A methanol to olefins review: Diffusion, coke formation and deactivation on SAPO type catalysts

Microporous and Mesoporous Materials 164 (2012) 239–250

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A methanol to olefins review: Diffusion, coke formation and deactivation on SAPO type catalysts D. Chen a,⇑, K. Moljord a,b, A. Holmen a,⇑ a b

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Trondheim N-7491, Norway Statoil Research Center, Postuttak, Trondheim N-7005, Norway

a r t i c l e

i n f o

Article history: Available online 6 July 2012 Keywords: MTO SAPO-34 Coke formation Deactivation

a b s t r a c t The catalytic conversion of methanol to lower olefins (MTO) is a promising way of converting natural gas and coal to chemicals and fuels with methanol as an intermediate. Coke formation is a major cause of deactivation in the MTO processes and the present contribution deals with the progress on the study of adsorption, diffusion and reaction including deactivation due to coke formation during MTO. Design of SAPO-34 to achieve high activity and selectivity is discussed in terms of the two most important parameters, namely crystal size and operating temperatures. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction The catalytic conversion of methanol to lower olefins (MTO) is a promising way of converting natural gas and coal to chemicals via methanol [1,2]. Three demonstration coal to olefin (CTO) units (Shenhua Ningxia Coal Industry Group’s 500,000 ton/year methanol-to-propylene (MTP), 2010; Shenhua Group (Baotou)’s 600,000 ton/year MTO unit, 2010; and Datang International Power Generation’s 460,000 ton/year MTP unit, 2011) with total capacity of 1,560,000 tons have already achieved stable production. In addition, Henan Zhongyuan’s 200,000 tons/year CTO unit was started up in 2011. There are another nine CTO projects expected to come on stream before 2013 in China [3]. The MTO reaction has been studied over different types of zeolites or molecular sieves (ZSM5, SAPO-34, MOR, etc.) and at different reaction conditions. Several reviews have dealt with catalysts, reaction mechanism and processes of methanol to hydrocarbons including olefins [4–23]. The catalytic processes using zeolites often include side reactions leading to the formation of carbonaceous material with catalyst deactivation as a result, defined as ‘coke’. Coke deposition is known to be the major cause of deactivation in the MTO reaction over zeolites, and both catalyst activity and selectivity are influenced by coke deposition [22,24,25]. More exact knowledge about the mechanism and kinetics of coke formation is a basis for improving these catalytic processes. The present contribution will comprehensively review the progress on the study of adsorption, diffusion and reactions including ⇑ Corresponding authors. Tel.: +47 73594151; fax: +47 73595047 (A. Holmen), tel.: +47 73593149; fax: +47 73595047 (D. Chen). E-mail addresses: [email protected] (D. Chen), [email protected] (A. Holmen). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.06.046

deactivation due to coke formation during MTO. Various topics, such as adsorption and diffusion, reaction pathways leading to coke formation, location of coke, effects of zeolite crystal size, cage structure and composition, acidic site density and strength, as well as the effect of coke on catalyst activity and selectivity, mostly on SAPO type of catalysts are included. The proposed reaction mechanisms on SAPO-34 are summarized and revised. The rational design of SAPO-34 to achieve high activity and selectivity to olefin, and the olefin capacity is discussed in terms of the two most important parameters, namely crystal size and operating temperatures.

2. Effects of diffusion in the MTO reactions on SAPO-34 It has long been recognized that the crystal size of zeolites can influence the performance both of H-ZSM5 [26–32], H-SAPO-34 [25,33–45] and H-STA-7 [37] in the MTO reaction. It has been shown that long life can be obtained by using small crystals. For H-ZSM5, small crystals resulted not only in a long life time, but also increased selectivity to aromatics [28]. However, the crystal size did not change the selectivity on SAPO-34 at identical coke content [25,35,36]. A detailed kinetic study of MTO on different sized crystals of SAPO-34 in a tapered element oscillating microbalance (TEOM) reactor has provided information of the crystal size effects on adsorption, diffusion, reactions, coke formation and deactivation for olefin formation [25,33,46]. The differently sized crystals were synthesized in a controlled manner to obtain identical acid site density and distribution [34]. The effect of intracrystalline diffusion on methanol conversion to olefins (MTO) over SAPO-34 was elucidated by performing reactions on crystal sizes in the range of 0.25–2.5 lm at identical conditions [25,33]. The conversion of

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The effect of SAPO-34 crystal size on the olefin formation was studied in DME conversion to light olefins (DTO) using the TEOM reactor to eliminate the effects of DME formation on the olefin formation on SAPO [46]. Similar to the MTO reaction, the DME conversion was lower and deactivation was faster on larger crystals. An increase of DME conversion with coke content was very obvious on 0.25 lm crystals, but not on 2.5 lm crystals (Fig. 2). Olefin formation during the DTO reaction was treated as a first order reaction [46]. The first order reaction rate constant was calculated by Eq. (1) on a carbon basis:

MeOH and DME to olefins as well as coke formation was found to be influenced by the crystal size. The highest reaction (Fig. 1B) and coking rates (Fig. 1A) were found on 0.4–0.5 lm crystals. Dimethyl ether (DME) diffusion seems to play an important role in the formation of olefins. The smallest crystals resulted in a relatively large amount of DME escaping the pores of SAPO-34 (Fig. 1D) before being converted to olefins, hence giving lower olefin yields (Fig. 1C). Coking rates increased with increasing crystal size at low coke contents, levelling off at a lower level for the largest crystals than for the smaller at high coke contents. The deactivating effect of the coke formed was larger on the larger crystals. MTO can be considered as a consecutive reaction where olefins are formed from DME:Methanol ? DME ? olefins The methanol to DME ratio was found to increase with coke formation on the catalysts and the ratio was in a range of 1 to 9 which is far from the equilibrium ratio (0.47 at 698 K) [47]. The SAPO-34 external surface is relative large for the small crystals. The possible contribution of the external surface in the MTO reaction was tested by the selective coke deposition on the external surfaces from 1butanol [48]. i-Butanol dehydrates to i-butene on acidic sites on the external surface, but i-butanol and i-butene moleculeas are too large to enter the pores of SAPO-34. The methanol to DME ratio increased with increasing external coke content (up to 1.5 wt.%). A small amount of external coke 0.65 wt.% increased slightly the olefin formation by increasing the resistance of DME diffusion out from the crystals. The results point out that both the external and internal surface contributed to the DME formation, but the reaction between methanol and DME is not in equilibrium. The SAPO-34 crystal size determines the residence time of DME in the crystal, thus the degree of conversion to olefins.

