Catalytic Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different Bulk Topologies

CHINESE JOURNAL OF CATALYSIS Volume 30, Issue 10, October 2009 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2009, 30(10): 1049–1057.

RESEARCH PAPER

Catalytic Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different Bulk Topologies Zeeshan NAWAZ1, TANG Xiaoping1, ZHU Jie1, WEI Fei1,*, Shahid NAVEED2 1

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

2

Department of Chemical Engineering, University of Engineering and Technology Lahore, Lahore 54890, Pakistan

Abstract: Three SAPO-34 catalysts, 100% SAPO-34, 30% SAPO-34, and meso-SAPO-34, with different bulk topologies were prepared. The catalysts were characterized by N2 adsorption, scanning electron microscopy, X-ray diffraction, and infrared spectroscopy techniques. The pore size, total acidity, and internal cage structure of the catalysts were almost identical, but they had different bulk appearances. The role of the bulk topology/structure of the catalysts was studied using 1-hexene cracking. On 30% SAPO-34, the surface acidity and diffusion rate decreased due to blocking by binder, which adversely affected catalytic activity. 100% SAPO-34 gave better cracking ability and higher propylene selectivity because of suitable acid sites and effective shape selectivity, respectively. In order to study the effect of diffusion, meso-SAPO-34 was used. The different bulk structure gave different feed conversion and selectivity profiles. A superior control of the stereochemistry was observed in the cracking by the meso-SAPO-34 and 100% SAPO-34 catalysts, in which enhanced diffusion mass transport played an appreciable role. Most of the propylene was produced by the direct cracking pathway by the ȕ-scission carbenium ion mechanism. Hydrogen transfer reactions became significant at higher conversions. Decreasing the residence time to a certain extend is an appropriate way to obtain high propylene yield and selectivity. Activity and selectivity patterns for 1-hexene cracking to propylene were compared to justify superior SAPO-34 topology for 1-hexene cracking to propylene. Key words: 1-hexene; catalytic cracking; propylene; diffusion; SAPO-34

Propylene is a traditional petrochemical building block. There is a need at this time to enhance propylene production to meet the increasing market demand. Propylene is the word’s second largest petrochemical commodity and the principal raw material for the production of many important petrochemicals, especially polypropylene. According to market statistics, it is forecasted that propylene demand for 2010 will be about 85–92 million tons with an annual growth rate of 5.8% [1–6]. About 64% of propylene is produced as a by-product from the steam cracking of naphtha. Fluid catalytic cracking (FCC) units typically produce around 3%–7% propylene, depending on the feed composition, operating conditions, and catalysts [7]. Therefore, at this time, purposely developed propylene production technologies are considered the better option to meet the production-demand gap [8,9]. Among these propylene technologies, the transformation of higher olefins to lower

olefins is the most economical and viable route to produce propylene because of the cheap and easy availability of the raw material [10]. A solid acid zeolite is used in the petrochemical industry for cracking applications [4,10,11]. The cracking reactions of olefins over zeolites have been studied extensively and explained in terms of carbenium ion mechanism [12], while the superiority of these catalysts were explained in terms of shape selectivity [13]. The concept of shape-selective catalytic cracking was mainly shown in two ways: improved conversion of raw materials and increased desired product selectivity [13]. Therefore, catalysts having selective pores for desired products and that do not allow higher hydrocarbons to form are highly desirable for olefin cracking. In earlier studies, this concept of shape selectivity was studied for SAPO-34 catalysts [14]. It was widely accepted that 1-hexene cracked over Brønsted acid

Received date: 30 March 2009. *Corresponding author. Tel: +86-10-62785464; Fax: +86-10-62772051; E-mail: [email protected] Foundation item: Supported by Higher Education Commission, Islamabad, Pakistan (2007PK0013) and the National Natural Science Foundation of China (20606020, 20736004, 20736007). Copyright © 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved. DOI: 10.1016/S1872-2067(08)60137-0

