Dominant reaction pathway for methanol conversion to propene over high silicon H-ZSM-5

Chemical Engineering Science 66 (2011) 4722–4732

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Dominant reaction pathway for methanol conversion to propene over high silicon H-ZSM-5 Wenzhang Wu a, Wenyao Guo b, Wende Xiao a,b,n, Man Luo b a b

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2011 Received in revised form 14 June 2011 Accepted 17 June 2011 Available online 24 June 2011

The reaction pathway for propene formation in methanol to propene (MTP) process over a high silica H-ZSM-5 catalyst has been investigated in a fixed bed reactor by comparing the experimental results from three kinds of feeding: alkene only, methanol only and mixed alkene and methanol. The results show that alkene methylation with methanol is dominant for the case of methanol and individual C3–C6 alkenes co-feeding, C2¼ is almost un-reactive. C7¼ cracks to propene and butene immediately whether co-fed with methanol or not, and C6¼ cracks to propene readily when reacted alone. Oligomerization occurs but is suppressed by the co-fed methanol for light alkenes of C2–C5. Methylation-cracking has been verified as the main reaction mechanism of a typical MTP process in which recycling of C2¼ and C4¼ –C6¼ to the reactor inlet is required. Based on the relative reactivities of alkenes towards methylation and inter-conversion, a reaction scheme has been presented including a cycle composed of a consecutive methylation from C4¼ through C5¼ to C6¼ and further to C7¼ , the b-scission of hexene and heptene for propene, and the a-scission of hexene for ethene as well. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Zeolites Chemical reactors Catalysis Model reduction Methanol to propene Reaction pathway

1. Introduction Methanol to hydrocarbons (MTH) processes based on the acidic zeolite catalysts provide promising alternative routes for producing hydrocarbons independent of petroleum, since methanol can be easily produced from coal and natural gas which are much richer in resource than oil. Depending on different product selectivities due to different process conditions, catalyst structure and acidity choice, the abbreviations MTG (methanol to gasoline) (Keil, 1999), MTO (methanol to olefins with similar yields of ¨ ethene and propene in the product mixture) (Stoker, 1999) and MTP (methanol to propene with much higher yield of propene over ethene) (Bach et al., 2004; Rothaemel et al., 2006; Birke et al., 2006) are well accepted. Nowadays, MTP process is more desired than MTO due to the increasing and stronger market demand for propene than for ethene (Plotkin, 2005). Compared with the MTO process based on the silicoaluminophosphate catalyst SAPO-34, MTP is based on the aluminosilicate zeolite H-ZSM-5 with a Si/Al ratio high up to 200 (Liu et al., 2009) and can achieve a very high propene yield and strong resistance to coke deactivation.

n Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. Tel./fax: þ86 21 3420 3788. E-mail address: [email protected] (W. Xiao).

0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.06.036

The most fundamental problem for progressing industrial application of a new chemical process is to understand its reaction mechanism. For a MTH reaction system, it is generally accepted that light olefins are primarily formed from methanol, and the other hydrocarbons including the higher olefins, paraffins and aromatics are then formed from the resulted light olefins. Many mechanistic hypotheses have been proposed to illustrate the light olefin formation from methanol (Derouane et al., 1978; Dessau, ¨ 1986; Haw et al., 2003; Stoker, 1999; Svelle et al., 2006), but the reaction mechanism remains greatly controversial. Early researchers (Derouane et al., 1978; Mole and Whiteside, 1982; Chang and Chu, 1983) focused mainly on the formation of the first carbon– carbon bonds from methanol and whether or not ethene was the initial olefin. But more and more studies indicated that light olefins are mainly formed indirectly and the formation of the primary olefin from direct coupling of methanol is insignificant (Dessau, 1986; Wang et al., 2003, Haw et al., 2003, 2006). Thus indirect pathway for the light olefins formation from methanol drew more attention. Dessau (1986) proposed an indirect reaction pathway, hereafter termed methylation-cracking pathway, in which olefins were consecutively methylated by methanol from ethene to form higher olefins and the later underwent cracking to form light olefins in return. This pathway suggested propene and butene as the primary kinetic olefinic products and implied that the reaction sequence was characterized by the autocatalysis. In the last decade, another indirect route, known as ‘‘hydrocarbon pool’’ mechanism

