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dimethyl ether to light olefins over SAPO-34: Comprehensive comparison of the products distribution and catalyst performance
Journal of Natural Gas Science and Engineering 21 (2014) 532e539
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Methanol/dimethyl ether to light olefins over SAPO-34: Comprehensive comparison of the products distribution and catalyst performance Mohammad ghavipour*, Reza Mosayebi Behbahani, Reza Bagherian Rostami, Alireza Samadi Lemraski Catalyst Research Group, Gas Research Center, Gas Engineering Department, Petroleum University of Technology, Ahwaz 63431, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2014 Received in revised form 11 September 2014 Accepted 12 September 2014 Available online
Dimethyl Ether (DME) as a possible reactant for production of light olefins was compared with methanol over a synthesized SAPO-34 catalyst at 400e460 C and the products' distributions as a function of time on stream and reaction temperature were reported and analyzed. Physiochemical characteristics of the synthesized SAPO-34 catalyst were determined by BET, TPD, XRD, SEM and XRF techniques. The optimum reaction temperature range for methanol conversion to olefins (MTO) seemed to be 400e460 C ¼ with the light olefins' selectivity of 77e78.5% and C¼ 2 /C3 molar ratios from 1.6 at 400 C to 2.4 at 460 C. Products' distributions in DME to olefins (DTO) reaction were different from MTO reaction specially at methane and light olefins' formation rates. Life time of the catalyst increased while using dimethyl ether as the feed but due to high rates of methanol and methane formation, the light olefins' selectivities were not as high as those of MTO reaction. © 2014 Elsevier B.V. All rights reserved.
Keywords: MTO DTO SAPO-34 Light olefin Dimethyl ether Methanol
1. Introduction Light olefins (ethylene and propylene) which are among the main feed stocks of petrochemical complexes are produced majorly as the by-products in fluidized catalytic cracking or steam cracking of naphtha. By shortage of petroleum and high price of crude oil, on-purpose technologies (i.e. the main goals of these processes are the production of light olefins) such as ethane or propane dehydrogenation, metathesis, methanol to propylene (MTP) and methanol to olefins (MTO) have been accelerated because of their lowcost non-petroleum feed stock (CAP, 2012). The conversion of synthesis gas to methanol over the bifunctional catalyst of Cu/ZnO/AL2O3 has been established. Synthesis gas can be produced from abundant fossil resources such as coal or natural gas (Gasification of coal or steam reforming of natural gas) and even from renewable resources like bio mass or any carbon-containing industrial waste by partial combustion. Hence, MTO and MTP processes have received wide attention in last years as indirect new technologies to produce industrially valuable
* Corresponding author. Tel./fax: þ98 6115550868. E-mail addresses: [email protected], (M. ghavipour). http://dx.doi.org/10.1016/j.jngse.2014.09.015 1875-5100/© 2014 Elsevier B.V. All rights reserved.
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¼ € olefins (StOcker, 1999); and MTO process due to its flexible C¼ 2 /C3 production based on the market demand, and its higher light olefins' selectivity than MTP process is more favorable. Silicoaluminophosphate molecular sieve (SAPO) that was invented by Union Carbide Corporation in 1980s (Lok et al., 1984a, 1984b) is the finalized catalyst for MTO process and because of high rate of coke deposition on the catalyst, fluidized bed reactor with continuous regeneration has been designed for this process. Among SAPOs family catalysts, researcher's interests have been focused on small pore molecular sieves such as SAPO-34 (pore size 3.4e3.6⁰A) due to its narrow range of product distribution with high selectivity to C2eC4 olefins in the MTO reaction. These narrow pores just allow the straight chain molecules such as primary alcohols, linear paraffins and light olefins to pass and restrict the diffusion of branched isomers and aromatics (Arstad and Kolboe, 2001). Based on the literatures of UOP Co. (the licensor of MTO process) (Eng et al., 1998a, 1998b; Chen et al., 2005) it has been claimed that the C¼ 2/ ¼ ¼ C¼ 3 mass ratio could be changed at the range of 0.75e1.5 (C2 /C3 molar ratio of 1.125e2.25) by simply adjusting the operating conditions. Reactor temperature seems to be the most affecting parameter. A few articles have been published on investigating the products' distribution while changing the reactor temperature. Some researchers (Dubois et al., 2003; Obrzut et al., 2003) have reported the MTO products at the temperatures of 300e500 C. The
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products' distributions have been reported as the average of the products during the catalyst life time and the products amounts changing by time over the catalyst have not been reported at each ¼ temperature. C¼ 2 /C3 molar ratios at 400 and 450 C were 1.3 and 2.8 and catalyst life times by weight hourly space velocity (WHSV) ¼ 0.5 h1 were 24 and 12 h, respectively. By varying the C¼ 2/ C¼ 3 ratio, the catalyst lift time declined dramatically, and this is against the UOP claims. At the present study, amounts of all the products have been listed at tables by time and at each temperature. This will help the good understanding of the reaction trends by time and the catalyst behavior by increasing the coke content at different reaction temperatures. The possibility of dimethyl ether (DME) conversion to light olefins over SAPO-34 was also tested and compared to MTO process because of the two following reasons: Firstly, although methanol can be directly converted to olefins over ZSM-5 (Mei et al., 2008), methanol to propylene technology (MTP) by Lurgi Co. contains two main sections (Koempel and Liebner, 2007); methanol conversion to equilibrium mixture of methanol, dimethyl ether and water over g-Al2O3 and subsequent reaction of this mixture over ZSM-5 catalyst to produce propylene as the main product and LPG, gasoline and other hydrocarbons as the by-products. It is believed that producing the intermediate DME separately, and its consequence conversion to olefins can lengthen the ZSM-5 catalyst life time and improve its performance. Analogies between MTO and MTP reaction mechanisms gave us an idea that DME could be also a possible feed for olefin production over SAPO-34. Secondly, in recent years, DME has attained much attention as a cost-effective substitute fuel in domestic appliances or diesel engines (Semelsberger et al., 2006, Marchionnaa et al., 2008) and much effort has been put into its economic and large-scale synthesis. DME is now synthesized commercially by the dehydration of methanol; but its direct production from synthesis gas has been proven to be more economical that the two-step process (i.e. syngas to methanol and methanol to DME) (Topp-Jorgense, 1985; Hansen et al., 1993; Ogawa et al., 2003; Aguayo et al., 2005; ~ a et al., 2008). In direct DME synthesis one product of each Eren reaction is a reactant for another reaction. This creates a strong driving force for the overall reaction and allows overcoming the equilibrium limitations of the methanol synthesis. By large-scale production of DME in early future, investigation on possibility of DME conversion to light olefins (DTO) seems to be reasonable and interesting. So far, a few works have been reported on DTO conversion over SAPO-34 (Song et al., 2002; Zhou et al., 2008; Jie et al., 2010; Cui et al., 2013; Lee et al., 2014), but the products' distribution and the catalyst performance have not been monitored at different reactor temperatures and have not been compared to MTO reaction at the same time over the same catalyst. Based on our knowledge, the only specialized study on comparison of MTO and DTO reactions over the SAPO-34 catalyst has been published recently by Li et al., 2014. Although the proper range of weight hourly space velocity (WHSV) for MTO reaction over SAPO-34 even in industrial scale has been realized to be 1e3, but they have arranged the WHSVs of methanol and DME to be 32 and 23 gfeed/gcat.h, respectively. These improper reaction conditions caused rapid deactivation of the catalysts (10e20 min) in MTO and DTO reactions. As a result, the products' distribution was not analyzed concisely. They have reported that the two reactions have the same product selectivities but different reactivities and catalyst life times. The present study has analyzed the products in MTO and DTO reactions at a suitable reaction temperature range and WHSV and has compared the two reactions from different points of view.
