conversions of 1-butene less than 75%. Such results indicatethat the different active sites work for tungsten-modified P/HZSM-5. The tungsten doping shows little effect on ethene selectivityupon the 1-butene conversion, as shown in Fig. 8b. Fig. 8c revealsthe origin of the increase in propene selectivity, i.e., the tungstendoping shrinks the selectivity to C5+ hydrocarbons, which are fromthe oligomerization of butenes.

According to the mechanisms of 1-butene cracking reported [2–6], the transformation mechanism involves three successive steps:dimerization of butenes, skeletal isomerization of dimers, andcracking of octene isomers. And the dimerization to produce octenes

is the limiting step of the butene transformation process [2]. The b-scission mechanism involving two tertiary carbocations (frommultibranched hydrcarbons) is the fast pathway in the long-chainhydrocarbon cracking [28]. The isomerization of small tungstenoxide particles in modified zeolite does not affect the conversion ratemuch, as can be deduced from the comparison of HZSM-5 and W/HZSM-5. The obtained enhanced performance over W–P/HZSM-5catalysts should be ascribed to the change of the network of 1-butene reaction for synergistic catalysis effect of induced tungstenand phosphorus. The new acid sites that result from the interactionbetween phosphorus and tungsten lead to the new environment ofactive sites. Hence the product distribution varies with the acidicproperties of W–P/HZSM-5. Steamed P-W-Z catalyst showsenhanced propene selectivity and 2P-W-Z gives very high propeneselectivity. The ratio of propene to propane, however, seemsunaffected by the tungsten modification, as shown in Fig. 8d.

The total amount of coke deposition after reaction was deter-mined by thermogravimetric analysis and is listed in Table 3. The

N. Xue et al. / Applied Catalysis A: General 352 (2009) 87–9494

weight losses by the combustion of coke species revealed that thetungsten modification resulted in a small increase of cokedeposition, due to the increase of butene conversion by the co-effect of tungsten and phosphorus.

4. Conclusions

The impregnated tungsten species on HZSM-5 interact weaklywith the zeolite and affect the property of 1-butene crackingonly very slightly. The interaction between tungsten andphosphorous helps the high dispersion of tungsten on P/HZSM-5. The highly dispersed tungsten species aggregate to smalltungsten oxide clusters during steam treatment at high tempera-ture. The synergistic interaction between the tungsten andphosphorous enhances the catalytic performance of W–P/HZSM-5 catalyst for the 1-butene cracking by affecting the network of the1-butene cracking reaction. The W–P/HZSM-5 is a promisingcatalyst with hydrothermal stability, high activity and propeneselectivity for 1-butene cracking.

Acknowledgements

The project is supported by the Ministry of Science andTechnology of China (Grant No. 2003CB615804), the Natural ScienceFoundation of China (Grants Nos. 20773062 and 20673054).

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