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(Dahl and Kolboe, 1993, 1994, 1996; Haw et al., 2000, 2003; Song et al., 2001; Wang et al., 2003, 2006) is most favorable. This mechanism involves methanol addition to the hydrocarbon pool, which subsequently rearranges and splits off alkenes in a parallel way (Chen et al., 2007). The hydrocarbon pool has been described as a catalytic scaffold composed of larger organic molecules such as multiply methylated benzenes or their protonated counterparts. The hydrocarbon pool mechanism bring out some questions: where does the pool come from? How can it govern the selectivities of the light olefins? Several previous researchers attributed the original pool to the impurities in the methanol feed and those adsorbed in the catalyst micropores (Song et al., 2002; Haw et al., 2006; Jiang et al., 2006). Recently, Dewaele et al. (1999) and Svelle’s group (Bjørgen et al., 2007, 2009; Svelle et al., 2006, 2007) demonstrated that ethene formation is mechanistically separated from the formation of C3–C6 alkenes during MTH over H-ZSM-5. According to Svelle et al. (2006), ethene is formed from lower methylbenzenes while propene and higher alkenes are formed mainly from alkene methylation and inter-conversion reactions. Besides, alkenes can also undergo oligomerization reactions (Espinoza et al., 1983; Espinoza, 1986), especially at high methanol conversions. Some studies (Espinoza, 1984; Park and Froment, 2004) suggested that oligomerization of alkenes were more important than methylation. It can be seen that the above mentioned three pathways, i.e., methylation-cracking, hydrocarbon pool and oligomerizationcracking, have been verified experimentally for the MTH reaction system over the H-ZSM-5 catalyst in some specific experimental conditions, which means they may play the role synchronously. Park and Froment (2001a, 2001b) presented a detailed MTO mechanism with 726 elementary steps including protonation, deprotonation, hydride and methyl-shift, methylation, oligomerization and cracking, and pointed out that the primary olefins are ethene and propene which are formed from an intermediate of oxonium methylide, a product of methoxy ion interacting with a basic site of the catalyst, and the methylation proceeds based on the carbonium ion chemistry. By elaborate parameter reduction, they obtained a kinetic model with an independent parameter number high up to 33 for the MTO reaction system over the H-ZSM-5 catalyst. Nevertheless, under the specific reaction conditions, it can be conceived that one pathway can be dominant while the others can be seriously suppressed. Knowledge of the dominant reaction pathway at a certain range of reaction conditions is of major importance in kinetic modeling of such a complicated reaction system. If all the occuring reaction details are included without reasonable and viable simplification, the derived kinetic model will become too intricate to use in the reactor design and scale-up. In a MTP process with multiple-stage adiabatic fixed bed reactors which was originally developed by Lurgi A.G. (Bach et al., 2004; Birke et al., 2006; Koempel and Liebner, 2007), as schematically shown in Fig. 1, all alkenes other than propene are recycled back to the reactor inlet for a dominant propene production with a molecular propene yield about 65% over less than 1% of ethene (the balance is composed of LPG and gasoline). Thus a situation of co-feeding methanol and the alkenes appears and complicates the understanding of the reaction mechanism involved. Accordingly, the reaction pathway for propene formation from the co-feeding methanol and alkene mixture becomes one of the most imperative research topics. Obviously, one way to investigate the reaction pathway in the MTP reaction system is to conduct co-reactions of methanol with individual C2–C6 alkenes. Product distributions from co-reaction experiments can mirror the relevant reactions involved. Over H-ZSM-5 catalyst, there has been published several experimental results on co-reaction of methanol with ethene (Wu and Kaeding,

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Fig. 1. Schematic diagram of a MTP process.

1984; Tau et al., 1990; Svelle et al., 2004), propene (Tau and Davis, 1993; Svelle et al., 2005), butene (Svelle et al., 2005), and hexene (Behrsing et al., 1986). Svelle et al. (2004, 2005) presented the kinetics of the co-reactions of individual C2–C4 alkenes with methanol over H-ZSM-5 under the conditions with a low Si/Al ratio of 45 and a low temperature of 350 1C, and found that alkene methylation with methanol to higher alkenes was dominant over the interconversion of the alkenes and the apparent reaction rates of ethene, propene and butene take the ratio of 1:17:50, and behave as zero-order with respect to methanol and one-order to alkene. In this study, we performed co-reaction of methanol and individual alkene over a high silica H-ZSM-5 under the conditions closed to the practical ones: Si/Al¼ 200, temperature of 460 1C and alkenes of C2–C7 in order to elucidate thoroughly the dominant reaction mechanism of the practical MTP process. Three kinds of experiments with different feeding modes (alkene only, methanol only and mixed alkene and methanol) have been carried out, and the initial reaction rates were used to discriminate relative importance of the published mechanism propositions.

2. Experimental 2.1. Catalyst and reagents H-ZSM-5 (Si/Al¼ 200) catalyst was a gift from Shanghai Fuxu Molecular Sieve Co., Ltd. The sample had a crystal size of submicron (300–800 nm) and was pressed into tablets, crushed and sieved into a fraction of 200–400 mm. To accurately control the reaction conditions, n-alcohol of C2–C7 were used as the precursor of the corresponding alkene, since the alcohols dehydrate virtually instantaneously to the corresponding alkene and water over H-ZSM-5 catalyst under the reaction temperature. Considering the influence of water introduced by n-alcohol, the equal molar water was co-fed with methanol in the methanol feed only case. All n-alcohols (AR, 499%) were supplied by Sinopharm Chemical Reagent Co., Ltd. 2.2. Experimental setup and procedure The experimental set-up is shown in Fig. 2. The experiments were performed in a U-shaped tube reactor with an internal diameter of 5 mm. The reactor is made of titanium and immersed

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Fig. 2. Schematic diagram of the experimental set-up.

in a molten salt bath to meet the isothermal requirement. The catalyst bed was diluted with inert oxide beads (2 vol. of beads inert/1 vol. of catalyst). In each run, the amount of catalyst used was about 100 mg. It took about an hour for the catalyst to achieve steady state after a change in operation conditions. Catalyst deactivation was not noticed during the operation. The reactor effluent was analyzed on-line by a GC equipped with a HP-PLOTQ capillary column (30 m  0.32 mm i.d.) and a FID detector. Under the experimental circumstances, the main products were C2–C7 olefins whereas the formation of paraffins and aromatics was ignored. For simplification, methanol and DME was taken as a single species and the DME formed in product was accounted as un-reacted methanol. Product concentration in the reactor effluent was expressed in terms of partial pressure. The partial pressure of component i in the reactor effluent is calculated by the following equation: ðGHC xi =Mi Þ Pi ¼ P0 Ft

ð1Þ

where P0 is defined as total pressure at the reactor outlet, GHC is the total mass flow rate of all hydrocarbon species, xi is the mass fraction of species i in the hydrocarbon mixture and is measured by GC with FID detector. Mi stands for the molecular weight of component i, and Ft for the total molar flow rate of all components including water and nitrogen at the reactor outlet. In this study, due to the large amounts of nitrogen introduced in the system, the change of total molar flow rate between reactor inlet and outlet caused by reaction was neglected.