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

sites by the protonation of the double bond to form a tri-coordinated carbenium ion. This carbeniun ion undergoes beta-scission to form a light olefin and smaller carbenium ion. Previously, Y-zeolite and ZSM-5 zeolite catalysts have been extensively studied for FCC model feed compound (1-hexene) cracking [10,15]. Y-zeolite is highly acidic and promotes both cracking and hydrogen transfer reactions [16]. ZSM-5 zeolite has a large pore size that fails to exhibit shape selectivity for propylene [16–18], and its strong acidity promotes surface reactions. However, the silico-aluminophosphate zeolite molecular sieve, SAPO-34, has a small pore size and weak acidity [14,15]. SAPO-34 zeolite is composed of oxygen atoms that form an elliptical cage, and the three-dimensional CHA structure [9,19,20]. The internal pore size, orifice diameter, and the cage diameter are 0.34 nm, 0.43 nm, and 0.67 nm × 1.0 nm, respectively [21]. In the present study, SAPO-34 catalysts of three different bulk configurations were prepared to explore the effect of the bulk topology (structure) experimentally. We also report a one-step synthesis of meso-SAPO-34 zeolites without mesopore templates. The natural and cheap layered material, kaolin, was used as the basic raw material as silicon and aluminum sources. Previously, much attention has been given to justify catalytic performance by considering acidic properties and shape selectivity. However, no discussion has been found in the open literature about the bulk topological structure of SAPO-34 on the performance of olefin cracking to propylene. The experimental results were compared and discussed on the basis of the opportunities offered by the different bulk structures that increased diffusion rate and surface acidity and promoted surface reactions.

sized from the non-mesopore template, kaolin. The kaolin, P2O5, and H2O were mixed in the molar ratio of 1.5:1:500. The mixture was stirred, aged, autoclaved, filtered, washed, dried, and calcined at 600 oC for 4 h. Finally, a silico-alumino phosphate catalyst with a slit shape named meso-SAPO-34 was obtained. The nano-scaled confinement environment provided by the naturally layered starting material allowed the zeolite crystals to grow in a novel hierarchical structure with high crystallinity and excellent hydrothermal stability [22]. 1.2 Catalyst characterization The morphology of the catalysts was obtained using a JEOL JSM-7401F scanning electron microscope (SEM). The X-ray diffraction (XRD) patterns of powdered catalysts were obtained on a powder X-ray diffractometer (Rigaku-2500) with a copper anode tube. KĮ radiation was used with a beam monochromator and the spectra were scanned at a rate of 5o/min from 2ș = 5o–40o. The infrared (IR) spectra of adsorbed ammonia were recorded using a NEXUS apparatus (Nicolet, USA) at 100 oC to measure the acidic properties of the catalysts. BET surface area was measured using N2 adsorption/desorption isotherms determined at –196 oC on an automatic analyzer (Autosrb-1-C). The specific pore sizes were determined from the BET data. The amount of coke formed on the catalysts during cracking was determined by O2-pulse experiments using a gas chromatographic system equipped with a TCD detector. The experiments were carried out at 750 o C by injecting pulses of pure oxygen (99.99%). The CO2 formed was continuously measured by a TCD detector. 1.3

1

Catalyst performance test

Experimental

1.1 Catalyst preparation The 100% SAPO-34 catalyst was prepared by the mixing of Al2O3:P2O5:SiO2:TEA:H2O = 1:1:0.5:2:100 (molar ratio). The synthesis involves stirring, aging at room temperature for 24 h, autoclaving, filtration, drying, and finally calcination at 600 oC. In order to increase the attrition resistance of SAPO-34 to meet the commercial requirements of fluidization catalysts, 30% SAPO-34 was prepared. The 30% SAPO-34 catalyst was prepared with a mixture of pure SAPO-34 zeolite, kaolin, and silicon solution with a weight ratio of 30%, 40%, and 30%, respectively. The SAPO-34 catalyst was first prepared as stated above, which was then followed by the above recipe to get 30% SAPO-34. The specially shaped meso-SAPO-34 was designed and prepared using kaolin (a combined source of aluminum and silicon), phosphorus, template, and de-ionized water mixed together and stirred to obtain a uniform crystallization solution [22]. Highly crystalline meso-SAPO-34 zeolite was synthe-