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2. Experimental 2.1. Catalyst preparation The sources of Al, P and Si for the gel preparation were aluminum iso propoxide (AIP 99%, Aldrich), phosphoric acid (H3PO4 85%, Merck) and Ludox (40% aqueous solution, Aldrich) and the template was tetra ethyl ammonium hydroxide (TEAOH 40%, Aldrich) which results the smallest crystals of catalyst among the various templates for SAPO-34 synthesis. According to the articles that have studied the gel composition of SAPO-34 synthesis, catalysts with Si/Al ratios of 0.13e0.22 have shown higher crystallinity, longer lifetime and lower rate of activity loss (Izadbakhsh et al., 2009). For pure SAPO-34 crystals, phosphorous content should be at the range of 0.7 n(P2O5)/n(Al2O3) 1.2 and because low H2O content in the gel leads to a higher ion concentration, favoring the higher nucleation rate; the best H2O range has been measured to be 25 n(H2O)/n(Al2O3) 100 (Guangyu et al., 2012). Hence, the gel composition was selected as 1 Al2O3/1 P2O5/0.3 SiO2/2 TEAOH/60 H2O. In a typical synthesis, 20.43 g AIP was hydrolyzed in 29 g water at 80 C for 3 h. 2.25 g Ludux and 36.82 g TEAOH solution were poured into and mixed for 3 h. Finally, 11.53 g acid phosphoric was added, and the resulting 100 ml gel was stirred for 24 h for aging, after pH measurement (7e7.3). The gel was transferred into 120 ml Stainless steel Teflon-lined autoclave. The autoclave was placed at a programmable furnace. The temperature was raised to 200 C at a slope of 1 C/min and maintained for 30 h. The resulting slurry was washed with deionized water 3 times and the catalyst crystals was gathered by centrifuging and dried at 110 C overnight and calcined at 600 C for 5 h. The gel composition and the starting materials of the present study are the same as those of example 35th in Lok et al., 1984b, but the order of addition of materials is different. With the procedure of the patent, the final gel pH was about 9e10 and the final resulting powder was completely amorphous and no SAPO-34 crystal was formed even after adjusting the gel pH to 7 by addition of several droplets of H2SO4. Different sequences of reagents' addition were tested and the best catalyst was formed with the procedure which was explained previously. 2.2. Catalyst activity test Thanks to our good experience in methanol dehydration to DME over g-Al2O3 (Ghavipour and Behbahani, 2014), DTO reaction was simulated by initial conversion of methanol to DME over gAl2O3 at 300 C with equilibrium conversion of about 85% and then the equilibrium mixture (molar composition ~ 42% DME, 42% H2O and 16% CH3OH) was cooled down to 2 C to condense the unreacted methanol and water and finally, DME with a minimum concentration of 98.5% mole was sent to DTO reactor. In the case of MTO reaction, the vaporized methanol was sent directly to the MTO reactor. To simulate the same reaction conditions for MTO and DTO reactions for comparison of catalyst life times, the feed loads on the catalysts in the two reactions should be the same and since methanol and DME do not have same carbon content, the WHSV's must be adjusted according to the same carbon based content (WHSV of 1.0gMeoH/gCat.h for MTO, WHSV of 0.72gDME/ gCat.h for DTO that result the same carbon-based WHSV of 0.375 gCarbon/gCat.h for both reactions). A schematic diagram of the experimental reactor setup is illustrated in Fig. 1. As it can be seen, liquid methanol was injected via a metering pump (0.01e9.99 ml/ min) to an evaporator maintained at 200 C. The vaporized methanol was mixed with nitrogen as the carrier gas that prevented condensation through the lines and was controlled by a gas flow controller (0e500 ml/min AALBORG Germany) at a constant flow of 60 ml/min. The mixed gas was directed through a heat
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Fig. 1. Schematic diagram of the experimental reactor setup.
traced line to a fixed bed reactor (stainless steel, inside diameter of 12 mm and length of 20 cm) contained 3 g of catalyst in powder form with glass wools in both sides of the catalyst bed. A K-type thermocouple was located at the center of the catalyst bed to monitor the reaction temperature which was adjusted via an electrical furnace surrounded the stainless steel reactor. The MTO reaction was conducted at temperature range of 370e460 C with intervals of 30 C at WHSV ¼ 1 gMeOH/gcat.h. The products left the MTD reactor before and after the condenser and the products of MTO/DTO reactor were sent to a gas chromatograph (Agilant 6890) equipped with flame ionization detector (FID) and HP-PLOT Q column at the oven program of: 60 C held 5 min # 20 C/min # 200 C held 10 min.