3. Results The main objective of the present work was to study the main reaction pathway through which propene was formed in MTP process where methanol was co-reacted with C2–C6 alkenes except propene. Thus co-reaction experiments of methanol with individual C2–C6 alkenes were carried out. When methanol was co-reacted with alkene, the reactions involved can be sorted into three types: inter-conversion reactions (including oligomerization and cracking) of the alkene (Espinoza et al., 1983; Espinoza, 1986), methanol conversion reaction alone to hydrocarbons ¨ (Stoker, 1999) and methylation reaction of alkene with methanol to higher alkenes (Behrsing et al., 1986; Dessau, 1986). We first

Table 1 Conversion of the alcohols to the corresponding alkenes at space time of 0.20 gcat h/mol. Feed

Ethanol 1-Propanol 1-Butanol

Conversion 95 to alkenes (%)

98

100

1-Pentanol 1-Hexanol 1-Heptanol 100

100

100

probed the reaction of individual C2–C6 alkenes alone to evaluate the significance of alkene inter-conversion reactions in the MTP system. 3.1. Reaction of alkene alone Transformation of individual C2–C6 alkenes on H-ZSM-5 was performed at 460 1C in the fixed bed reactor. Since C7 alkenes might be important intermediates responsible for propene formation, the conversion of C7 alkene (1-heptene) was also conducted. For each alkene, the corresponding n-alcohol with the same carbon number was used as a precursor. In each run, the partial pressure of n-alcohol at the reactor inlet was kept at 10 kPa by diluting the feed with nitrogen. The amount of catalyst used was 100 mg and the total feed rate was varied to make the space time (W/Falcohol) range from 0.7 to 8.0 gcat h/mol. A test of the dehydration reaction of alcohols at the space time as short as 0.20 gcat h/mol over the catalyst in the current set of reaction conditions was also carried out. The conversion of each alcohol to the corresponding alkenes or to their cracking products over the catalyst was over 95%, as is shown in Table 1. Among these alcohols, ethanol and 1-propanol were dehydrated to ethene and propene. For each C4–C7 alcohol, alkenes comprising several skeletal isomers were formed instantaneously. But for 1-hexanol and 1-heptanol, 7 wt% of propene and 35 wt% of propene and butene among the alkene products were formed, respectively. Clearly, the dehydration reaction rates of the alcohols over the catalyst are very fast, and the alcohols are converted to the corresponding alkenes instantaneously. 3.1.1. Conversion of the alkene As for the transformation of alkene alone over the H-ZSM-5 catalyst, the oligomerization of light alkene to higher one and

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cracking of the higher alkene to the light ones can be inferred to be the main mechanism. For the light alkenes, such as ethene and propene, the oligomerization plays an important part; for butene and pentene, the oligomerization and then cracking occur; in the hexene and heptene conversion, the cracking is certainly the main reaction. The conversion of individual C2–C7 alkenes as a function of space time is depicted in Fig. 3. Here, alkene conversion is defined as the fraction of the alkene that is converted to other products. The conversion of individual C2–C5 alkenes increases gradually with an almost linear dependence on space time during the scope and range from 0.7 to 8.0 gcat h/mol, which means the reaction rate is nearly constant and takes the zero order with respect to the alkene. The reaction rates of C2–C5 alkenes when reacted

Fig. 3. Conversion of alkene versus space time for individual C2–C7 alkene reaction alone. Feed: 10 kPa of individual C2–C7 alcohols, temperature: 460 1C.

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alone can be evaluated by the slope of the corresponding conversion curves at limiting space time of zero, which are 0.003, 0.056, 0.083 and 0.09 mol/(gcat h), respectively. C6 and C7 alkenes exhibit much higher reactivity than C2–C5 alkenes, which shows that the cracking takes place much more easily than the oligomerization. By extrapolation of the conversion curve of hexene and heptene, one can find that the initial rate of heptene cracking is about 0.98 mol/(gcat h), and that of hexene about 0.33 mol/(gcat h), almost 340 and 110 times more than the rate of ethene oligomerization, respectively. In a word, the inter-conversion reactivities of C2–C7 alkenes agree with the following ranking: heptene4hexenebpentene4butene4 propeneb ethene, corresponding to a ratio of 340:110:30:28:19:1. 3.1.2. Product distributions Product partial pressures as a function of space time for conversion of individual C2–C7 alkenes are shown in Fig. 4. The main compounds in the reactor effluent in each case except the case of heptene conversion were C2–C6 alkenes, without detectable amounts of C7 alkene formed in the conversion of individual C2–C6 alkenes. Product partial pressures for ethene conversion are shown in Fig. 4(A). It can be seen that the oligomerization of ethene is very slow and can be considered as virtually un-reactive. The main products of ethene conversion detected are butene and hexene, which are certainly come from the dimerization and tri-merization of ethene, respectively, and the amounts of propene and pentene formed from ethene transformation can be ignored. Propene exhibits a reaction rate of about 20 times more than ethene when reacted alone over H-ZSM-5 catalyst, as shown in Fig. 3. The product partial pressures for propene conversion are shown in Fig. 4(B). It can be found that the main product when the space time is less than 2 gcat h/mol is hexene, which is

Fig. 4. Product distributions versus space time for individual C2–C7 alkene reaction alone. Feed: 10 kPa of individual C2–C7 alcohols, A ¼ethanol, B ¼ 1-propanol, C ¼ 1-butanol, D¼ 1-pentanol, E ¼ 1-hexanol, F ¼ 1-heptanol; temperature: 460 1C.