The catalytic cracking experiments were carried out in a continuous flow quartz reactor integrated with a controlled temperature setup. A measured amount of catalyst was put into the reactor in order to obtain the desired weight hourly space velocity (WHSV), and the cracking activity was analyzed. The feed was introduced into the reactor over the range of WHSV, time on stream (TOS), and temperature. The product distribution was analyzed by an online gas chromatograph (7890-II) equipped with an FID detector. The feed conversion and product yield and selectivity were calculated and presented in weight percentage. 1-Hexene (98.1%, from Johnson Matthey) was used as the feed.

2

Results and discussion

2.1 Catalyst characterization results The BET measurements showed that there was no change in the internal pore structure and sizes of the differently prepared

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057 BET surface area of different SAPO-34 catalysts Surface area (m2/g)

100% SAPO-34

460.29

30% SAPO-34

397.88

Meso-SAPO-34

524.13

30% SAPO-34 Meso-SAPO-34 5

10

15

20

25

30

35

40

45

2T/( o ) Fig. 2.

XRD patterns of different SAPO-34 catalysts.

30% SAPO-34

Meso-SAPO-34

100% SAPO-34

1450

1500

1550

1600



SAPO-34 catalysts, while the external surface area varied as shown in Table 1. The preparation recipes for the SAPO-34 catalysts had a large impact on the structural topology integration and the bulk structure (Fig. 1). The 100% SAPO-34 prepared with Si and Al sources gave distinct cube shape morphology. 30% SAPO-34 prepared by mixing 100% SAPO-34 was in the form where kaolin surrounded or camouflaged the original texture of SAPO-34. When kaolin was used as the source of Si and Al in the same recipe, distinct slits appeared on the catalyst surface (Fig. 1), which was named meso-SAPO-34 catalyst. The structures of these prepared SAPO-34 topologies were determined from the XRD patterns shown in Fig. 2. It has long been known that diffusion can affect the activity of zeolites. In general, observations made with a higher adsorption enthalpy of the feed and higher activity per acid site gave lower activation energy. These recent developments in zeolites suggested possibilities for propylene enhancement through catalyst modifications. The Brønsted acid sites of SAPO-34 are responsible for cracking and are dependent on Si atom incorporation into the framework [23–26]. The structure, acidity, and other catalytic properties of SAPO-34 depend on the number and distribution of Si in the framework, which was determined by the synthesis recipe [26–32]. These SAPO-34 zeolites of different bulk structures have intrinsically equal acid strength as shown in Fig. 3. It was observed that both Brønsted and Lewis acid sites were in the order 100% SAPO-34 > Meso-SAPO-34 > 30% SAPO-34, while this difference was negligibly small. Therefore, it can be concluded that the overall acidity did not affect the performance of the SAPO-34 catalysts. Moreover, a higher surface area provided more surface acid sites. These significantly played a role in cracking reaction dynamics and were experimentally quantified.

100% SAPO-34

Intensity

Catalyst

Transmittance

Table 1

1650

1700

1

Wavenumber (cm ) Fig. 3.

IR spectra of ammonia adsorbed of different SAPO-34 catalysts.

2.2 Reaction performance evaluation The cracking was enhanced due to the shape selectivity [13] and suitable acidity [14]. It has been shown in our previous studies that SAPO-34 acidity was sufficient and suitable for the conversion of higher olefins to lower olefins (particularly propylene), in comparison to Y-zeolite and ZSM-5 catalysts [14]. The small pore size of SAPO-34 actively plays a role in giving a shape selectivity effect for propylene [13]. Still, there

Fig. 1. SEM images of different SAPO-34 catalysts. (a) 100% SAPO-34; (b) 30% SAPO-34; (c) Meso-SAPO-34 [22].