characterization tests can lead to long catalyst life time and high light olefins' selectivity in MTO reaction. 3.2. Activity tests results The products' distributions of the MTO/DTO reactions over synthesized SAPO-34 catalyst have been drawn at Fig. 4. In MTO reaction, at 400 C methane mole fraction reduces slightly by time on stream and remains almost constant to the end of the reaction time but at higher temperatures it escalates by time on steam and has more intense slope of increase as the temperature rises. Ethylene formation increases by time and by raising the temperature. Ethane value has not a clear trend by time and temperature and oscillates between 1 and 3 mol percent. Propylene production
3. Results and discussion 3.1. Catalyst characterization results The X-ray diffraction (XRD) patterns of the synthesized SAPO-34 catalyst and the reference pattern (Lok et al., 1984b) have been shown at Fig. 2. As it can be seen, the identifier diffraction peaks of the reference catalyst have been appeared on the XRD pattern of the synthesized catalyst proving the SAPO-34 phase formation. The scanning electron microscopy (SEM) of the prepared SAPO34 catalyst shows the uniform cubic crystals of SAPO-34 at the range of 1e3 micron (Fig. 3). The results of other characterization tests such as BET, temperature programmed desorption (TPD) and X-ray fluorescence (XRF) have been tabulated at Table 1. Descending amount of phosphorous from 49.82 %wt in the starting gel to 41.59 %wt in the final catalyst can evidently show the mechanism of phosphorous replacement by silicon and hydrogen to produce bridging hydroxyl group (-SiOHAl-) as Brønsted acidic sites in the neutral framework of alumino phosphate (AlPO4) molecular sieve (Ashtekar et al., 1994; Sastre et al., 1997; Vomscheid et al., 1994). The TPD results also shows two moderate weak and medium acid sites and a slight amount of a strong acid site at desorption temperature of 450e600 C. The positive effects of low silicon content and as a sequence, moderate acid sites on descending the propane formation and enhancing the catalyst life time in MTO reaction have been reported (Wilson and Barger, 1999; Strohmaier, 2004; Cao et al., 2004). Proper physicochemical characteristics of the prepared catalyst such as uniform catalyst sizes, small crystals, high surface area (437 m2/gCat) and moderate acid sites as proven by the
Fig. 2. The XRD patterns of a) the synthesized catalyst b) the reference SAPO-34.
M. ghavipour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 532e539
535
Fig. 3. The SEM images of synthesized SAPO-34 catalyst at different orders of magnitude.
goes up by time at temperature of 400 C and declines by time at þ higher temperatures. Amounts of Propane, C¼ 4 complexes and C5 þ fraction drop by time at any temperature. C5 fraction decrease as the temperature goes up. This could be explained by the fact that, increasing the temperature causes the heavier components (Cþ 5 ) to be cracked in to lighter ones. It has been proven earlier that at higher temperatures, the amount of coke formation increases (Chen et al., 2012), but longer catalyst life time at 460 C compared to 400 C should be due to the types of coke form at different temperatures. As the Cþ 5 fraction at 460 C has a lower amount in
comparison to 400 C, the coke deposits on the catalyst surface at 460 C may contains lighter components with shorter chains that cannot block the catalyst pores rapidly and in spite of the higher amount of coke content at 460 C, the catalyst life time increases. At 400 and 430 C, the catalyst life times were 8 and 7 h, respectively and at 460 C longest catalyst life time was recorded. DTO reaction over the synthesized SAPO-34 was performed at 400, 430 and 460 C and the molar distributions of products have been also illustrated at Fig. 4. In DTO reaction, ethane, propane, þ methanol, C¼ 4 complexes and C5 fraction go down by time on
Table 1 The XRF, BET and TPD results of the synthesized SAPO-34 catalyst. XRF Comp. Al Si P
BET Starting gel 43.40% wt 6.78% wt 49.82% wt
TPD
Final catalyst 48.20% wt 10.21% wt 41.59% wt
2
Surface area (m /gCat) Average particle size (mm) Micropore volume (cm3/gcat)
437 1.9 0.23
Temperature range (C)
mmolt-Butylamin/gcat
150e300 350e450 450e600
0.187 0.121 0.035
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Fig. 4. Molar products distribution of MTO and DTO reactions at different temperatures.
stream while methane and ethylene go up as the reaction proceeds. Propylene increases at the reaction beginning and then decreases. By raising the reaction temperature, methane, ethane, propane, C¼ 4 complexes and Cþ 5 fraction decline and light olefins formation increases (Fig. 4d, e, f). Li et al. (2014) have reported that the MTO and DTO reactions have similar selectivities, but our findings are in good agreement to the previous article (Chen et al., 1999) that reported there are substantial differences in methane and light olefins selectivities in these two reactions. Methane formation in DTO reaction is higher than MTO reaction and has a different trend as it increases by decreasing the temperature. As opposed to the research expounding to elucidate the mechanism of CeC bond formation in MTO reaction, mechanistic investigation of methane formation in MTO reaction have been reported in a few literature (Kirmse, 1964; Mihail et al., 1983; Hutchings and Roger Hunter, 1987) in which methane could arise via a radical pathway
involving DME. Mihail et al. (1983) developed a kinetic model in the case of MTO reaction which methane formation proceeds via reaction of carbene intermediate generating by DME, with molecular hydrogen. The mechanism of methane formation in this kinetic model has been mentioned below:
CH3 OCH3 /2 : CH2 þ H2 O : CH2 þ 2H2 /CH4 Also, another possible reaction mechanism leading to methane formation proposing by Obrzut et al. (2003) is as follow:
CH3 OCH3 þ H2 O/2CO þ 4H2 2CO þ 2H2 /CO2 þ CH4
M. ghavipour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 532e539
537
Fig. 5. Olefins selectivity (ethylene, propylene and butylenes) of MTO and DTO reactions versus time on stream at different temperatures.
The above mechanism also shows that methane production is DME dependent. Based on above, it is expected that methane production would be higher in DTO reaction than in MTO. Light olefins selectivities have been depicted at Fig. 5. As it can be seen, in DTO reaction light olefins' selectivities enhances by temperature dramatically due to simultaneous increase of ethylene and propylene. On the other hand, in MTO reaction the selectivity to light olefins increases slightly, because by rising the temperature ethylene production goes up and propylene production falls down. In both of the reactions, olefin selectivity increases by time on stream. MTO reaction seems to be more selective in olefins production than DTO and two of its main reasons are DME conversion to methanol and high amount of methane formation in DTO reaction. ¼ In both of the reactions, C¼ 2 /C3 molar ratio enhances by time on stream and by raising the temperature as shown in Fig. 6. Although, Dubois et al. (2003) have reported that in MTO reaction, the catalyst life time at 400 C is twice the life time at 450 C, but completely different results were recorded here. Based on the UOP/Hydro license (Eng et al., 1998a, 1998b; Chen et al., 2005) there should be a
certain range of operating condition that without any activity loss ¼ or catalyst life time depletion, the C¼ 2 /C3 ratio can be considerably changed to meet the market demand and based on the average values presented at Table 2 and 400e460 C with acceptable ¼ catalyst life time is the optimum range of MTO reaction with C¼ 2 /C3 molar ratio of 1.6e2.4. By passing the time of stream on the catalyst and partial deposition of coke causing more diffusion constrains for heavier hydrocarbons, the selectivity to light olefins specially ethylene enhances rapidly (Bos et al., 1995; Qi et al., 2007). At 400 C, it took 2.5 h to reach the light olefins' molar fraction higher than 70%, but at 430 and 460 C, this was happened after several minutes (Fig. 5). Qi et al. (2007) have also found the best light olefins selectivity over SAPO-34 catalyst by coke content of about 5.7% wt. At industrial MTO process, to have the highest amount of light olefins' selectivity, the catalysts are partially regenerated and the profitable amount of coke is maintained on the catalyst surface; so, the first unwanted selectivity of SAPO-34, for instance, at 400 C should not be taken into account, and the actual products' distribution is after a while of reaction. Hence, the average molar fractions of products
¼ Fig. 6. C¼ 2 /C3 molar ratios of MTO and DTO reactions versus time on stream at different temperatures.