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expected to be formed from dimerization of propene. The formation of butene and pentene at short space time can be neglected but the partial pressure of butene increases gradually with space time longer than 2 gcat h/mol. Clearly, the appearance of butene is due to the secondary reactions of propene and hexene, via oligomerization and cracking. Comparing with ethene and propene, butene displays evident reactivity when reacted alone. Product partial pressures for butene conversion versus space time are depicted in Fig. 4(C). Propene and pentene constitute the main part of products which are the expected cracking products of C8 species formed from butene dimerization as their partial pressures are almost the same at relative short space time. With the increase of the space time, the partial pressure of pentene is lower than that of propene due to the secondary reaction of pentene, and ethene and hexene can be detected in trace amounts. Thus the primary reactions for butene are written as follows: 2C4¼ "C8¼

ð2Þ

C8¼ "C3¼ þ C5¼

ð3Þ

When 1-hexene is reacted over H-ZSM-5 solely, the results are displayed in Fig. 4(E). It can be seen that the b-cracking reaction to produce propene and the a-cracking reaction to produce both butene and ethene take place simultaneously, and the former prevails much over the latter. A secondary reaction between propene and hexene to form C9 species, which cracks to butene and pentene may occur readily. This makes the partial pressure of butene higher than that of ethene. With the increase of the space time, b-cracking reaction approaches the equilibrium state as the propene partial pressure tends to be constant with a conversion of about 50%. Therefore, the primary reactions of hexene are given by "2C3¼

ð4Þ

C6¼ "C2¼ þ C4¼

ð5Þ

C6¼ þ C3¼ "C4¼ þC5¼

ð6Þ

C6¼

For heptene conversion, at relatively short space time lower than 1.5 gcat h/mol, propene and butene have the same partial pressure, as shown in Fig. 4(F), which reveals that the primary and the dominant reaction is the b-cracking of heptene as given by Eq. (7). When the contact time increases from 1.5 gcat h/mol, the butene pressure decreases moderately, and the pressures of pentene, ethene and hexene increase slowly, which are resulted mainly from the dimerization of butene and the followed cracking. Furthermore, the heptene cracking has an equilibrium conversion of about 95% at the testing conditions. C7¼ "C3¼ þ C4¼

ð11Þ

The dimerization of pentene (Eq. (9)) is followed by b-scission (Eq. (11)) and g-scission (Eq. (10)) of the dimer simultaneously, which forms hexene plus butene and heptene plus propene, respectively. The resulted heptene is converted solely according to b-scission mechanism producing both butene and propene as shown in Eq. (7), and hexene can be converted based on both band a-scission producing two moles of propene and one mole butene plus one mole ethene, respectively, see Eqs. (4) and (5). A notable phenomenon exists that the partial pressures of propene and butene are almost the same during the entire investigated space time range, and are about three times more than ethene. This fact means that the reaction rate of Eq. (11) is twice that of Eq. (10), the oligomerization reaction, Eq. (9), prevails much over the direct cracking reaction, Eq. (8), and ethene is certainly due to reaction (10) as a result of hexene cracking. Consequently, from the results of alkene reaction alone reported above, one can find that, the oligomerization of ethene can be ignored, oligomerization and then cracking occur obviously for propene, butene and pentene with propene and butene being the main products, cracking takes place significantly for hexene and heptene with the main products as same as those of butene and pentene transformation. Propene and butene can be obtained with almost the same amount in the cases of pentene and heptene conversions, but an amount of propene about five times more than butene in hexene conversion. In addition, a small amount of ethene is produced by the a-cracking of hexene. 3.2. Co-reaction of alkene and methanol Co-reaction of methanol and individual C2–C7 alkenes over H-ZSM-5 was performed in the fixed bed reactor at 460 1C. Analogous to the situations of alkenes reaction alone, the corresponding n-alcohol with the same carbon number was used as a precursor for each alkene. Both the initial methanol and the alcohol partial pressures in each case were 10 kPa. The space time (W/Falcohol) was varied from 0.7 to 8.0 gcat h/mol by adjusting the total feed rate. As for the base point for comparison, reactions with a feed mixture consisting of 10 kPa methanol and 10 kPa water were also conducted in the same space time range at 460 1C. ¨ It is well accepted that (Schoenfelder et al., 1994; Stoker, 1999), over H-ZSM-5 catalyst, a rapid reversible reaction of methanol dehydration to dimethyl ether (DME) and water proceeds in parallel with the methanol conversion to the hydrocarbons, so DME is reckoned in the unconverted methanol as the reactant for the experimental data correlation.

ð7Þ

As far as pentene conversion is concerned, propene and butene are formed in moderate amounts, and constitute the dominant parts of products as shown in Fig. 4(D). Propene and butene are the expected products from oligomerization of pentene followed by cracking. Hexene was detected in minor amounts in the whole space time range. Ethene was present in significant amounts in the effluent, contrary to that detected in the propene and butene conversion cases. When pentene is reacted over H-ZSM-5 catalyst, direct b-scission of pentene may occur (Bortnovsky et al., 2005). Thus the following reactions are probably involved in the pentene transformation C5¼ "C2¼ þ C3¼

ð8Þ

¼ 2C5¼ "C10

ð9Þ

¼ "C6¼ þ C4¼ "C2¼ þ 2C4¼ C10

¼ C10 "ðC3¼ þC7¼ Þ or ðC6¼ þ C4¼ Þ"2C3¼ þ C4¼

ð10Þ

3.2.1. Conversion of methanol The conversions of methanol as a function of space time when methanol was fed alone and with the individual alkenes are depicted in Fig. 5. It can be seen that, methanol displays very low reactivity when it is fed alone at the relatively short space time, such as less than 1.0 gcat h/mol. The conversion of methanol was only 0.49% at a space time of 0.91 gcat h/mol, and an induction time of about 1 gcat h/mol appeared obviously implying the autocatalysis effect which had been reported in many previous ¨ studies (Stoker, 1999). Co-feeding of C2–C7 alkenes accelerates the methanol conversion to hydrocarbons evidently. By calculation of the slopes for the conversion curves at the limiting space time, one can obtain the initial methanol consumption rates of 0.062, 0.063, 0.27, 0.52, 0.098 and 0.20 mol/(gcat h), which are 12.4, 12.6, 54, 104, 19.6 and 40 times more than the rate of methanol feeding alone (i.e., 0.005 mol/ (gcat h)), respectively. The difference of conversion rate between the

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Fig. 5. Conversion of MeOH versus space time with methanol and alkene co-fed. Feed: 10 kPa methanol with or without 10 kPa of respective C2–C6 alcohols; temperature: 460 1C.