A

kapp ¼  lnð1  XÞ

F A0 qC C A0 W

ð1Þ

where X is the conversion of oxygenates, CA0 (kmol/m3) is the initial reactant concentration calculated from the ideal gas law, qc is the catalyst density (800 kg/m3), W (kg) is the catalyst weight and FAo (kmol/s) is the molar flow rate of the reactant. The apparent rate constant kapp (s1) is smaller than the intrinsic rate constant k, if diffusion effects cause the effectiveness factor g to be less than unity. Assuming the SAPO-34 crystals to be spherical, the effectiveness factor is given as:

g¼

 1 1  U tanhU U 3



ð2Þ

where Thiele modulus / = R(k/D)1/2, R is the crystal radius and D is the diffusivity of the reactant. When the observed rate constants differ for two crystals with radius R1 and R2, the parameters g1, g2 and /1, /2 can be uniquely determined by the method of triangulation in the ln g and ln / plot [46]. The intrinsic rate constant k and the diffusivity D were thus

B

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Conversion (mol%)

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15 20 25 Time on stream (min)

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D 80 Selectivity to DME (mol%)

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0 0

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Fig. 1. (A) Coke formation as function of time and (B) the conversion of methanol-dimethyl ether to olefins as a function of the amount of methanol fed to the catalyst (g/gcat, h) (C) conversion to olefins and (D) selectivity to DME as a function of coke content during the MTO reaction over SAPO-34 at 698 K WHSV = 385 g/gcat, h and a methanol pressure of 0.08 bar with different crystal sizes. 0.25 lm(j), 0.4 lm (d), 0.5 lm (N) and 2.5 lm () [25].

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10

5

0

0

5

10 Coke (wt%)

15

Fig. 2. DME conversion versus coke contents over 0.25 lm (h) and 2.5 lm ()crystals at WHSV 395 h1, 698 K and PDME: 8 kPa. Lines: predicted by model (Eqs. (1) and (4)) [46].

estimated, assuming that k and D are identical for the different crystals. This method was applied to calculate k and D on SAPO34 containing coke, based on the assumption that the coke was randomly deposited inside the crystals for all the samples. As shown in Fig. 3A, the DME diffusivity (m2/s) decreased more rapidly than the intrinsic reaction rate constant due to coke formation, as described in Eq. (3):

DDME ¼ 7:25  1011 expð0:44CÞ

ð3Þ

where C is the coke content (wt.%) and the effective DME diffusivity on fresh SAPO-34 is 7.25  1011 m2/s. In a similar way, the MeOH diffusivity and the rate constant of methanol conversion were determined as a function of coke content based on the methanol conversion on SAPO-34 with different crystal sizes (0.25–2.5 lm) [49]. The effective methanol diffusivity of 1.1  108 m2/s estimated indirectly from the kinetic data is comparable with the steady-state diffusivity of 3  109 m2/s measured at low temperatures [49]. The effective diffusivity of methanol decreased by almost three orders of magnitude from the fresh catalyst to that with a coke content of 15 wt.%. Such decrease in the effective diffusivity of methanol with increasing coke content has been described by the percolation theory, where changes in the diffusion path with coke blockage of cavities were simulated by Monte Carlo method in a 3-D network of SAPO-34 cubic crystals [49]. Recently, the percolation theory was successfully extended to predict the deactivation behavior of MTO’s porous catalyst in a fixed bed reactor [50]. The reactor model predicted well the loss in catalyst activity with time-on-stream. The Thiele modulus and the effectiveness factor (Fig. 3B) were calculated based on the intrinsic rate constants and effective

250

2 -11

DME Diffusivity (10

SAPO-34 has a high catalytic activity for the MTO reaction, but suffers fast deactivation due to coke formation [24,25,34,35,48, 49,52–67]. The rate of deactivation is so high that it is difficult to decouple it from the kinetics of the main reactions. Also the high heat of reaction (100 to 300 cal/g depending on the selectivity to olefins [4] makes the kinetics of this reaction difficult to measure in conventional laboratory reactors. Coke deposition during MTO on SAPO-34 (about 2 lm) was studied in detail in a TEOM reactor as a function of temperature, space velocity and partial pressure of methanol [24,25,48,62] and crystal size [25]. The effect of space velocity on coke formation and deactivation was investigated at 698 K in a WHSV range of 57–384 g/(g of catalyst h) with a methanol partial pressure of 7.2 kPa at atmospheric pressure. Because the catalysts underwent

B

300

6

m /s)

A

3. Coke formation on SAPO-34

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4

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100 50

0 0

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10 Coke (wt%)