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Table 2 Average molar distribution of Products in MTO/DTO reaction at the specified period with the acceptable light olefins production rate (light olefins selectivity more than 70%). Outlet components (mole %) C1 C¼ 2 C2 C¼ 3 C3 DME MeOH C¼ 4 Cþ 5 Sum ¼ C¼ 2 & C3 ¼ ¼ C¼ 2 & C3 & C4 ¼ (C¼ /C )ratio 2 3 Light olefins selectivity Conversion Olefins selectivity Time on stream (h)
MTO 400 C
MTO 430 C
MTO 460 C
10.7 50.8 1.6 23.5 2.0 0.2 5.9 4.4 1.0
100.0
e
100.0
100.0
78.4 83.6 2.4 78.4 99.5 87.4 10.5
e e e e e e e
71.0 74.2 2.2 78.0 99.9 84.1 2.5
74.3 78.8 2.2 78.1 99.8 86.2 7.0
8.1 55.3 2.6 23.1 3.1 0.4 1.1 5.2 1.1
100.0
100.0
78.8 85.4 1.6 77.0 99.1 87.7 6.0
79.8 86.6 1.9 76.9 99.4 88.0 6.0
SAPO-34 catalyst with Proper physicochemical characteristics was synthesized, and its activity in MTO and DTO reactions was assessed at 400e460 C. Products distribution by time on stream and by raising the temperature was analyzed for both of the reactions and in summary, the following three important point were concluded: firstly, in DTO reaction at higher temperatures, the equilibrium of DME hydrolysis shifts toward methanol, which makes methanol partial pressure to be higher, and consequently the kinetics and selectivities of the DTO and MTO reactions are much more similar. Secondly, as the DME to methane reaction is more exothermic than DME to methanol, so at low temperatures DME to methane reaction is more favorable and methane molar fraction increases dramatically that is in complete contrast to methane formation in MTO reaction. Thirdly, as MTO reaction is so fast over SAPO-34 catalyst that does not allow the escape of the bigger molecules and catalyst deactivation happens immediately, the use of DME as the reactant is useful in holding the partial pressure of methanol low in the reactor, from which better catalyst stability can be achieved. Generally, it can be say that the induction time of DTO reaction is longer and its catalyst deactivation is gradually. As evidence, the Cþ 5 fraction in DTO reaction is much lower than that of MTO reaction and coke deposition due to creation of heavy hydrocarbons happened slower and catalyst life time is longer than MTO. Besides, in DTO reaction, the light olefins' selectivities are lower because of two main reasons that are DME conversion to methanol and high amount of methane formation. In conclusion, by taking all of these into account, DME does not seem to be a competitive feed for olefins production in comparison to methanol.
DTO 460 C
19.6 48.9 2.0 22.1 1.5 0.1 1.9 3.2 0.7
3.6 52.4 2.0 27.3 3.8 0.8 1.5 6.8 1.8
4. Conclusion
DTO 430 C
e e e e e e e e e
3.0 48.4 2.4 30.4 3.1 2.4 2.2 6.6 1.7
in this period of time on stream with acceptable light olefin selectivities were listed at Table 2. This period was 10.5 h at 460 C and 6 h for 400 and 430 C. The highest light olefins' molar fraction of 79.8 was reported at 430 C. As shown in Table 2, amounts of þ ¼ ¼ propane, C¼ 4 complexes, C5 fraction and C2 /C3 molar ratios are less than MTO reaction. Although catalyst life time increases in DTO reaction but light olefins' selectivities are not competitive to the MTO reaction. At 400 C the light olefins' molar fraction does not even exceed 40% and periods of time on stream with light olefins' molar fraction higher than 70% were 2.5 and 7 h for 430 and 460 C as presented in Table 2.
DTO 400 C
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M. ghavipour et al. / Journal of Natural Gas Science and Engineering 21 (2014) 532e539 Izadbakhsh, A., Farhadi, F., Khorasheh, F., Sahebdelfar, S., Asadib, M., Feng, Y.Z., 2009. Effect of SAPO-34's composition on its physico-chemical properties and deactivation in MTO process. Appl. Catal. A 364, 48e56. Jie, Z., Yu, C., Zeeshan, N., Wang, Y., Wei, F., 2010. In situ synthesis of SAPO-34 Zeolites in kaolin microspheres for a fluidized methanol or dimethyl ether to olefins process. Chin. J. Chem. Eng. 18, 979e987. Kirmse, W., 1964. Carbene Chemistry. Academic press, New York. Koempel, H., Liebner, W., 2007. Lurgi's methanol to propylene (MTP(R)) report on a successful commercialization. Stud. Surf. Sci. Catal. 261e267. Lee, S.G., Kima, H.S., Kima, Y.H., Kang, E.J., Lee, D.H., Park, C.S., 2014. Dimethyl ether conversion to light olefins over the SAPO-34/ZrO2composite catalysts with high lifetime. J. Ind. Eng. Chem. 20, 61e67. Li, Y., Zhang, M., Wang, D., Wei, F., Wan, Y., 2014. Differences in the methanol-toolefins reaction catalyzed by SAPO-34 with dimethyl ether as reactant. J. Catal. 311, 281e287. Lok, B.M., Messina, C.A., Patton, R.L., Gajek, R.T., Cannan, T.R., Flanigen, E.M., 1984a. Silicoaluminophosphate molecular sieves: another new class of microporous crystalline inorganic solids. J. Am. Chem. Soc. 106, 6092e6093. Lok, B.M., Patton, R.L., Gajek, R.T., Cannan, T.R., Flanigen, E.M., 1984b. Crystalline silicoaluminophosphates. U. S. Pat. 4440871. Marchionnaa, M., Patrini, R., Sanfilippob, D., Migliavacca, G., 2008. Fundamental investigations on di-methyl ether (DME) as LPG substitute or make-up for domestic uses. Fuel Process Technol. 89, 1255e1261. Mei, ChS., Wen, P.Y., Liu, Z.Ch, Liu, H.X., Wang, Y.D., Yang, W.M., Xie, Z.K., Hua, W.M., Gao, Z., 2008. Selective production of propylene from methanol: mesoporosity development in high silica HZSM-5. J. Catal. 258, 243e249. Mihail, R., Straja, S., Maia, G., Musca, G., Pop, G., 1983. Kinetic model for methanol conversion to olefins. Ind. Eng. Chem. Process Des. Dev. 22, 532.