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

Table 2

100 90 Feed conversion (%)

is a need to further intensify the SAPO-34 texture and topology to maximize propylene from the olefin route. Therefore, in the present study, three distinct SAPO-34 bulk topologies (100% SAPO-34, 30% SAPO-34, and meso-SAPO-34) were characterized and their performance was experimentally determined. Extensive experiments were carried out under identical operating conditions to determine the performance of the three distinct SAPO-34 bulk topologies for 1-hexene catalytic cracking. The emphasis has been given to maximize propylene. First, the thermal cracking of 1-hexene was experimentally determined at 550 oC and 0.1 MPa. A negligibly small amount of conversion (>2%) was seen for all prepared catalyst samples. Therefore, in the present study, the contribution of thermal cracking to the overall reaction was neglected. The shape selectivity can be explained by very simple thermodynamic analyses, by considering the impact of the zeolite topology on the free energy landscape, i.e., the free energies of formation of the molecules involved in the reactions [13]. Previously, a number of catalysts were stated to be excellent and their performance was justified by this concept [14]. In the present study, however, we focus on how a small change in the bulk structure of the catalyst affected the overall performance of the reaction. Hexene has 17 isomers, with distinct thermodynamic equilibrium compositions in the course of the cracking reaction. Moreover, olefin isomerization was very quick and immediately reached thermodynamic equilibrium [33]. Because n-hexene has a smaller kinetic diameter, it can easily diffuse into the channels of SAPO-34, but with the iso-hexene isomers, diffusion is much slower owing to their large kinetic diameter. The product stayed on the surface of the catalyst and favored hydride transfer (HT) reactions that ultimately reduced the catalyst efficiency to produce light olefins. The reaction in the internal channels and ability to diffuse directly affected the conversion rate of 1-hexene. It has been reported in the literature that with the increase in carbon chain, the isomerization rate remained unchanged [34], while the cracking rate increased rapidly [35]. Therefore, the main objective in using SAPO-34 as a cracking catalyst for 1-hexene was to prevent the isomerization of the feed and HT reactions, and to enhance propylene in a selective way. The control of stereo-chemistry is needed for selective olefin conversion to light olefins. Modification of the bulk structure such as with meso-SAPO-34 can give intensified diffusion, as molecules diffuse through the pores via various diffusion mechanisms. The higher feed conversion was due to the higher surface acidity at the cost of lower propylene selectivity.

80

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

70 60 50 40 30

1

2

3

4

5

TOS (min) Fig. 4. Feed conversion over different SAPO-34 catalysts at 550 oC.

The comparison of 1-hexene conversion over the prepared SAPO-34 catalyst topologies at 550 oC is shown in Fig. 4. For catalysts with an identical pore size but differently shaped bulk structure (outer topology), there was a drastic effect in enhancing the feed conversion. On 30% SAPO-34, most of the active sites were covered inside the kaolin and silica, which may block the pore mouths to some extent. On the other hand, 100% SAPO-34 (distinct cube shape) provided immediate contact to acid sites and showed relatively better conversion. Meso-SAPO-34 provided exceptional diffusion pathways so that a maximum of the feed reached the pores, and it demonstrated a superior conversion above 97% at TOS = 1 min. This enhancement was attributed to better diffusion and surface acidity. Significant differences in conversion with TOS were noted. With further time on stream, the conversion dropped for all catalysts. This behavior was mainly due to gradual catalyst deactivation. The stability of meso-SAPO-34 was impressive with a very slow deactivation rate as shown in Table 2. This was calculated by: deactivation rate = (Xi – Xf)/Xi. The rapid deactivation rate of the catalysts was mainly due to the fast surface reaction that led to coke formation, blocked pores, and decreased acid site contacts. The changes with TOS of desired product selectivity for 1-hexene catalytic cracking were also experimentally determined at 1–5 min. The results are shown in Fig. 5. Figure 5 could mislead readers about the complete reaction performance, and to avoid this, olefin performance envelop (OPE) was compared. The lower selectivity for propylene using meso-SAPO-34 was due to higher ethylene, butene, and propane contents. The meso-SAPO-34 catalyst had a

Deactivation rates of different SAPO-34 bulk topologies

Catalyst

Xi/%

Xf/%

Deactivation rate (%)

Si/%

Sf/%

Coke*

100% SAPO-34

83.21

47.85

42.49

94.69

87.99

0.69

30% SAPO-34

60.07

41.12

31.55

90.12

89.54

0.76

Meso-SAPO-34

97.4

88.31

9.33

82.79

89.13

0.47

Xi, Xf, Si, and Sf were the initial conversion at 1 min, final conversion at 5 min, initial total olefin selectivity, and final total olefin selectivity, respectively. * Measured in wt% using O2-pulse analysis.