0 15

1

Effectiveness Factor, η

15

diffusivities at different coke contents. The effectiveness factor for the smallest crystals was larger than 0.95 for coke content less than 4 wt.%, and decreased to 0.8 for about 14–15 wt.% coke.[46] This corresponds with a situation where the reaction changed from a kinetically controlled regime to a diffusion influenced regime, due to the intracrystalline coke deposition.[46] All the effectiveness factors were less than 0.8 on the 2.5 lm crystals, indicating that DTO was influenced by diffusion on these crystals. As more coke was deposited, the effectiveness factor decreased much faster on the larger crystals than on the smaller crystals, which resulted in the faster deactivation on the 2.5 lm crystals than on 0.25 lm crystals (Fig. 2). It can therefore be concluded that small SAPO-34 crystals are preferred to reduce the diffusion limitation, thus reducing the deactivation rate or the deactivating effects of the coke formed. All the results clearly point out that crystal size is a very important parameter for controlling coke formation and catalyst deactivation in the MTO process. The crystal size of SAPO-34 has recently been controlled between 1.5 and 7 lm by hydrothermal synthesis using a mixture of tetraethyl–ammonium hydroxide (TEAOH) and morpholine (Mor) as the structure-directing agents [36]. The smaller SAPO-34 crystals showed a longer catalyst lifetime in the methanol-to-olefins reactions. The amount of coke deposited on deactivated SAPO-34 catalyst decreased with decreasing crystal size, indicating that the effectiveness of the catalyst could be improved by reducing the crystal size of SAPO-34 [36]. Crushing large crystals (5–20 lm) to 1 lm by ball milling increased the life time more than three times [35]. Formation of a reaction zone near the external surface caused by diffusion control in MTO has recently also been directly confirmed by combined in situ UV/Vis microscopy and a confocal fluorescence microscope [51].

Intrinsic Rate Constant (1/s)

Conversion of DME (mol%)

20

0.1 0.1

1 10 Thiele Modulus, Φ

100

Fig. 3. (A) Effective DME diffusivity and intrinsic rate constant of DTO as a function of coke content at 698 K and (B) effectiveness factor as a function of Thiele Modulus [46].

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20

Coke (wt%)

15

10

5

0

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40

80

120

CAMF (g/gcat)

rapid deactivation, the MTO reaction was investigated by using 3 min interrupted pulses with GC analysis carried out after 2 min for each pulse (the integrated pulse method). Coke deposition is often studied as a function of time on stream [52,68]. Marchi and Froment [61] studied deactivation during MTO over SAPO-34 as a function of the cumulative amount of methanol fed to the catalyst. The interpretation of the measured coke content as a function of time on stream and as a function of the cumulative amount of methanol fed to the catalyst could lead to contradictory results [24]. It was found that the cumulative amount of methanol fed to the catalyst is a better parameter to describe the coke deposition at different conditions, which has also recently been reported by Qi et al., [69]. Fig. 4 presents coke deposition at different space velocities. A lower space velocity resulted in a higher coking rate, as a result of high average concentration of olefins and low average concentration of oxygenates (MeOH/DME mixtures) in the reactor. It indicates that the rate of coke deposition is related to the concentration of olefins, which is in good agreement with the results reported for methanol conversion to gasoline (MTG) over HZSM-5 [70]. Aguayo [34] observed also a higher coke content at the outlet than at the inlet of a fixed-bed reactor in methanol conversion over HZSM-5 [68]. At space velocities less than 82 g(gcat h)1, the change in coke deposition with space velocity is not significant, since the oxygenate conversion is almost complete. This explains the observation that the coking rate is identical regardless of space velocity in the range of 8–45 g(gcat h)1[71]. The coke deposition can be well correlated to the cumulative amount of hydrocarbons formed per gram of catalyst, regardless of space velocity, as shown in Fig. 5 [24]. From Fig. 5, it is straightforward to obtain the average/accumulated coke selectivity (g coke/g hydrocarbon formed) by dividing the coke content by the amount of hydrocarbon formed. In addition, the catalyst capacity (g hydrocarbon formed/g catalyst) can be obtained by the amount of hydrocarbon formed at the coke content of 18%, where the catalyst was almost completely deactivated. The catalyst capacity and the coke selectivity are 29.3 and 0.006 at 673 K compared to 4.0 and 0.045 at 823 K, respectively. Higher temperatures favoured the reactions leading to coke formation more than the reactions leading to olefins, hence a faster deactivation, in good agreement with the observation on H-ZSM-5 [70]. However, the coking rate was found to decrease with increasing temperature in the low temperature range of 613 to 693 K on SAPO-34 [52], which was explained by increased cracking of coke precursors. The identical conversion to olefins was obtained when methanol pressure and weight hourly space velocity (WHSV) were increased

Fig. 5. Coke formation versus the amount of hydrocarbon formed (CAHF) at different temperatures on SAPO-34 (particle size ca. 2 lm): 823 K: h (PMeOH = 13 kPa, WHSV = 270 g/gcat, h); 773 K: s (PMeOH = 13 kPa, WHSV = 270 g/ gcat, h); 698 K and PMeOH = 7.2 kPa: WHSV = ⁄57, j82, N 113, d 270 and  385; 673 K: 4(PMeOH = 7.2 kPa, WHSV = 385 g/gcat, h) [24].

15

Coke (wt%)

Fig. 4. Coke formation during MTO over SAPO-34 (particle size ca. 2 lm) versus cumulative amount of methanol fed to the catalysts(g/gcat) at 698 K and a methanol partial pressure of 7.2 kPa for different space velocities WHSV (g/(gcat, h): h: 57, ⁄: 82,4: 113, e: 385 [24].

10

5

0

0

5

10

15

Time on stream (min) Fig. 6. Coke formation versus time on stream at different methanol partial pressures on SAPO-34 (particle size ca. 2 lm). }: 15 kPa 384 g/(gcat h), d: 30 kPa and 768 g/(gcat h), N: 60 kPa and 1538 g/(gcat h), h: 83 kPa and 2558 g/(gcat h) [24].

by an identical factor, indicating a first order reaction with respect to methanol partial pressure. However, coke formed much faster at the higher methanol pressures and higher WHSVs (Fig. 6), which can be ascribed to the higher amount of hydrocarbon formed. At industrial relevant conditions (high methanol pressures and high WHSVs), a coke content of about 10 wt% was achieved within 90 s. Such fast reactions would require a reactor design providing short contact times as well as continuous catalyst regeneration.