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Obrzut, D.L., Adekkanattu, P.M., Thundimadathil, J., Liu, J., Dubois, D.R., Guin, J.A., 2003. Reducing methane formation in methanol to olefins reaction on metal impregnated SAPO-34 molecular sieve. React. Kinet. Catal. Lett. 80, 113e121. Ogawa, T., Inoue, N., Shikada, T., Ohno, Y., 2003. Direct dimethyl ether synthesis. J. Nat. Gas. Chem. 12, 219e227. Qi, G., Xie, Z., Yang, W., Zhong, S., Liu, H., Zhang, Ch, Chen, Q., 2007. Behaviors of coke deposition on SAPO-34 catalyst during methanol conversion to light olefins. Fuel Process Technol. 88, 437e441. Sastre, G., Lewis, D.W., Richard, C., Catlow, A., 1997. Modeling of silicon substitution in SAPO-5 and SAPO-34 molecular sieves. J. Phys. Chem. B 101, 5249e5262. Semelsberger, T.A., Borup, R.L., Greene, H.L., 2006. Dimethyl ether (DME) as an alternative fuel. J. Power Sources 156, 497e511. Song, W.G., Marcus, D.M., Fu, H., Ehresmann, J.O., Haw, J.F., 2002. An of studied reaction that may never have been: direct catalytic conversion of methanol or dimethyl ether to hydrocarbons on the solid acids HZSM-5 or HSAPO-34. J. Am. Chem. Soc. 124, 3844e3845. € StOcker, M., 1999. Methanol-to-hydrocarbons: catalytic materials and their behavior. Microporous Mesoporous Mater. 29, 3. Strohmaier, K.G., 2004. Synthesis of molecular sieves of CHA framework type. U. S. Pat. 6835363 B1. Topp-Jorgense, J., 1985. Process for the preparation of catalysts for use in ether synthesis. U. S. Pat. 4536485. Vomscheid, R., Briend, M., Peltre, M.J., Man, P.P., Barthomeuf, D., 1994. The role of the template in directing the Si distribution in SAPO Zeolites. J. Phys. Chem. 98, 9614e9618. Wilson, S., Barger, P., 1999. The characteristics of SAPO-34 which influence the conversion of methanol to light olefins. Micropor Mesopor Mater. 29, 117e126. Zhou, H., Wang, Y., Wei, F., Wang, D., Wang, Z., 2008. In situ synthesis of SAPO-34 crystals grown onto a-Al2O3 sphere supports as the catalyst for the fluidized bed conversion of dimethyl ether to olefins. Appl. Catal. A 341, 112e118.
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Methanol/dimethyl ether to light olefins over SAPO-34: Comprehensive comparison of the products distribution and catalyst performance Mohammad ghavipour*, Reza Mosayebi Behbahani, Reza Bagherian Rostami, Alireza Samadi Lemraski Catalyst Research Group, Gas Research Center, Gas Engineering Department, Petroleum University of Technology, Ahwaz 63431, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2014 Received in revised form 11 September 2014 Accepted 12 September 2014 Available online
Dimethyl Ether (DME) as a possible reactant for production of light olefins was compared with methanol over a synthesized SAPO-34 catalyst at 400e460 C and the products' distributions as a function of time on stream and reaction temperature were reported and analyzed. Physiochemical characteristics of the synthesized SAPO-34 catalyst were determined by BET, TPD, XRD, SEM and XRF techniques. The optimum reaction temperature range for methanol conversion to olefins (MTO) seemed to be 400e460 C ¼ with the light olefins' selectivity of 77e78.5% and C¼ 2 /C3 molar ratios from 1.6 at 400 C to 2.4 at 460 C. Products' distributions in DME to olefins (DTO) reaction were different from MTO reaction specially at methane and light olefins' formation rates. Life time of the catalyst increased while using dimethyl ether as the feed but due to high rates of methanol and methane formation, the light olefins' selectivities were not as high as those of MTO reaction. © 2014 Elsevier B.V. All rights reserved.
Keywords: MTO DTO SAPO-34 Light olefin Dimethyl ether Methanol
1. Introduction Light olefins (ethylene and propylene) which are among the main feed stocks of petrochemical complexes are produced majorly as the by-products in fluidized catalytic cracking or steam cracking of naphtha. By shortage of petroleum and high price of crude oil, on-purpose technologies (i.e. the main goals of these processes are the production of light olefins) such as ethane or propane dehydrogenation, metathesis, methanol to propylene (MTP) and methanol to olefins (MTO) have been accelerated because of their lowcost non-petroleum feed stock (CAP, 2012). The conversion of synthesis gas to methanol over the bifunctional catalyst of Cu/ZnO/AL2O3 has been established. Synthesis gas can be produced from abundant fossil resources such as coal or natural gas (Gasification of coal or steam reforming of natural gas) and even from renewable resources like bio mass or any carbon-containing industrial waste by partial combustion. Hence, MTO and MTP processes have received wide attention in last years as indirect new technologies to produce industrially valuable
* Corresponding author. Tel./fax: þ98 6115550868. E-mail addresses: [email protected], (M. ghavipour). http://dx.doi.org/10.1016/j.jngse.2014.09.015 1875-5100/© 2014 Elsevier B.V. All rights reserved.