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

70 100% SAPO-34 30% SAPO-34 Meso-SAPO-34

80

70

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

60 Propylene yield (%)

Propylene selectivity (%)

90

50 40 30 20

TOS = 1 min

10

60

0 60 1

2

3

4

5

TOS (min) Fig. 5. Propylene selectivity over the SAPO-34 catalysts at 550 oC.

significant advantage with a higher desired product yield and less coke formation than 100% SAPO-34 (Table 2), while it had lower propylene selectivity. The amount of coke formed during cracking reaction was largely influenced by the surface area of the catalyst. That was why meso-SAPO-34 had the lower deactivation rate due to lower coke formation. The effect of temperature on 1-hexene catalytic cracking reaction was investigated over the temperature range of 450–550 oC. The results are shown in Fig. 6. It is obvious that with the increase in temperature, conversion increased. The decrease of the hydrogen transfer coefficient with reaction temperature indicated that the higher temperature favors monomolecular. The propylene yields from 1-hexene cracking over the different SAPO-34 catalysts were compared at 450–550 oC and at TOS = 1 and 5 min. The yield of propylene increased with temperature because of higher conversions. The propylene yield over 30% SAPO-34 was almost constant with an identical growth pattern with TOS. Over 100% SAPO-34, propylene yield sharply declined with TOS at the designated temperatures due to quick deactivation. Owing to the decreased rate of hydrogen transfer reactions, the product paraffin content decreased and the total olefin content increased. However, pore blockage may change the whole scheme. Hexene cracking mainly followed two pathways. At lower temperatures, 1-hexene converted to propylene via indirect cracking and therefore butene, pentene, and propylene were quite selective. With increased temperature, direct cracking became dominant (1-hexene split into two propylene). Further increase in temperature enhanced cracking rate and produced propylene by both routes. On the other hand, the free radicals mechanism increased coke and/or may cause some amount of propylene to further crack. Generally it was observed that, with a short residence time and increased space velocity, the conversion of raw materials declined. The increase of space velocity did not affect the total olefin selectivity because of limited hydrogen transfer reaction and/or other secondary reactions. It was concluded that a lower catalyst to

Propylene yield (%)

50

50 40 30 20 10 0

TOS = 5 min 450

475

500 Temperature (oC)

525

550

Fig. 6. Propylene yield over the SAPO-34 catalysts at various TOS.

feed ratio was important over meso-SAPO-34, while a higher catalyst to feed ratio was required for better results over 100% SAPO-34. However, it is highly desirable during olefin cracking to prevent even a small amount of olefins from converting to paraffin. The problem of a lower conversion is solvable, but to reprocess paraffin into olefins is too difficult. Therefore, the catalyst of higher conversion and lower desired product selectivity was not recommended. Pure SAPO-34 demonstrated a superior catalytic performance and proved its excellent catalytic ability for 1-hexene cracking. It should be noted that the ethane, propane, and butane were the second most abundant products that had a significant impact on total olefin yield. Accordingly, the olefin performance envelop (OPE) was drawn in order to correlate the propylene yield and selectivity with the wide range of corresponding conversions. This is shown in Fig. 7. It can be seen from the OPE that 100% SAPO-34 had a superior propylene selectivity even at higher conversions than 30% SAPO-34 and meso-SAPO-34. The OPE further suggested that with the increase in conversion, propylene selectivity tended to decline and this trend became more serious for 30% SAPO-34. The good agreements between catalyst performance were also observed with total olefin and total paraffin OPE, as shown in Fig. 8. It is seen that at high conversion, olefins transformed to paraffins. The 30% SAPO-34 catalyst showed a strong affinity towards paraffins due to the higher surface reaction rates and slower diffusivity. While using 100% SAPO-34 and meso-SAPO-34, at above 80% conversion a

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

100 (a) Yield of total olefins (%)

80 70 60 50

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

40 30

Yield of propene (%)

50 40 30 20

Cube-SAPO-34 30% SAPO-34 Meso-SAPO-34

10 0

0

10

20

30

40

50

60

70

80

OPE of propylene at 500 oC over different SAPO-34 catalysts.