4. Catalyst deactivation during the MTO reaction [22] 4.1. Deactivation of SAPO-34 during MTO by coke formation As discussed above, coke forms rapidly on SAPO-34 due to its large cages and narrow pores. Catalyst deactivation depends not only on the coke content but also on the nature and location of the coke molecules. SAPO-34 deactivated generally as a linear function of the coke content [24,62,72], but it deactivated faster at higher temperatures, indicating that larger molecules are formed at higher temperatures at an identical coke content [62]. More importantly, large SAPO crystals deactivated faster than small crystals with similar coke contents, possibly due to coke formation near the external surface of the catalyst particles [73], gradually blocking the diffusion path of oxygenates to the inner

D. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 239–250

core of the catalysts, in accordance with the observed lower coke content on larger SAPO-34 crystals. Removal of coke by combustion in air can completely recover the activity [74]. 4.2. Effects of pore structure on catalyst deactivation A large number of catalysts with different structures have been investigated in MTO aiming at increasing the olefin selectivity and life time. Effects of the catalyst structure on the activity and selectivity to olefins have been well documented in several reviews [5,16,20,75]. Unfortunately, most studies report only the conversion and deactivation with time on stream, and coke deposition and the deactivating or activating effect of coke could not be determined [37,53,61,76–84]. Silicoaluminophosphates (SAPOs) are a relatively new generation of crystalline microporous molecular sieves. They are synthesized by incorporating Si into the framework of the aluminophosphate (AlPO) molecular sieves. Djieugoue et al., studied four small pores SAPOs: Erionite-like SAPO-17, chabazite-like SAPO34, Levynite-like SAPO-35, and SAPO-18, whose structure is closely related to, but crystallographically distinct from, that of SAPO-34 [80]. The cage structures are presented in Fig. 7. The life time followed the order of SAPO-18 > SAPO34  SAPO-17 > SAPO-35. SAPO-35 has the smallest cage size and the lowest number (three) of 8-membered ring openings in each cage, which deactivated most rapidly. The lowest olefin yield or selectivity is reported on SAPO-35 [85], but the measurements could have been influenced by deactivation. In addition it should be mentioned that the number of acid sites is not the same, following an order of SAPO-34 > SAPO17 > SAPO-18 > SAPO-35. Therefore, the rapid deactivation of SAPO-35 cannot be explained by the high acid site density. It was suggested that the small cage is the main cause for the faster deactivation. Guisnet et al., have long recognized profound effects of the pore structure of zeolites on coke deposition and deactivation

Fig. 7. Cage structure of SAPO-34, SAPO-35, SAPO-17 and SAPO-18 [85].

243

[86–89]. There are several studies comparing H-ZSM-5 and HSAPO-34 [60,90,91], and the reported rate of coke deposition is less on ZSM-5 compared to SAPO-34, which can be explained by the ratio of cage size and pore size. A small cage size provides a space constraint for growth of coke molecules [25,33,92]. In this regard, it is expected that the coking rate should be lower on SAPO-35. The fast deactivation of SAPO-35 would probably be related to its structure of three 8-membered ring openings while SAPO-34, -17 and -18 have six 8-membered rings in each cage. The probability of molecular diffusion in and out of the cages is related to the number of ring openings in each cage and a small number of rings could cause more pronounced deactivation by a faster reduction in diffusion caused by blockage by coke molecules. A better stability of SAPO-18 could be explained by a different shape of the cage and less acidic sites [80]. The effect of the ratio of cage and pore openings was also investigated by Park et al., using four kinds of 8-membered ring (8-MR) small-port molecular sieves with CHA (SAPO-34), ERI (UZM-12), LTA (UZM-9), and UFI (UZM-5) topologies [77]. The ratio of cage and pore size and deactivation are in a same order of LTA > ERI > CHA. UV–Vis spectroscopic studies revealed that larger coke molecules are formed in larger cages, causing the faster deactivation. UFI and LTA have similar cage and pore size, LTA has a threedimensional (3-D) structure while UFI has a two-dimensional (2D) connectivity. Clearly a 3-D structure has better stability than a 2-D structure [77], which has better stability than the 1-D structure such as SAPO-41 [82], SAPO-5 [53,76,93] and SAPO-11 [61].

4.3. Effects of catalyst acidity and composition on deactivation Acid site density and acidic strength are expected to have significant effects on reactions leading to coke formation, such as oligomerization, cyclization and hydrogen transfer. These reactions are requiring strong acidic sites. Aguayo et al., have used calorimetric measurements of differential adsorption of ammonia (DSC) and temperature programmed desorption of ammonia (TPD) to investigate the total acidity and acidic strength distribution [84]. High acidic strength resulted in high reaction rates on SAPO-18 and SAPO-34 compared to SAPO-11 and Beta-zeolites. Considering the similarity of SAPO-18 and the SAPO-34 in the pore structure and the acidic strength, a greater density of acid sites of SAPO-34 might be responsible for the faster deactivation compared to SAPO-18. Similar observation was also reported by other groups [83,94–97]. Yuen et al., [97] performed comparative MTO tests on catalysts with CHA topology with varying acidic strength (SAPO-34 and SSZ-13, respectively) and acid site density. They observed that an intermediate acid site density (10% Al, vs. 18% Al and 3.3% Al, respectively, in H-SSZ-13) is advantageous for the stability of H-SSZ-13 catalyst. Recently, Bleken et al., have performed a similar study with carefully synthesized zeolites (SAPO-34 and SSZ13) with the same crystal size and acid site density to elucidate the effect of acidic strength on MTO [95]. Consistent with the results in [84,97], acidic strength decreases the energy barrier and thus increases the reaction rates. Ethylene increases in a shorter time on H-SSZ-13 than on H-SAPO-34, suggesting a faster coke formation on H-SSZ-13 due to its higher acidic strength. The acid site density has been long recognized to have profound effects on catalyst coke formation and deactivation [5,21]. A comparison of iso-structural H-SAPO-34 and H-chabazite (HCHA) in MTO showed a similar initial rate and selectivity, but a much faster deactivation on H-chabazite with higher acid site density than the low acid-site density analogues [98]. It is expected that the coking rate is much larger on HCHA. Zhu et al., noticed that less Si in CHA zeolites results in a prolonged catalyst life time due to the reduced acid sites [99].