[email protected]
¼ € olefins (StOcker, 1999); and MTO process due to its flexible C¼ 2 /C3 production based on the market demand, and its higher light olefins' selectivity than MTP process is more favorable. Silicoaluminophosphate molecular sieve (SAPO) that was invented by Union Carbide Corporation in 1980s (Lok et al., 1984a, 1984b) is the finalized catalyst for MTO process and because of high rate of coke deposition on the catalyst, fluidized bed reactor with continuous regeneration has been designed for this process. Among SAPOs family catalysts, researcher's interests have been focused on small pore molecular sieves such as SAPO-34 (pore size 3.4e3.6⁰A) due to its narrow range of product distribution with high selectivity to C2eC4 olefins in the MTO reaction. These narrow pores just allow the straight chain molecules such as primary alcohols, linear paraffins and light olefins to pass and restrict the diffusion of branched isomers and aromatics (Arstad and Kolboe, 2001). Based on the literatures of UOP Co. (the licensor of MTO process) (Eng et al., 1998a, 1998b; Chen et al., 2005) it has been claimed that the C¼ 2/ ¼ ¼ C¼ 3 mass ratio could be changed at the range of 0.75e1.5 (C2 /C3 molar ratio of 1.125e2.25) by simply adjusting the operating conditions. Reactor temperature seems to be the most affecting parameter. A few articles have been published on investigating the products' distribution while changing the reactor temperature. Some researchers (Dubois et al., 2003; Obrzut et al., 2003) have reported the MTO products at the temperatures of 300e500 C. The
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products' distributions have been reported as the average of the products during the catalyst life time and the products amounts changing by time over the catalyst have not been reported at each ¼ temperature. C¼ 2 /C3 molar ratios at 400 and 450 C were 1.3 and 2.8 and catalyst life times by weight hourly space velocity (WHSV) ¼ 0.5 h1 were 24 and 12 h, respectively. By varying the C¼ 2/ C¼ 3 ratio, the catalyst lift time declined dramatically, and this is against the UOP claims. At the present study, amounts of all the products have been listed at tables by time and at each temperature. This will help the good understanding of the reaction trends by time and the catalyst behavior by increasing the coke content at different reaction temperatures. The possibility of dimethyl ether (DME) conversion to light olefins over SAPO-34 was also tested and compared to MTO process because of the two following reasons: Firstly, although methanol can be directly converted to olefins over ZSM-5 (Mei et al., 2008), methanol to propylene technology (MTP) by Lurgi Co. contains two main sections (Koempel and Liebner, 2007); methanol conversion to equilibrium mixture of methanol, dimethyl ether and water over g-Al2O3 and subsequent reaction of this mixture over ZSM-5 catalyst to produce propylene as the main product and LPG, gasoline and other hydrocarbons as the by-products. It is believed that producing the intermediate DME separately, and its consequence conversion to olefins can lengthen the ZSM-5 catalyst life time and improve its performance. Analogies between MTO and MTP reaction mechanisms gave us an idea that DME could be also a possible feed for olefin production over SAPO-34. Secondly, in recent years, DME has attained much attention as a cost-effective substitute fuel in domestic appliances or diesel engines (Semelsberger et al., 2006, Marchionnaa et al., 2008) and much effort has been put into its economic and large-scale synthesis. DME is now synthesized commercially by the dehydration of methanol; but its direct production from synthesis gas has been proven to be more economical that the two-step process (i.e. syngas to methanol and methanol to DME) (Topp-Jorgense, 1985; Hansen et al., 1993; Ogawa et al., 2003; Aguayo et al., 2005; ~ a et al., 2008). In direct DME synthesis one product of each Eren reaction is a reactant for another reaction. This creates a strong driving force for the overall reaction and allows overcoming the equilibrium limitations of the methanol synthesis. By large-scale production of DME in early future, investigation on possibility of DME conversion to light olefins (DTO) seems to be reasonable and interesting. So far, a few works have been reported on DTO conversion over SAPO-34 (Song et al., 2002; Zhou et al., 2008; Jie et al., 2010; Cui et al., 2013; Lee et al., 2014), but the products' distribution and the catalyst performance have not been monitored at different reactor temperatures and have not been compared to MTO reaction at the same time over the same catalyst. Based on our knowledge, the only specialized study on comparison of MTO and DTO reactions over the SAPO-34 catalyst has been published recently by Li et al., 2014. Although the proper range of weight hourly space velocity (WHSV) for MTO reaction over SAPO-34 even in industrial scale has been realized to be 1e3, but they have arranged the WHSVs of methanol and DME to be 32 and 23 gfeed/gcat.h, respectively. These improper reaction conditions caused rapid deactivation of the catalysts (10e20 min) in MTO and DTO reactions. As a result, the products' distribution was not analyzed concisely. They have reported that the two reactions have the same product selectivities but different reactivities and catalyst life times. The present study has analyzed the products in MTO and DTO reactions at a suitable reaction temperature range and WHSV and has compared the two reactions from different points of view.
533
2. Experimental 2.1. Catalyst preparation The sources of Al, P and Si for the gel preparation were aluminum iso propoxide (AIP 99%, Aldrich), phosphoric acid (H3PO4 85%, Merck) and Ludox (40% aqueous solution, Aldrich) and the template was tetra ethyl ammonium hydroxide (TEAOH 40%, Aldrich) which results the smallest crystals of catalyst among the various templates for SAPO-34 synthesis. According to the articles that have studied the gel composition of SAPO-34 synthesis, catalysts with Si/Al ratios of 0.13e0.22 have shown higher crystallinity, longer lifetime and lower rate of activity loss (Izadbakhsh et al., 2009). For pure SAPO-34 crystals, phosphorous content should be at the range of 0.7 n(P2O5)/n(Al2O3) 1.2 and because low H2O content in the gel leads to a higher ion concentration, favoring the higher nucleation rate; the best H2O range has been measured to be 25 n(H2O)/n(Al2O3) 100 (Guangyu et al., 2012). Hence, the gel composition was selected as 1 Al2O3/1 P2O5/0.3 SiO2/2 TEAOH/60 H2O. In a typical synthesis, 20.43 g AIP was hydrolyzed in 29 g water at 80 C for 3 h. 2.25 g Ludux and 36.82 g TEAOH solution were poured into and mixed for 3 h. Finally, 11.53 g acid phosphoric was added, and the resulting 100 ml gel was stirred for 24 h for aging, after pH measurement (7e7.3). The gel was transferred into 120 ml Stainless steel Teflon-lined autoclave. The autoclave was placed at a programmable furnace. The temperature was raised to 200 C at a slope of 1 C/min and maintained for 30 h. The resulting slurry was washed with deionized water 3 times and the catalyst crystals was gathered by centrifuging and dried at 110 C overnight and calcined at 600 C for 5 h. The gel composition and the starting materials of the present study are the same as those of example 35th in Lok et al., 1984b, but the order of addition of materials is different. With the procedure of the patent, the final gel pH was about 9e10 and the final resulting powder was completely amorphous and no SAPO-34 crystal was formed even after adjusting the gel pH to 7 by addition of several droplets of H2SO4. Different sequences of reagents' addition were tested and the best catalyst was formed with the procedure which was explained previously. 2.2. Catalyst activity test Thanks to our good experience in methanol dehydration to DME over g-Al2O3 (Ghavipour and Behbahani, 2014), DTO reaction was simulated by initial conversion of methanol to DME over gAl2O3 at 300 C with equilibrium conversion of about 85% and then the equilibrium mixture (molar composition ~ 42% DME, 42% H2O and 16% CH3OH) was cooled down to 2 C to condense the unreacted methanol and water and finally, DME with a minimum concentration of 98.5% mole was sent to DTO reactor. In the case of MTO reaction, the vaporized methanol was sent directly to the MTO reactor. To simulate the same reaction conditions for MTO and DTO reactions for comparison of catalyst life times, the feed loads on the catalysts in the two reactions should be the same and since methanol and DME do not have same carbon content, the WHSV's must be adjusted according to the same carbon based content (WHSV of 1.0gMeoH/gCat.h for MTO, WHSV of 0.72gDME/ gCat.h for DTO that result the same carbon-based WHSV of 0.375 gCarbon/gCat.h for both reactions). A schematic diagram of the experimental reactor setup is illustrated in Fig. 1. As it can be seen, liquid methanol was injected via a metering pump (0.01e9.99 ml/ min) to an evaporator maintained at 200 C. The vaporized methanol was mixed with nitrogen as the carrier gas that prevented condensation through the lines and was controlled by a gas flow controller (0e500 ml/min AALBORG Germany) at a constant flow of 60 ml/min. The mixed gas was directed through a heat
534
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Fig. 1. Schematic diagram of the experimental reactor setup.