Fig. 6. Conversion of alkene versus space time with methanol and alkene co-fed. Feed: 10 kPa of respective C2–C6 alcohols and 10 kPa of methanol; temperature: 460 1C.

co-feeding alkene and methanol case and feeding sole methanol case may be used to represent the overall methylation rate under the conditions of co-feeding, where the overall methylation rate includes that of the co-fed alkene itself (the first methylation) and those of secondary alkenes (the second, the third, etc.). Accordingly, the overall methylation rates of C2–C7 alkenes are 0.057, 0.058, 0.265, 0.515, 0.093 and 0.195 mol/(gcat h), respectively, which implies that methylations of butene and pentene are much more rapid than those of ethene and propene, and when a mixture of C2–C5 alkenes is co-fed with methanol, the dominant methylation reactions are certainly those of butene and pentene. Moreover, it can be found that methanol conversion rate consecutively increases with the increase of the number of methyl groups for light alkenes up to the pentene, but the rates of hexene and heptene approach those of propene and butene, respectively. In the cases of hexene and heptene co-feeding, the cracking reactions of the corresponding alkenes may take place primarily to produce propene for hexene and both butene and propene for heptene, because their reactivities are high up to 0.33 and 0.98 mol/(gcat h), respectively, as described in Section 3.1.1. Therefore, the methylations of hexene and heptene approach those of propene and butene, respectively. The further insight into the comparison of the related methylation rates shows that the methylation of the alkene can consecutively proceed to hexene but terminate at heptene since the conversion curve of hexene is above that of propene, but the curve of heptene underneath that of butene. Furthermore, the approximate ‘‘S’’ shapes are observed for methanol conversion curves of all the feeding cases except the pentene. This phenomenon is similar to the so-called autocatalysis effect. When methanol is fed alone or with individual C2–C4 alkenes, higher alkenes are produced as space time increases, which are more reactive towards methylation than the reactants. But no autocatalysis behavior was observed for co-reaction of methanol and pentene, as the reactivity of pentene towards methylation is the highest, revealing that pentene may contribute most to the methanol conversion to hydrocarbons when a mixture of C2–C7 alkenes is co-reacted with methanol.

From Fig. 6, the co-reaction rates of the alkenes at the limiting space time can be extracted as 0.008, 0.048, 0.20, 0.38, 0.15 and 0.84 mol/(gcat h) from ethene to heptene, respectively. As the initial consumption rates of methanol are 0.057, 0.058, 0.265, 0.515, 0.093 and 0.195 mol/(gcat h) for C2–C7 alkene co-reaction, respectively, as shown in Section 3.2.1, the rate ratios of methanol to individual alkenes which represent the molecular ratios in the co-reactions are about 7.1, 1.2, 1.3, 1.35, 0.26 and 0.23, respectively. The large molecular ratio in ethene co-reaction implies the inter-conversion of methanol itself to the alkenes is co-catalyzed by the co-fed ethene and the ethene methylation reaction is insignificant. But for propene, butene and pentene co-reactions, the primary methylation of the raw reactant followed by the secondary methylation of the product must dominate as the corresponding molecular ratios are only about 1.2, 1.3 and 1.35, respectively, and the inter-conversion of methanol itself can be neglected. In contrast to the light alkenes, the molecular ratios in the co-reactions of hexene and heptene are less than unity, only about one-fourth mole of methanol to one mole of the corresponding alkenes, which reveals that the cracking reactions are much faster than the methylation reactions. For heptene, there exists no methylation as will be discussed later, and the reactions involved in hepteneþmethanol co-reaction system contain only the cracking of heptene to give propene and butene. The consumption of methanol therefore arises mainly through methylation of these products due to the cracking reaction. Furthermore, when heptene is reacted alone, its initial rate of cracking is 0.98 mol/(gcat h), as given in Section 3.1.1, but 0.84 mol/(gcat h) when co-reacted with methanol. The reduction of 0.14 mol/ (gcat h) is probably due to the inhibition of adsorbed methanol to the heptene adsorption on the catalyst surface. This inhibition effect is alleviated as the concentration of methanol decreases with the increase of the space time. When the space time is longer than 3 gcat h/mol, the corresponding methanol conversion is more than 90%, and further conversion of heptene co-reaction up to the level of reaction alone was observed. In hexene co-reaction case, both cracking and methylation reactions take place. The total initial consumption rate of hexene is 0.15 mol/(gcat h), nearly two times less than the initial cracking rate of 0.33 mol/(gcat h) when hexene is reacted alone (see Section 3.1.1). The probable cause of reduction in hexene reactivity when co-reacted with methanol is the competitive adsorption between hexene and methanol, and the inhibition is more severe than for heptene reaction. Nevertheless, another reason that may be considerable is the methylation rate of hexene by methanol is relatively low in nature.

3.2.2. Conversion of alkene The alkene conversions as a function of space time when the individual alkene and methanol are co-reacted are shown in Fig. 6. For the alkene, the inter-conversion of itself, such as oligomerization and/or cracking, takes place besides the methylation with methanol.