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Incorporating metal into the SAPO framework has been recognized as a way of improving the olefin selectivity and prolonging the catalyst life time since the reports from Inui and Kang [100,101], as part of the large research efforts on metallosilicates [102–104] in his group. Various Me-APSO-34 (Me@Ni [85,101,105–121], Co, Mn, Fe [106,109,113,114], Cu [79] and Ti, Cr, Cu, Zn, Mg, Ca, Sr, and Ba [119,121]) have been tested in MTO. Incorporating metals into the framework of zeolites has proven to reduce the acid density, leading to a higher selectivity to ethylene, reduced coke formation and thus higher catalyst life time [85,101,121]. Co-feeding of water increased the olefin selectivity and reduced coke formation and prolonged the catalyst life time. This can be explained by strong acidic sites being occupied by polar water molecules [61,122,123].

0 3.2 wt %

-0.01 6 wt %

-0.02

dx/dT (1/ °C)

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13.3 wt %

-0.03

5. The role of coke in the MTO reactions and for the deactivation -0.04

Fig. 2 shows that the DME conversion rate increased slightly with coke formation up to about 5 wt.% coke. Coke has obviously a promoting effect besides a deactivating effect for the formation of olefins [124]. However, when coke was formed directly from propene as the feed, no active function of coke can be detected in the sequential MTO and DTO reactions, where the cooking time and the volume of the coke were identical during the DTO reaction and the propene conversion. It has been concluded that the coke formed in MTO and DTO can be classified into two categories: Unreactive coke formed from adsorbed alkenes having a deactivating effect on DTO and MTO, and reactive coke formed from oxygenates having a promoting effect on the DTO and MTO reactions [25,46,48]. The results suggested that coke formed both as a primary product from the MeOH–DME mixture and as a secondary product from olefins [62]. The nature of coke molecules is definitely of importance for understanding the promoting and deactivating effect of coke molecules. Many different techniques have been used to characterize coke molecules. By comparing the real and the apparent density of coke it was concluded that both types of coke are located inside the cages [125], which is in good agreement with observations obtained by synchrotron powder X-ray diffraction (PXRD) study [126]. Temperature programmed oxidation (TPO) revealed two peaks (Fig. 8), indicating two types of coke. [125] The coke corresponding to the low temperature peak is referred to as soft coke with a relatively high hydrogen to carbon ratio, while the one corresponding to the high temperature peak is referred to as hard coke with a relatively low hydrogen to carbon ratio. The ratio of soft to hard coke decreased concurrently with increasing the coke content. The most widely used technique to analyze the coke composition is the post-extraction method [127,128]. Coked catalysts are dissolved in hydrofluoric acid (15% HF), and the organic materials residing in the pores thus liberated are extracted by CCl4 and the extract are analyzed by GC–MS [129]. This technique can provide 13C and 12C distribution in retained carbon after isotopic exchange experiments [130–132]. The most abundant molecules in coke generated during MTO were found to be aromatics with multi-methyl groups, which are typically equilibrated with isotopic C in the isotopic exchange experiments. These findings have led to the development of the hydrocarbon pool mechanism [12,54,130]. It was found that the coke composition changed from monoaromaticts to two rings, three rings and four rings aromatics with increasing coke content [131]. The hydrogen content decreased with increasing number of aromatic rings. As a result, the coke became harder and harder with increasing coke content, as indicated in Fig. 8. SAPO-34 has small pore openings (8-membered rings) and large cages (11 Å long and 6.5 Å wide), so not only aromatics, but also branched olefins and paraffins could also be trapped in the cage

17 wt %

-0.05

-0.06 200

300

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500

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700

Temperature (°C) Fig. 8. TPO spectrum for different coke contents of 3.17, 6, 13.3 and 17 wt.%. The coke was formed during MTO over SAPO-34 (particle size ca.2 lm) at 698 K and a methanol partial pressure of 7.2 kPa [125].

as coke at typical MTO conditions. The post-extraction method has a drawback that the small molecules and reactive surface entities cannot be detected due to their volatile nature. Different techniques should be employed to provide more detailed information on all coke molecules. In recent years different advanced spectroscopic techniques have been used to investigate coke formation in MTO, such as Raman [60,126], IR, C-13 NMR and UV–Vis spectroscopic methods [133]. UV-Raman spectroscopic study [60] showed the coke band at 1632 cm1 appeared when methanol was introduced at 523 K, which can be ascribed to the C@C stretching vibration of olefinic species. When the temperature was increased to 773 K, the new bands at 1392, 2822 and 2987 cm1 were attributed to the symmetric deformation mode of CHx (x = 23), and the symmetric and asymmetric stretching vibrations of CHx, respectively. The results suggest that the nature of the coke molecules involved both olefin and polyolefin species. UV–Vis spectroscopy is also a useful tool to study the aromatic structure in coke. Aromatics with 1–2 benzene rings were detected in CHA (SAPO-34), while 3–4 benzene rings were detected in ERI (UZM-12) and UFI (UZM-5) [77]. Coke formation has clearly been demonstrated as a transition-state shape selective reaction, and the cage space limit the size of coke molecules. FTIR, UV–Vis, ESR, and NMR spectroscopy are suitable methods for in situ investigations of zeolites and reactions catalyzed by these materials. During the past decade, an increasing number of research groups have been dealing with the development and application of new techniques allowing in situ studies under batch and continuous-flow conditions. Hunger [134] has recently reviewed in situ spectroscopic studies in zeolite catalysis. Very recently, a method using simultaneous synchrotron powder X-ray diffraction (PXRD) and Raman spectroscopy with online analysis of products by mass spectrometry has been developed to study MTO on SAPO-34 under real working conditions. These technologies provide information on the nature of surface species and

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A

B

3.E-04

1.4

Mass response (g/ 100 gcat)