traced line to a fixed bed reactor (stainless steel, inside diameter of 12 mm and length of 20 cm) contained 3 g of catalyst in powder form with glass wools in both sides of the catalyst bed. A K-type thermocouple was located at the center of the catalyst bed to monitor the reaction temperature which was adjusted via an electrical furnace surrounded the stainless steel reactor. The MTO reaction was conducted at temperature range of 370e460 C with intervals of 30 C at WHSV ¼ 1 gMeOH/gcat.h. The products left the MTD reactor before and after the condenser and the products of MTO/DTO reactor were sent to a gas chromatograph (Agilant 6890) equipped with flame ionization detector (FID) and HP-PLOT Q column at the oven program of: 60 C held 5 min # 20 C/min # 200 C held 10 min.
characterization tests can lead to long catalyst life time and high light olefins' selectivity in MTO reaction. 3.2. Activity tests results The products' distributions of the MTO/DTO reactions over synthesized SAPO-34 catalyst have been drawn at Fig. 4. In MTO reaction, at 400 C methane mole fraction reduces slightly by time on stream and remains almost constant to the end of the reaction time but at higher temperatures it escalates by time on steam and has more intense slope of increase as the temperature rises. Ethylene formation increases by time and by raising the temperature. Ethane value has not a clear trend by time and temperature and oscillates between 1 and 3 mol percent. Propylene production
3. Results and discussion 3.1. Catalyst characterization results The X-ray diffraction (XRD) patterns of the synthesized SAPO-34 catalyst and the reference pattern (Lok et al., 1984b) have been shown at Fig. 2. As it can be seen, the identifier diffraction peaks of the reference catalyst have been appeared on the XRD pattern of the synthesized catalyst proving the SAPO-34 phase formation. The scanning electron microscopy (SEM) of the prepared SAPO34 catalyst shows the uniform cubic crystals of SAPO-34 at the range of 1e3 micron (Fig. 3). The results of other characterization tests such as BET, temperature programmed desorption (TPD) and X-ray fluorescence (XRF) have been tabulated at Table 1. Descending amount of phosphorous from 49.82 %wt in the starting gel to 41.59 %wt in the final catalyst can evidently show the mechanism of phosphorous replacement by silicon and hydrogen to produce bridging hydroxyl group (-SiOHAl-) as Brønsted acidic sites in the neutral framework of alumino phosphate (AlPO4) molecular sieve (Ashtekar et al., 1994; Sastre et al., 1997; Vomscheid et al., 1994). The TPD results also shows two moderate weak and medium acid sites and a slight amount of a strong acid site at desorption temperature of 450e600 C. The positive effects of low silicon content and as a sequence, moderate acid sites on descending the propane formation and enhancing the catalyst life time in MTO reaction have been reported (Wilson and Barger, 1999; Strohmaier, 2004; Cao et al., 2004). Proper physicochemical characteristics of the prepared catalyst such as uniform catalyst sizes, small crystals, high surface area (437 m2/gCat) and moderate acid sites as proven by the
Fig. 2. The XRD patterns of a) the synthesized catalyst b) the reference SAPO-34.
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535
Fig. 3. The SEM images of synthesized SAPO-34 catalyst at different orders of magnitude.
goes up by time at temperature of 400 C and declines by time at þ higher temperatures. Amounts of Propane, C¼ 4 complexes and C5 þ fraction drop by time at any temperature. C5 fraction decrease as the temperature goes up. This could be explained by the fact that, increasing the temperature causes the heavier components (Cþ 5 ) to be cracked in to lighter ones. It has been proven earlier that at higher temperatures, the amount of coke formation increases (Chen et al., 2012), but longer catalyst life time at 460 C compared to 400 C should be due to the types of coke form at different temperatures. As the Cþ 5 fraction at 460 C has a lower amount in
comparison to 400 C, the coke deposits on the catalyst surface at 460 C may contains lighter components with shorter chains that cannot block the catalyst pores rapidly and in spite of the higher amount of coke content at 460 C, the catalyst life time increases. At 400 and 430 C, the catalyst life times were 8 and 7 h, respectively and at 460 C longest catalyst life time was recorded. DTO reaction over the synthesized SAPO-34 was performed at 400, 430 and 460 C and the molar distributions of products have been also illustrated at Fig. 4. In DTO reaction, ethane, propane, þ methanol, C¼ 4 complexes and C5 fraction go down by time on
Table 1 The XRF, BET and TPD results of the synthesized SAPO-34 catalyst. XRF Comp. Al Si P
BET Starting gel 43.40% wt 6.78% wt 49.82% wt
TPD
Final catalyst 48.20% wt 10.21% wt 41.59% wt
2
Surface area (m /gCat) Average particle size (mm) Micropore volume (cm3/gcat)
437 1.9 0.23
Temperature range (C)
mmolt-Butylamin/gcat
150e300 350e450 450e600
0.187 0.121 0.035
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Fig. 4. Molar products distribution of MTO and DTO reactions at different temperatures.
stream while methane and ethylene go up as the reaction proceeds. Propylene increases at the reaction beginning and then decreases. By raising the reaction temperature, methane, ethane, propane, C¼ 4 complexes and Cþ 5 fraction decline and light olefins formation increases (Fig. 4d, e, f). Li et al. (2014) have reported that the MTO and DTO reactions have similar selectivities, but our findings are in good agreement to the previous article (Chen et al., 1999) that reported there are substantial differences in methane and light olefins selectivities in these two reactions. Methane formation in DTO reaction is higher than MTO reaction and has a different trend as it increases by decreasing the temperature. As opposed to the research expounding to elucidate the mechanism of CeC bond formation in MTO reaction, mechanistic investigation of methane formation in MTO reaction have been reported in a few literature (Kirmse, 1964; Mihail et al., 1983; Hutchings and Roger Hunter, 1987) in which methane could arise via a radical pathway
involving DME. Mihail et al. (1983) developed a kinetic model in the case of MTO reaction which methane formation proceeds via reaction of carbene intermediate generating by DME, with molecular hydrogen. The mechanism of methane formation in this kinetic model has been mentioned below:
CH3 OCH3 /2 : CH2 þ H2 O : CH2 þ 2H2 /CH4 Also, another possible reaction mechanism leading to methane formation proposing by Obrzut et al. (2003) is as follow:
CH3 OCH3 þ H2 O/2CO þ 4H2 2CO þ 2H2 /CO2 þ CH4
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537
Fig. 5. Olefins selectivity (ethylene, propylene and butylenes) of MTO and DTO reactions versus time on stream at different temperatures.