90 85

100% SAPO-34 30%-SAPO-34 Meso-SAPO-34

80 75

(b) 20 Cube-SAPO-34 30%-SAPO-34 Meso-SAPO-34

15 10 5 0

90 100

0

10

20

30

40

50

60

70

80

90

100

Conversion (%)

Conversion (%) Fig. 7.

(a)

95

70

(b) Yield of total parafins (%)

Selectivity for propene (%)

90

Fig. 8.

Yield versus conversion at 500 oC over different SAPO-34 cata-

(a) Selectivity; (b) Yield.

lysts. (a) Total olefins; (b) Total paraffins.

sharp increase in paraffin content was obtained. Therefore, the optimum conversion for 1-hexene cracking to produce propylene was recommended to be less than 80% for both 100% SAPO-34 and Meso-SAPO-34. It was further verified that the paraffins were secondary products. A rigorous analysis was devised to quantify the reaction behavior. This used the by-products analysis and detailed OPE of ethylene, propane, butane, butene, pentane, and dry gas shown in Fig. 9. Among these by-products, the highest contribution was due to butene and its yield was more than 15% at 80% conversion. The amount of undesired by-products, propane, butane, and pentane, were also more with the increase in conversion. Above 90% conversion, Fig. 9 seems strange for all catalysts. However, no methane was recorded. At higher conversions, the amount of paraffin was high for all catalysts, but it was clear that the overall performance of 100% SAPO-34 was still superior by accommodating paraffins effectively. In the early reactions, the increase in paraffins with conversion was due to hydrogen transfer reactions. Carbon-hydrogen bond braking has different response to different carbon number atoms, and it is further affected due to their concentration [36]. Secondary products were derived from the primary products in a similar manner. This early trend of the system with increasing conversion rate generated hexane and propane. It is observed that ethylene, propane, and butane amounts increased with diffusion rate. Therefore, it can be concluded that the reaction was not only diffusion limited but that the thermodynamic constraints were still dominant.

The mechanism of alkene cracking over solid acidic catalysts and also their kinetics were proposed by a number of researchers [12,15,16]. Our focus in the present study was to characterize reaction behavior by the operating conditions to enhance propylene production. Overall, the active centers in these reactions are proton acidic centers on the catalyst surface, and the reactive species are carbenium ions. In direct cracking, the carbon chain breaks at the center to give two propylene molecules, while only a small amount of 1-hexene cracking proceeds via two-way reactions. With the further reaction time of raw materials, the secondary reactions start, such as after cracking, hydrogen transfer reactions generate the corresponding paraffins. 1-Hexene mainly cracks into two propylene molecules via direct cracking and also has parallel co-production of ethylene, butane, and pentene. However, over 99% of the cracking occurs by the direct cracking route [15] and bimolecular reactions are negligible. Generally, cracking can take place on Brønsted acid sites via protolytic cracking or on Lewis acid sites via a classical ȕ-scission mechanism. The activation energy for protolytic cracking is lower than that for ȕ-scission [37]. It has long been reported that at higher temperatures and lower pressures, the adsorption of hydrocarbons was decreased and the monomolecular mechanism was enhanced [38]. SAPO-34 has weak acid sites (see Fig. 3), and therefore most of the reaction proceeds via the ȕ-session carbenium ion mechanism (Scheme 1). There is still controversy regarding the nature of the acid sites for sustained hydride transfer and how it may enhance alkyla-

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

8 6 4 2 0

20

40 60 80 Conversion (%)

30

2

0

20

40 60 80 Conversion (%)

15 10 5 0

20

40 60 80 Conversion (%)

100

8 6 4 2 0

2 1

0

20

40 60 80 Conversion (%)

100

0

20

40 60 80 Conversion (%)

(f)

10

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

Yield of dry gas (%)

20

(c)

12 (e)

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

3

0

100

10

Yield of pentane (%)

Yield of butene (%) Fig. 9.