1.2 Mass response, m t (g)

1

2.E-04 2 3

1.E-04

4 Quartz

0.E+00 0

50

100

150

200

250

300

350

400

a

1.0 0.8 b

0.6 0.4

c

0.2 0.0 -0.2

0

10

20

Time on Stream (s)

30

40

50

Time (s)

Fig. 9. (A): Transient mass response during four successive 3 min pulses of methanol. For comparison, the mass response from a pulse (3 min) into a bed of quartz is shown. (B) Transient mass response curves for a 6 s pulse of methanol into the catalyst bed (a), into a quartz bed (c), and the corrected curve for the mass response when the pulse was injected into SAPO-34 (b). 5.5 mg of SAPO-34 at T = 698 K, WHSV = 385 g/(g of catalyst  h) and pMeOH = 7.2 kPa [64].

coke molecules, the location of coke and also the kinetics [126]. The combined confocal fluorescence microscopy and UV/Vis microscopy made it possible to study coke formation in large HZSM-5 and H-SAPO-34 crystals during the methanol-to-olefin (MTO) in a space- and time-resolved manner [51]. The build-up of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualizes the formation of coke molecules and their precursors inside the catalyst grains. In the HSAPO-34 crystals, formation of the fluorescent species during the course of the reaction is limited to the near-surface region due to diffusion limitation of the reaction. It was found that the coke front moved towards the middle of the crystal during the MTO reaction. In-situ measurements of adsorption and coke formation during MTO using TEOM [15] provides valuable information on the nature of coke. The mass increased rapidly after the injection of methanol due to adsorption. The maximum mass gained in the 6 s pulse (Fig. 9B) is similar to the adsorbed amount of methanol extrapolated from the isotherm measured at low temperatures [64]. The coke formed immediately after injection without any induction period (Fig. 9A and B). The mass slightly decreased with time on stream after switching off methanol, which might be a result of cracking of coke molecules. 6. Effects of coke on shape selectivity The reaction network of MTO has been identified by a study of product yields as a function of MeOH and DME concentrations [62,72], the resultant reaction network is shown in Fig. 10[62,72]. All the olefin products were identified as the primary products of the MeOH–DME mixture, while the papafins (C2 and C3) were identified as secondary products of the olefins. The methane was suggested as both the primary and secondary product of the large molecules such as coke. The reaction between the surface methoxy group and coke molecules could contribute to the high selectivity of methane at high coke contents [62,72]. The changes in selectivities of different hydrocarbons with the coke content are shown in Fig. 11 A for WHSV = 57 h1. Fig. 11 B shows variations in relative selectivities at the same conditions, where the initial selectivity was obtained by extrapolation to zero coke content from Fig. 11 A. These results clearly show that the selectivity to ethylene increases with increasing coke content. The following decrease in selectivity of the olefins with coke content was found:C6 > C5 > C4,C3. There are generally three types of shape selectivities, namely reactant, product and transition-state shape selectivity, making

Methane Ethene

MeOH

DME

Propene

Coke

C4

Paraffins

C5 C6 Coke Fig. 10. The reaction network of MTO on SAPO-34 [62].

zeolites very attractive for many applications. Both product shape selectivity and transition-state shape selectivity have been reported to governing the product distribution in the MTO reaction. Product shape selectivity plays an obvious role in obtaining high selectivity to olefins in SAPO-34 compared to ZSM-5. The 8-membered ring openings limit aromatics diffusion out of the cages. However, there is still a debate about the role of shape selectivity with respect to the selectivity of C2–C4 olefins on SAPO-34. We have studied MTO and DTO on different SAPO-34 crystal sizes [25,46], and found that the olefin selectivity was identical at a certain coke content, regardless of the crystal size. However, the ethylene selectivity increased and the selectivities to propene and C4–C6 decreased concurrently with increasing coke content. More importantly, the change in product selectivity with the coke content was identical, regardless of crystal size. It has been proven that the propene to ethylene ratio changed linearly with the free space inside the pores on the coked samples [25]. It led to a conclusion that transition-state selectivity governs the product selectivity [25]. The conclusion was supported by Haw and co-workers [123], who performed a systematic study of MTO using combined GC analysis and solid-state MAS 13C NMR, and observed an increase in ethylene-to-propene ratio with increasing flush duration after stopping methanol flow, correlating to a decrease in the number of methyl groups in the polymethylbenzene intermediates. The authors concluded that the formation of olefins is governed by transition-state shape selectivity [135]. However, Dahl et al., reached an opposite conclusion with product shape selectivity as dominating for olefin distribution [34], using ethanol and 2-propanol as probe molecules on different sized crystals (being the same as used in [25]). Diffusion of molecules is normally slow in the case

D. Chen et al. / Microporous and Mesoporous Materials 164 (2012) 239–250

A

60

Selectivities of hydrocarbons (mol%)

246

50

B 1.4 1.2 1

40 0.8 S/S0

30 20

0.4

10 0

0.6

0.2 0

5

10 15 Coke (wt%)

20

0

0

5

10 15 Coke (wt%)

20

Fig. 11. Changes of selectivity to olefins (A) and relative selectivities (B) as a function of the coke content on SAPO-34 (2 lm) at 698 K, WHSV = 57 h1 and a methanol partial pressure of 7.2 kPa. j: Methane, d: ethylene, : propene, N: butene, : C5, +: C6.[62].

of similar size of molecules as channel size, and it has been defined as the configurational diffusion. Dahl et al. found that ethanol conversion was not limited by the ethanol diffusion while 2-propanol conversion was controlled by 2-propanol diffusion. However, direct extrapolation from the results in ethanol and 2-propanol reactions to MTO is not straightforward. Firstly, the diffusivity of 2-propanol can be rather different from propene due to their different structure. Secondly, reaction rate of the dehydration of 2-propanol and the propene formation rate in the MTO reaction could be very different. Barger [136] has also proposed a product shape selectivity by comparing measured ethylene to propene (C2/C3) ratios and the thermodynamic predicted ratio in the gas phase at different temperatures. It was found that the measured ethylene to propene ratio was much higher than the equilibrium ratio, but the experimental C2/C3 ratio increased at a same rate as the equilibrium C2/C3 ratio with increasing temperature. Recently, Hereijgers also reached a conclusion of product shape selectivity by analysis of 13C in retained molecules inside the cages of SAPO-34 [132]. However, equilibrium effects between olefins and formation of aromatics from olefins might add complexity to the discussion. All the results pointed out that the selectivity at the same conversion level is very different on coked SAPO-34, compared to fresh SAPO-34 [24,62,63]. The formation of olefins with different sizes would go through different sized intermediates and the reaction with the larger intermediates deactivates faster. Coke deposition itself is also suppressed by coke molecules in the cages through transition-state shape selectivity, and the change in coke selectivity is rather similar as for the large olefins with increasing coke content.