The above mechanism also shows that methane production is DME dependent. Based on above, it is expected that methane production would be higher in DTO reaction than in MTO. Light olefins selectivities have been depicted at Fig. 5. As it can be seen, in DTO reaction light olefins' selectivities enhances by temperature dramatically due to simultaneous increase of ethylene and propylene. On the other hand, in MTO reaction the selectivity to light olefins increases slightly, because by rising the temperature ethylene production goes up and propylene production falls down. In both of the reactions, olefin selectivity increases by time on stream. MTO reaction seems to be more selective in olefins production than DTO and two of its main reasons are DME conversion to methanol and high amount of methane formation in DTO reaction. ¼ In both of the reactions, C¼ 2 /C3 molar ratio enhances by time on stream and by raising the temperature as shown in Fig. 6. Although, Dubois et al. (2003) have reported that in MTO reaction, the catalyst life time at 400 C is twice the life time at 450 C, but completely different results were recorded here. Based on the UOP/Hydro license (Eng et al., 1998a, 1998b; Chen et al., 2005) there should be a
certain range of operating condition that without any activity loss ¼ or catalyst life time depletion, the C¼ 2 /C3 ratio can be considerably changed to meet the market demand and based on the average values presented at Table 2 and 400e460 C with acceptable ¼ catalyst life time is the optimum range of MTO reaction with C¼ 2 /C3 molar ratio of 1.6e2.4. By passing the time of stream on the catalyst and partial deposition of coke causing more diffusion constrains for heavier hydrocarbons, the selectivity to light olefins specially ethylene enhances rapidly (Bos et al., 1995; Qi et al., 2007). At 400 C, it took 2.5 h to reach the light olefins' molar fraction higher than 70%, but at 430 and 460 C, this was happened after several minutes (Fig. 5). Qi et al. (2007) have also found the best light olefins selectivity over SAPO-34 catalyst by coke content of about 5.7% wt. At industrial MTO process, to have the highest amount of light olefins' selectivity, the catalysts are partially regenerated and the profitable amount of coke is maintained on the catalyst surface; so, the first unwanted selectivity of SAPO-34, for instance, at 400 C should not be taken into account, and the actual products' distribution is after a while of reaction. Hence, the average molar fractions of products
¼ Fig. 6. C¼ 2 /C3 molar ratios of MTO and DTO reactions versus time on stream at different temperatures.
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Table 2 Average molar distribution of Products in MTO/DTO reaction at the specified period with the acceptable light olefins production rate (light olefins selectivity more than 70%). Outlet components (mole %) C1 C¼ 2 C2 C¼ 3 C3 DME MeOH C¼ 4 Cþ 5 Sum ¼ C¼ 2 & C3 ¼ ¼ C¼ 2 & C3 & C4 ¼ (C¼ /C )ratio 2 3 Light olefins selectivity Conversion Olefins selectivity Time on stream (h)
MTO 400 C
MTO 430 C
MTO 460 C
10.7 50.8 1.6 23.5 2.0 0.2 5.9 4.4 1.0
100.0
e
100.0
100.0
78.4 83.6 2.4 78.4 99.5 87.4 10.5
e e e e e e e
71.0 74.2 2.2 78.0 99.9 84.1 2.5
74.3 78.8 2.2 78.1 99.8 86.2 7.0
8.1 55.3 2.6 23.1 3.1 0.4 1.1 5.2 1.1
100.0
100.0
78.8 85.4 1.6 77.0 99.1 87.7 6.0
79.8 86.6 1.9 76.9 99.4 88.0 6.0
SAPO-34 catalyst with Proper physicochemical characteristics was synthesized, and its activity in MTO and DTO reactions was assessed at 400e460 C. Products distribution by time on stream and by raising the temperature was analyzed for both of the reactions and in summary, the following three important point were concluded: firstly, in DTO reaction at higher temperatures, the equilibrium of DME hydrolysis shifts toward methanol, which makes methanol partial pressure to be higher, and consequently the kinetics and selectivities of the DTO and MTO reactions are much more similar. Secondly, as the DME to methane reaction is more exothermic than DME to methanol, so at low temperatures DME to methane reaction is more favorable and methane molar fraction increases dramatically that is in complete contrast to methane formation in MTO reaction. Thirdly, as MTO reaction is so fast over SAPO-34 catalyst that does not allow the escape of the bigger molecules and catalyst deactivation happens immediately, the use of DME as the reactant is useful in holding the partial pressure of methanol low in the reactor, from which better catalyst stability can be achieved. Generally, it can be say that the induction time of DTO reaction is longer and its catalyst deactivation is gradually. As evidence, the Cþ 5 fraction in DTO reaction is much lower than that of MTO reaction and coke deposition due to creation of heavy hydrocarbons happened slower and catalyst life time is longer than MTO. Besides, in DTO reaction, the light olefins' selectivities are lower because of two main reasons that are DME conversion to methanol and high amount of methane formation. In conclusion, by taking all of these into account, DME does not seem to be a competitive feed for olefins production in comparison to methanol.
DTO 460 C
19.6 48.9 2.0 22.1 1.5 0.1 1.9 3.2 0.7
3.6 52.4 2.0 27.3 3.8 0.8 1.5 6.8 1.8
4. Conclusion
DTO 430 C
e e e e e e e e e
3.0 48.4 2.4 30.4 3.1 2.4 2.2 6.6 1.7
in this period of time on stream with acceptable light olefin selectivities were listed at Table 2. This period was 10.5 h at 460 C and 6 h for 400 and 430 C. The highest light olefins' molar fraction of 79.8 was reported at 430 C. As shown in Table 2, amounts of þ ¼ ¼ propane, C¼ 4 complexes, C5 fraction and C2 /C3 molar ratios are less than MTO reaction. Although catalyst life time increases in DTO reaction but light olefins' selectivities are not competitive to the MTO reaction. At 400 C the light olefins' molar fraction does not even exceed 40% and periods of time on stream with light olefins' molar fraction higher than 70% were 2.5 and 7 h for 430 and 460 C as presented in Table 2.
DTO 400 C
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