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As for a reliable estimation of the methylation reactivties of the individual alkenes, the Cn þ 1 alkene yields for the Cn alkene and methanol co-reaction are calculated in addition to their initial methanol conversion rates, which are shown in Table 2. At the shortest space time, when co-reacted with methanol, propene yield is 1.92% from ethene, butene 4.24% from propene, pentene 15.69% from butene, hexene 19.48% from pentene, and heptene 4.48% from hexene. Because the formation of Cn þ 1 alkene from the sole Cn alkene reaction under the same reaction conditions can be ignored, the reactivties towards methyaltion for C2–C6 alkenes are 0.018, 0.04, 0.155, 0.25 and 0.041 mol/(gcat h) by dividing the yield with W/Falcohol, respectively. This estimation procedure has been proposed by Sevelle et al. (2005), and is reliable for the lower alkenes, as can be seen in Table 2, because the Cn þ 1 alkene is the main product from the methylation of Cn alkene. But for ethene, the yield of propene in the co-reaction of ethene and methanol exceeds the conversion of ethene by a factor of 2.3, which indicates that propene was produced mainly from methanol rather than ethene and the real reactivity of ethene towards methylation is over-estimated. Thus the reactivity of ethene towards methylation is adjusted to 0.0077 mol/(gcat h) by dividing ethene conversion with W/Fethanol. For butene, pentene and hexene, these figures may under-estimate the real reactivities, as the secondary reactions of Cn þ 1 are remarkable. The reactivity of butene towards methylation is updated to 0.19 mol/(gcat h) by adding the yield of pentene; that of pentene to 0.26 mol/(gcat h), which is obtained by taking into account the yield of C7¼ ; and that of hexene to 0.12 mol/(gcat h) by taking into account of the yield of C4¼ because of the rapid cracking rate of C7¼ . A comparison of the estimated reactivities of the individual alkenes towards methylation with methanol is shown in Fig. 7, which further reveals that a maximum reactivity exists at C5 for C2–C7 alkenes. The ratios among the reactivities of C2–C6 alkenes towards methylation are 1:5:24:33:15. This sequence of reactivities of alkenes is also in accordance with that found by Svelle et al. (2005) based on a H-ZSM-5 with a Si/Al ratio of 45. In their study, ratios of reactivities for ethene, propene and butene methylation are 1:17:50, respectively, with the difference from this work attributed to the difference in the Si/Al ratio and the reaction temperature.

pressures as a function of space time. C2–C7 alkenes were the major alkene products in the reactor effluent in each co-reaction test with the exception of cases for co-reaction of methanol with ethene, propene and butene, where only trace amount of C7 was detected. The product partial pressures versus space time for ethene coreacted with methanol are depicted Fig. 8(A), and those for methanol reacted alone are shown in Fig. 9 for comparison. It can be seen that the product partial pressures are comparable with these two kinds of feeding, except that propene partial pressure in the ethene and methanol co-reaction system is in slightly excess during the whole investigated space time. Since ethene is relatively un-reactive when reacted alone as described previously, the excess formation of propene demonstrates the methylation of ethene. However, this reaction plays only a minor role in the reaction with a mixture of methanol and ethene. That is, methylation of ethene by methanol can be neglected, if any. This result also agrees well with the extremely low reactivity of ethene towards methylation as described in Section 3.2.2. Propene undergoes methylation by methanol evidently when co-reacted with methanol. Fig. 8(B) describes the product partial pressures as a function of space time with equal molar methanol and propene as co-feed. Butene dominated the products at short space time (o2 gcat h/mol) with butene exceeding pentene by a factor of almost 8. Since the formation of butene from propene inter-conversion alone at short space time was insignificant as described previously, the excessive formation of butene demonstrates methylation of propene. As space time increases, considerable amounts of pentene were formed due to subsequent methylation of butene, thus the excess of butene in the products

3.2.3. Product distribution of alkene and methanol co-reaction The product distribution reveals the underlying reaction pathway involved in co-reaction of alkene and methanol. For each methanolþ alkene co-feed, Fig. 8 depicts the product partial

Fig. 7. Methylation reactivity of alkene with various carbon number.

Table 2 Yields for different feeds at the shortest test space time. Feed

a

Ethanol þ MeOH Ethanol 1-Propanol þ MeOH 1-Propanol 1-Butanol þMeOH 1-Butanol 1-Pentanol þ MeOH 1-Pentanol 1-Hexanol þ MeOH 1-Hexanol 1-Heptanol þMeOH 1-Heptanol a b

W/Falcohol (gcat h/mol)

1.06 1.06 1.05 1.05 1.01 1.01 0.78 0.78 1.08 1.08 0.83 0.83

XAlkene (%)

0.82 0.32 5.00 5.10 20.37 8.18 29.32 7.48 16.53 35.33 69.44 81.10

XMeOH (%)

6.56 – 6.64 – 27.73 – 40.83 – 10.62 – 16.76 –

Yield

b

(%)

C2¼

C3¼

C4¼

C5¼

C6¼

C7¼

– 0.06 0.11 0.18 0.11 1.13 1.37 0.69 3.73 0.81 0.93

1.92 0.03 – – 2.67 4.72 12.77 4.13 13.97 47.23 64.39 77.74

0.28 0.09 4.24 0.61 – – 6.45 3.51 7.25 9.37 59.28 76.41

0.18 0.01 0.46 0.32 15.69 3.52 – – 1.24 5.05 11.07 3.38

0.05 0.02 0.38 1.84 3.60 0.13 19.48 1.31 – – 2.60 1.68

0.00 0.00 0.00 0.00 0.11 0.00 0.59 0.05 4.48 0.01 – –

The data for sole alkene feed has been corrected to that at the same space time for the corresponding co-fed case based on Fig. 3. The yield is defined as mol/(mol alcohol reactant).