4

(d)

25

0

6

0

100

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

8

(b) Yield of butane (%)

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

10

0

4

10 (a) Yield of propane (%)

Yield of ethylene (%)

12

100

100% SAPO-34 30% SAPO-34 Meso-SAPO-34

8 6 4 2 0

0

20

40 60 80 Conversion (%)

100

Yield versus conversion at 500 oC over different SAPO-34 catalysts. (a) Ethylene; (b) Propane; (c) Butane; (d) Butene; (e) Pentane; (f) Dry gas.

CH2=CH-CH2-CH2-CH2-CH3 + Z-O-H ĺ CH2-CH+- CH2-CH2-CH2-CH3…Z-O– CH2-CH+-CH2-CH2-CH2-CH3 ĺ CH2-CH+-CH3 + CH2=CH-CH3 Scheme 1 C-type ȕ-scission carbenium ion mechanism of 1-hexene cracking.

tion activities [39]. The trend of a continuous increase in propylene yield with conversion was obtained over 100% SAPO-34. The whole reaction system can be explained by the ȕ-scission carbenium ion mechanism [12,15]. The reaction of polymerization has been ruled out, as no C7 and higher hydrocarbons were obtained.

3

Conclusions

The influences of operating parameters on product distribution and conversion were systematically studied to explore the influence of shape and bulk structure on reaction performance. Due to the very high initial conversion rates, even at high WHSV, the complete OPE profiles were drawn for a clearer understanding of the reaction behavior on the prepared bulk topologies. 30% SAPO-34 showed a lower conversion and selectivity owing to limited acid sites and pore exposure. A higher surface retention time promoted hydride transfer reactions and led to quick catalyst deactivation. 100% SAPO-34 showed relatively better 1-hexene conversion and superior propylene selectivity. The cube shaped SAPO-34 was deactivated quickly in comparison with other catalysts under identical conditions. Over meso-SAPO-34, an exceptionally high

conversion and stability were observed due to its distinct bulk geometry and large surface area. However, propylene selectivity was relatively low with slightly higher paraffin content due to the higher surface acidity. Moreover, the slit topology delayed deactivation by retarding surface reactions. Most of the feed were converted to propylene via the C-type ȕ-scission mechanism (direct cracking). Increasing catalyst-to-oil ratio can enhance 1-hexene cracking and improve the yield of propylene to some extent even at higher temperatures. Increase in TOS will lead to secondary reactions and decrease the desired product selectivity with an overall decrease in olefins. The percentage selectivity of propylene and conversion showed a rapid increase with increasing reaction temperature up to 550 o C. The drastic increase in propylene selectivity over 100% SAPO-34 was attributed to the balance between a better exposure of the feed to the acid sites and pores. The acidity of SAPO-34 was sufficient for 1-hexene cracking and also its smaller pores will promote shape selectivity effect for light olefins. Paraffins were produced via surface reactions as secondary products and later will become coke precursors. It was further observed that a change in the bulk geometry in order to enhance diffusion will only improve conversion, while propylene selectivity gets adversely affected.