7. Reaction mechanism and pathways leading to coke formation The MTO reaction mechanism has been discussed in a few reviews [10,14,75]. At least 20 distinct mechanisms have been proposed during the last 25 years, but most of them do not account for the primary products or the kinetic induction period [12]. As noted by Chang [3], a large number of proposals have been made concerning the mechanism of formation of the primary hydrocarbon products. Some of the proposed mechanisms have been based on detailed experimental support, notably the carbene mechanism, the trimethyloxonium mechanism, the free radical mechanism and deprotonation of a surface bonded methyl oxonium ion to give a surface bonded oxonium methylide [4]. Recent experimental and theoretical work has established that methanol and dimethyl ether react on cyclic organic species contained in the cages or channels of the inorganic host. These organic reaction centers act as scaffolds

for the assembly of light olefins so as to avoid the high-energy intermediates required by all ‘‘direct’’ mechanisms. It turns out to be useful to consider each cage (or channel) with its included organic and inorganic species as a supramolecule that can react to form various species [12,95]. There are a lot of theoretic [137– 143] and experimental [14,51,78,95,131,135,144–149] efforts for identifying the hydrocarbon pool or organic reactive centers in MTO catalysts. Hexamethylbenzene has been found as the most active organic center to undergo paring reactions to split olefins with low energy barriers [12]. The essential feature of the reaction, ring contraction followed by expansion, provides the means to extend an alkyl chain and hence eliminate an olefin from methylbenzene. Reaction of the aromatic product by methanol or dimethyl ether under MTO conditions would regenerate the original methylbenzene, completing a catalytic cycle. Haw and coworkers have recently presented a complete catalytic cycle for supramolecular methanol-to-olefins conversion in H-ZSM-5 by linking theory with experiment [149]. However, the authors have pointed out that HSAPO-34 has different topologies and different compositions which will most likely lead to slightly different major catalytic cycles. The hydrocarbon pool mechanism seems to be gradually accepted for olefin formation in MTO, but there are still some experimental observations which are not easily explained by the hydrocarbon pool mechanism only. SAPO-34 has the chabazite structure, which has three cavities per unit cell. [150] From the composition of the unit cells, the number of cavities has been calculated to be 1.37 mmol of cavities/g. The number of the acidic sites of 1.1 mmol/g was measured by ammonia temperature programmed desorption [64], which is close to the number of the cage number. If each active site, namely in each cage, contains a hexamethylbenzene molecule, the coke content will be 18 wt.%. At this coke content, SAPO-34 was found to deactivate almost completely even in the case of no diffusion limitation on the 0.25 lm SAPO-34 crystals. The coke has much lower coke density at low coke contents and reaches a similar density as hexamethylbenzene when SAPO-34 is completely deactivated [125]. The activity increases with coke content from 0 to 5–6%, which might be explained by hydrocarbon pool mechanism by the increased formation of hexamethylbenzene in the cages. However, SAPO-34 has a quite high activity on fresh catalysts in the DTO reaction as shown in Fig. 4, where the coke content and density are low. Moreover, the mechanism for the formation of the hydrocarbon pool in the initial stage is not clear yet. The main experimental evidence of the hydrocarbon pool mechanism is from the analysis of the catalyst after conversion of the methanol in batch mode, and in situ NMR analysis. Considering that the aromatic ion is most likely formed from olefins following Hutchings and Hunters’s oxonium methyl ylide mechanism [151], Froment reconciled the hydrocarbon pool mechanism

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The selectivity to ethylene is much lower than the one on the coked SAPO-34, indicating a possibly different reaction mechanism. It could be similar to the suggested olefin cycles and cracking of olefins results in low selectivity of ethylene. The duel cycle reaction mechanism is also consistent with the results of the kinetic modelling of the DTO reaction. The activity curve with the coke content in Fig. 3A can only be described by a dual reaction model in Eq. (4) [46]:

rDME ¼ ð171:17ð1  0:0684CÞ þ 55:96 C expð0:0153C 2 Þg  C DME

Fig. 12. Suggested dual cycle concept for the conversion of methanol over H-ZSM5.[91].

[17]. The hexamethylbenzene could be the product of the conversion of primary carbenium ions, but the hexamethylbenzene and primary carbenium ions could equilibrate each other, something which could explain the isotopic distribution in products and retained coke molecules. This is consistent with the observation of the olefinic nature of coke [60] and the co-existence of aromatics and carbenium ions [14]. More results have recently revealed that alkylation and cracking reactions of C3+ olefins contribute significantly to the production of C3 olefins in H-ZSM-5, resulting in a rather low ethylene selectivity, as shown in Fig. 12 [91,152,153]. The similar dual cycle mechanism might be valid also for the MTO reaction in SAPO-34, at least on SAPO-34 with low coke contents. Very small size of methanol pulses (6 s) was used to study the coke formation, activity and selectivity [62]. Fig. 13 clearly illustrates a much lower activity and ethylene selectivity on the fresh catalysts (in the first pulse). The ‘‘coke’’ contents measured during the first pulse are rather close to the adsorption amounts extrapolated from the methanol adsorption isotherm. The site coverage is estimated to be about 0.23 at 698 K and a methanol pressure of 7.2 kPa, which is much higher than the methylbenzene measured in the post-extracted coke molecules (< 10% of total cage numbers) even at high coke contents [95]. It means that the oxygenates conversion in the first cycle cannot be explained by hydrocarbon pool.