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Fig. 8. Product distributions versus space time for individual C2–C7 alkene co-reaction. Feed: 10 kPa of individual C2–C7 alcohols, A ¼ ethanol, B ¼1-propanol, C ¼1-butanol, D¼ 1-pentanol, E ¼ 1-hexanol, F ¼1-heptanol; temperature: 460 1C.

Fig. 9. Product distributions versus space time for methanol conversion alone. Feed: 10 kPa of methanol, temperature: 460 1C.

became less pronounced and more higher alkenes (pentene and hexene) were formed. Clearly, the co-feeding of propene promotes methanol conversion and results in increased formation of C4–C6 alkenes. In contrast to the results observed in the ethene co-feeding case and the propene co-feeding case, much more amounts of alkene products were detected for co-reaction system of methanol and butene as revealed in Fig. 8(C). This indicates the higher reactivity of butene. At short space time ( o2 gcat h/mol), pentene dominated among the products, and significant amounts of hexene were also detected. Due to the low reactivity of methanol alone, the formation of pentene and hexene from methanol interconversion alone can be ignored. Meanwhile, the formation of pentene and hexene from butene inter-conversion alone is also

insignificant comparing to that from the mixture of butene and methanol co-feed, as can be seen from Fig. 4(C). Hence, pentene and hexene in this co-reaction system are formed predominantly through methylation. As space time increases, the subsequent methylation reactions of pentene become more important, yielding hexene and even heptene, which may crack into propene readily. Thus abundant amounts of propene were observed. Analogous experiments with mixed feed of methanol and pentene were also carried out under the same reaction conditions. Product partial pressures as a function of space time are shown in Fig. 8(D). At short space time ( o2 gcat h/mol), hexene was the main product. Moderate amounts of propene and butene were also observed and increased dramatically with space time. Since the formation of hexene from methanol or pentene inter-conversion alone was insignificant as described earlier, we think that hexene in this co-reaction case was methylation product from pentene. The formed hexene can undergo cracking to yield propene and also be methylated further to form heptene, which cracks rapidly to form propene and butene. Therefore large amount of propene and butene were detected at long space time. The product partial pressures as a function of space time in methanol conversion with hexene co-fed is given in Fig. 8(E). Propene and butene constituted the main products. Their partial pressures increased dramatically as space time increases, with propene exceeding butene by a factor of almost 2. Especially, moderate amounts of heptene were present at short space time ( o2 gcat h/mol) but decreased gradually with increasing space time. Heptene was detected in trace quantities in the previous co-feeding cases except the co-reaction system of pentene. The excessive heptene was certainly formed via methylation of hexene by methanol. But due to the high reactivity towards cracking, heptene cracked to equal molar propene and butene

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rapidly once formed. This behavior will be addressed in more detail later. Thus the amounts of heptene observed were less than that of hexene consumed by methylation. The significance of hexene methylation reaction at the shortest space time can be assessed by assuming that heptene and the major part of butene were methylation products from hexene. Because firstly, butene formed from methanol alone could be ignored due to the low reactivity of methanol conversion alone. Secondly, if butene was the product from hexene cracking, the ratio of propene to butene should be exceeding 5 as can be concluded from hexene cracking as described earlier; but it was not the case here. On the other hand, in the product mixture of this co-reaction system, the amount of propene is twice butene, as shown in Fig. 8(E). This result indicates cracking of hexene to form propene also play an important role beside methylation of hexene in the co-reaction system of hexene and methanol. Heptene goes through cracking when co-reacted with methanol under the current set of reaction conditions. Fig. 8(F) describes the product partial pressures versus space time for heptene co-reacted with methanol. Propene and butene were the most abundant components in the product mixture, and their partial pressures were higher than 6 kPa even at the shortest space time. In this co-reaction system, heptene exhibits much higher reactivity than methanol as can be concluded in Table 2. At the shortest space time, methanol conversion was only 16.76 wt%, while heptene conversion reached 69.44 wt%. Thus propene and butene were formed directly from heptene cracking. Our interpretation is supported by the observation that the molar ratio of propene and butene is close to unity at the shortest space time as shown in Fig. 8(F). As space time increased, moderated amount of pentene, the product from butene methylation, can be observed. It is thus concluded that in the heptene and methanol co-reaction system, heptene cracking is dominant, and methanol is consumed mainly through methylation of butene. In a word, according to the above results and analysis, when alkenes of C2–C6 are co-reacted with methanol over the high silica H-ZSM-5 zeolite, the inter-conversion reaction of methanol, ethene, propene, or butene, the methylation reaction of ethene or propene can be ignored, but the methylation of butene, pentene and hexene can be dominant together with the cracking reactions of hexene and heptene.

et al., 2007) presented a methylation-cracking mechanism including C3¼ –C6¼ . In this study, heptene is found to be the terminal higher alkene of the methylation by methanol, as it cracks completely to propene and butene once formed. This finding confirms the proposition by Dessau (1986). Furthermore, it is also shown that methylation reactivities of ethene and propene can be neglected comparing with that of C4–C6 alkenes. In a typical MTP process, where C2 and mixed C4–C6 alkenes are co-fed with methanol, parallel reactions of methanol with individual alkenes occur simultaneously. Reactions of methanol with C4–C6 alkenes must be dominant, while methylation reactions of ethene and propene must be weak. Moreover, the catalyst surface coverage is expected to increase as the size of the chain increases for alkene reaction (Borges et al., 2007), the expected higher surface coverage of C4–C6 alkenes will make the methylation reactions of ethene and propene more insignificant. Therefore, we would like to propose the following reaction scheme (as shown in Scheme 1) for the dominant reactions for propene formation in the MTP system involving C4–C6 alkene methylation, and hexene and heptene cracking: 4.2. Pathway for ethene formation The second problem is about the formation pathway of ethene in the MTP process as ethene is optionally co-fed. For the MTO process based on SAPO-34 catalyst, the hydrocarbon pool mechanism is well accepted by which ethene is formed in parallel with propene from the same hydrocarbon pool (Wang et al., 2006). For the MTO process based on H-ZSM-5 catalyst, Svelle and coworkers (Bjørgen et al., 2007; Svelle et al., 2007) supposed that ethene was formed exclusively in line with the hydrocarbon pool mechanism but propene with the methylation and cracking mechanism. In the MTP process, however, methanol and the co-fed higher alkenes can inhibit severely the ethene adsorption onto the zeolite surface, which makes little effect of ethene on the whole reaction system. As the higher alkenes coexist with methanol from the very beginning, the so-called hydrocarbon pool mechanism is certainly unnecessary. From the experimental results of the alkene feeding alone, as shown in Section 3.1, one can speculate that cracking of higher alkenes must be more