Zeeshan NAWAZ et al. / Chinese Journal of Catalysis, 2009, 30(10): 1049–1057

References 1 Nawaz Z, Tang X P, Zhang Q, Wang D Z, Wei F. Catal Commun, 2009, 10: 1925 2 Corma A, Melo F V, Sauvanaud L, Ortega F. Catal Today, 2005, 107–108: 699 3 Nawaz Z, Wei F. Ind Eng Chem Res, 2009, 48: 7442 4 Wei Y X, Zhang D Z, Xu L, Liu Z M, Su B L. Catal Today, 2005, 106: 84 5 Nawaz Z, Qing S, Gao J, Tang X P, Wei F. J Ind Eng Chem, 2009, in press 6 Li J Z, Qi Y, Liu Zh M, Liu G Y, Chang F X. Chin J Catal, 2008, 29: 660 7 Nawaz Z, Tang X P, Wei F. Braz J Chem Eng, 2009, in press 8 Houdek J M, Andersen J. In: ARTC 8th Annual Meeting. Malaysia: Kuala Lumpur, 2005 9 Nawaz Z, Tang X P, Wei F. Korean J Chem Eng, 2009, in press 10 Wei F, Tang X P, Zhou H, Zeeshan N. Petrochem Technol, 2008, 37: 979 11 Tang X D, Zhou H Q, Qian W Z, Wang D Z, Jin Y, Wei F. Catal Lett, 2008, 125: 380 12 Zhou H, Wang Y, Wei F, Wang D, Wang Z. Appl Catal A, 2008, 348: 135 13 Smit B, Maesen T L M. Nature, 2008, 451: 671 14 Nawaz Z, Zhu J, Wei F. In: The 6th International Bhurban Conference on Applied Sciences and Technology (IBCAST), Islamabad, Pakistan, 2009 15 Buchanan J S, Santiesteban J G, Haag W O. J Catal, 1996, 158: 279 16 Buchanan J S. Appl Catal A, 1998, 171: 57 17 Niccum P K, Miller R B, Claude A M, Silverman M A. In: NPRA Annual Meeting, San Francisco, 1998 18 Marchese L, Frache A, Gianotti E, Martra G, Causa M, Coluccia S. Microporous Mesoporous Mater, 1999, 30: 145 19 Lok B M, Messina C A, Patton R L, Gajek R T, Cannan T R,

Flanigen E M. J Am Chem Soc, 1984, 106: 6092 20 Such X, Pang K. Zeolite Materials with Porous Chemicals. Beijing: Science Press, 2004. 257 21 Nawaz Z, Tang X P, Yu C, Wei F. Arab J Sci Eng B, 2009, in press 22 Zhu J, Yu C, Wang Y, Wei F. Chem Commun, 2009: 3282 23 Schnabel K H, Fricke R, Girnus I, Jahn E, Loffler E, Parlitz B, Peuker C. J Chem Soc, Faraday Trans, 1991, 87: 3569 24 Xu L, Du A, Wei Y, Wang Y, Yu Z, He Y, Zhang X, Liu Z. Microporous Mesoporous Mater, 2008, 115: 332 25 Liu G, Tian P, Li J, Zhang D, Zhou F, Liu Z. Microporous Mesoporous Mater, 2008, 111: 143 26 Tan J, Liu Z, Bao X, Liu X, Han X, He C, Zhai R. Microporous Mesoporous Mater, 2002, 53: 97 27 Borade R B, Clearfield A. J Mol Catal, 1994, 88: 249 28 Prakash A M, Unnikrishnan S. J Chem Soc, Faraday Trans, 1994, 90: 2291 29 Ashtekar S, Chilukuri S V V, Chakrabarty D K. J Phys Chem, 1994, 98: 4878 30 Sastre G, Lewis D W, Catlow C R A. J Phys Chem B, 1997, 101: 5249 31 Vomscheid R, Briend M, Peltre M J, Man P P, Barthomeuf D. J Phys Chem, 1994, 98: 9614 32 Dumitriu E, Azzouz A, Hulea V. Microporous Mater, 1997, 10: 1 33 Pu M, Li Z h, Gong Y J, Wu D, Sun Y H. J Mater Sci Lett, 2003, 22: 955 34 Abbot J, Wojciechowski B W. Can J Chem Eng, 1985, 63: 462 35 Kissin Y V. Catal Rev Sci Eng, 2001, 43: 85 36 Lukyanov D B. J Catal, 1994, 147: 494 37 Jung J S, Kim T J, Seo G. Korean J Chem Eng, 2004, 21: 777 38 den Hollander M A, Wissink M, Makkee M, Moulijn J A. Appl Catal A, 2002, 223: 85 39 Feller A, Zuazo I, Guzman A, Barth J O, Lercher J A. J Catal, 2003, 216: 313