where C is the total coke content (wt.%) and g can be calculated by Eq. (2). The first term might relate to the olefin cycle reaction and the secondary term might be related to the hydrocarbon pool cycles. In summary, the MTO in SAPO-34 reaction could be explained by the dual cycle reaction mechanism, and the contribution of each cycle to the products depends on the coke accumulated, in particular the aromatics, inside the cages. On the fresh catalysts, the olefin cycle is dominating. With time on stream, the olefins gradually convert to aromatics via cyclization and dehydrogenation, which provide centers to convert methanol and DME to olefins via hydrocarbon pool. This explains the increased activity with coke content at the low coke contents. As mentioned above, coke is a primary product from MeOHDME pool plus a stable secondary product from olefins at high conversions of methanol. A parallel reaction pathway leading to the formation of olefins and coke (Fig. 10) might indicate similar intermediates for their formation [24]. This is consistent with the dual cycle mechanism (Fig. 12), with coke of olefinic nature at low coke contents, and of aromatic nature at high coke contents. 8. Rational design of SAPO-34 catalysts The coking rate and the coke location, thus the deactivating effect, were found to be significantly influenced by intracrystalline diffusion as well as temperatures [47]. The coking rate and conversion to olefins were much higher at 723 and 748 K than at 698 K, but the rates were very similar at 723 and 748 K on SAPO-34 of 0.25 and 0,5 lm, indicating strong diffusion limitations even on small crystals (0.25 lm). Diffusion limitations could have a positive effect on the olefin formation, but a strong diffusion limitation should be avoided in order to obtain high catalyst capacity and low coke selectivity as shown in Table 1 [47]. Table 1 shows that the coke selectivity is higher on the larger crystals, in particular at high temperatures. The coke selectivity was much higher on large crystals (2.5 lm) than on small crystals

50

25

Selectivity of C 2 (mol%)

Conversion to olefins (wt%)

30

20 15 10 5 0

ð4Þ

0

5

10 15 Coke (wt%)

20

40 30 20 10 0 0

5

10 15 Coke (wt%)

20

Fig. 13. Conversion to olefins and ethylene selectivity with coke content at 698 K but different partial pressures.}: 15 Kpa, 384 g/(g of cat) h, s: 30 kPa and 768 g/(g of cat) h, 4: 60 kPa and 1538 g/(g of cat) h, h: 83 kPa and 2558 g/(g of cat) h. The experimental data were collected by a series of 6 s pulse in a TEOM reactor [62].

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Table 1 Catalyst capacity and coke selectivity (see definitions below) on 0.25 and 2.5 lm crystals at different temperatures [47]. T (K)

698 723 748

0.25 lm

2.5 lm

C (wt.%)

CAHF

SC

C (wt.%)

CAHF

SC

14.6 12 12

16.1 14.5 10.6

0.91 0.83 1.13

13 13.6 14.6

14.6 14.2 8.7

0.9 0.94 1.67

5% oxygenate conversion to olefins was defined as the final state. Coke selectivity (SC) is an average value: C/CAHF (g coke/100 g hydrocarbon formed). Catalyst capacity is defined as the cumulative amount of hydrocarbon formed (CAHF) per gram catalyst until the final state.

60

References

Selectivity (mol %)

50 40 30 20 10 0 650

The catalyst coke content is directly related to the accumulative amount of hydrocarbons formed over the catalyst. Product selectivities are identical on SAPO-34 with different crystal sizes at a certain coke content. Coke deposition in the cages has profound effects on product selectivity by increasing the ethylene selectivity and decreasing the selectivity to larger molecules including coke, due to pronounced shape selectivity effects on this catalyst. The operating temperature is a key process parameter to adjust the product selectivity towards ethylene or propene. The crystal size is the most important parameter for the rational catalyst design. Relatively small size of SAPO-34 is required to avoid diffusion limitation to achieve a low coke selectivity and low deactivation. The optimized crystal size of SAPO-34 depends on the operating temperature.

700 750 800 Temperature (K)

850

Fig. 14. Selectivity with temperature during the MTO reaction on SAPO-34 (2 lm) at 7.2 kPa methanol partial pressure, 385 g/(g of cat) h and a coke content of 4.6 wt.%. : ethylene, j: propene, d: C4, ⁄: C5, N: methane [62].

(0.25 lm) at low coking contents even at 698 K. The coke selectivity is lower at 723 K than at 698 and 748 K on small crystal sizes. An optimum crystal size is therefore determined as a balance between the positive and the negative effects of coke deposition. The optimum crystal size is expected to vary with the temperature due to the changes in the effects of coke deposition with temperatures. A good selection of a crystal size is suggested to maintain the effectiveness factor of methanol conversion between 0.95 and 1, hence keep the methanol conversion free of diffusion limitations. As a result, crystal sizes in the range of 0.4–0.5 lm seems to be optimal at 698 K [25]. However, the crystal size should be lower than 0.25 lm at high temperatures (>698 K). The operating temperature is a key parameter to control the selectivity. Fig. 14 clearly demonstrates the dependence of the selectivity on the temperature. The selectivity of ethane increases and the selectivity of propane increased concurrently with increasing the temperature. The temperature could be adjusted to produce more ethylene or propene. 9. Conclusions SAPO-34 is still the most promising industrial catalyst for the MTO reaction, mainly due to its high selectivity to ethylene and propene. The main challenge is the rapid deactivation due to coke formation, and much research effort has been directed towards improving the stability of the catalyst. Fundamental studies on the reaction mechanism for olefin and coke formation, as well the effect of crystal size, cage topology, acidic strength and density on coke formation and deactivation have gained a much better understanding of the chemistry involved in this reaction, and form a good basis for further catalyst development for the process.

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