4. Discussions 4.1. Pathway for propene formation On the basis of the experimental results of alkene alone, methanol alone and co-reaction of methanol and alkene on H-ZSM-5, one can deduce that methylation-cracking pathway is the main reaction pattern in MTP process in which the co-feeding of methanol and several alkenes is operated. Firstly, the reactivity of methanol alone can be ignored in the current set of reaction conditions. Secondly, when methanol is co-reacted with individual C3–C6 alkenes, the measurements of product partial pressures suggest that methylation is dominant at the shortest space time. Alkene inter-conversion reactions involving C3–C7 alkenes are strongly suppressed in the presence of methanol. Ethene is relatively un-reactive in the presence of methanol. C7¼ cracks to propene and butene rapidly with or without methanol co-fed, acting like the terminal alkene. Therefore methylation-cracking pathway is the dominant reaction pattern in MTP. The methylation-cracking mechanism in MTO reaction over the H-ZSM-5 catalyst was primarily proposed by Dessau (1986) in which alkenes can be consecutively methylated from C2¼ to C7¼ , and extended to C8¼ by Park and Froment (2001a, 2001b). Recently, Svelle and coworkers (Bjørgen et al., 2007; Svelle

Scheme 1. Dominant reaction pathway for propene formation in MTP.

Fig. 10. Estimated initial ethene formation rates. Viod: alkene alone; solid: cofeeding. Conditions: as same as Fig.7.

W. Wu et al. / Chemical Engineering Science 66 (2011) 4722–4732

Scheme 2. Dominant reaction pathway for ethene formation in MTP.

reasonable for ethene formation, and find that the best candidate is hexene. This point of view can be firmly supported by the comparison of the estimated initial ethene formation rates under the conditions of the two feeding schemes, as depicted in Fig. 10. When the individual alkenes are reacted alone, the maximum rate appears at the point of hexene, and at pentene due to the rapid methylation by methanol to form hexene in the co-feeding cases. As a result, by an analogy with propene formation, we would also like to propose the following reaction pathway for ethene formation in the MTP process (Scheme 2), which includes a cycle composed of a consecutive methylation from C4¼ through C5¼ to C6¼ and C6¼ a-cracking releasing C4¼ . Nevertheless, it should be noted that, though the proposed dominant pathways for propene and ethene can be used to model reactions with mixed methanol and C2–C6 alkene as feed over high silica H-ZSM-5, at long contact time when methanol conversion approaches completion, the alkene inter-conversion reactions on the basis of oligomerization and cracking will become evident and thus make the final alkene product mixture probably approach the chemical equilibrium composition. Furthermore, the side reactions of the alkenes to form paraffins and aromatics constituting the gasoline and LPG fractions, which account for about 30% in weight in the total hydrocarbon products in the Lurgi’s MTP process (Koempel and Liebner, 2007), will also take place to a modest extent. Exploring the exact extent and the related reaction pathways of the side reactions is the aim of further studies already in progress.

5. Conclusion On the basis of the above demonstrated experimental results for the three feeding schemes, we are now in a position to draw a conclusion for the reaction pathways occurring in a typical MTP system over a high silica H-ZSM-5 zeolite. Methylation and cracking is the dominant reaction pathway when methanol was co-reacted with C2–C6 alkenes with both propene and ethene being the results of the higher alkene cracking. The predominant direct routes for propene consist of b-scission of hexene and heptene, and that for ethene is a-scission of hexene. This standpoint makes possible unifying the formation pathways of all the alkenes involved in the MTP system. According to the comparison of the initial reactivities of alkenes towards methylation and inter-conversion as well, a simplified reaction scheme has been presented, which includes a cycle composed of a consecutive methylation from C4¼ through C5¼ to C6¼ and further to C7¼ , the b-scission of hexene and heptene for propene, and the a-scission of hexene for ethene. Furthermore, the experimental observations have also shown that methylation reactivities of ethene and propene can be neglected comparing with that of C4–C6 alkenes, and the maximum reactivity appears at the point of C5 alkene. Nomenclature Falcohol

molar flow rate of individual alcohol at the reactor inlet, mol/h

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Fethanol, F1-propanol, F1-butanol, molar flow rate of ethanol, 1-propanol, 1-butanol, 1-pentanol, F1-pentanol, F1-hexanol, F1-heptanol, 1-hexanol and 1-heptanol at the reactor inlet, respectively, mol/h Ft total molar flow rate of all components including water and nitrogen at the reactor outlet, mol/h FMeOH molar flow rate of methanol at the reactor inlet, mol/h GHC dry base mass flow rate of all hydrocarbon species, g/h Mi molecular weight (dry basis) of component i, g/mol Pi the partial pressure of component i, kPa Po total pressure at the reactor outlet, kPa W catalyst weight, g xi mass fraction of component i in the reactor effluent XAlkene conversion of alkene XMeOH conversion of methanol and DME

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