Article Cite This: Energy Fuels XXXX, XXX, XXX-XXX
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Differences in Product Distribution Measured with Flame Ionization Detector Gas Chromatography and Thermal Conductivity Detector Gas Chromatography during the Dimethyl Ether-to-Olefins and Methanol-to-Olefins Processes Yuli Gao, Yingqian Cao, Sheng-Li Chen,* Ya Wang, Ruyue Zhu, Wei Sun, Qi Zhang, and Yu Fan State Key Laboratory of Heavy Oil Processing, Chemical Engineering Department, China University of PetroleumBeijing, Beijing 102249, People’s Republic of China S Supporting Information *
ABSTRACT: It is generally accepted that the products of dimethyl ether (DME)-to-olefins (DTO) and methanol-to-olefins (MTO) processes are hydrocarbons; therefore, gas chromatography (GC) equipped with a flame ionization detector (FID) rather than a thermal conductivity detector (TCD) is usually used to measure their product distribution for the higher response sensitivity of FID than TCD. Product distributions of DTO and MTO processes over SAPO-34 and metal-oxide-modified SAPO-34 catalysts were measured in this research work with GC equipped with FID and TCD. Results showed that product distribution over SAPO-34 measured with FID GC alone was similar to that measured with TCD + FID GC when the DME or methanol conversions were close to 100% and the main products were hydrocarbons, with only trace amounts of H2 and carbon oxides in the products. When using metal-oxide-modified SAPO-34 as the catalyst, it was found that the product distribution measured by TCD + FID GC was greatly different from that measured by FID GC alone. A lot of H2 and carbon oxides were detected with TCD + FID GC, which are missed when using the sole FID GC to analyze the products. The accurate measurement of the complete product distribution spectrum is of importance for not only the calculation of product yields, mass balance, and reaction heat but also the investigation of reaction mechanisms and reaction kinetics. It is strongly suggested that TCD + FID GC be used to analyze the product distribution of DTO and MTO processes, especially when SAPO-34 is promoted with other components. compounds9,10 if a suitable carrier gas is used. Possible products of carbon oxides and hydrogen can only be detected by TCD GC. Because most compounds have a thermal conductivity much less than that of H2 or He, to have the TCD signal as large as possible, H2 or He is usually used as the carrier gas. There were some investigators who used TCD GC to measure DTO/MTO product distribution, but most of them reported that there were no CO, CO2, and H211−15 or a trace amount of carbon oxides16−20 was found in the DTO/MTO products. The reasons for this are that it is not easy to detect all components of oxygen-containing hydrocarbons, hydrocarbons, CO, CO2, and H2 in a gas mixture containing N2 and H2O vapor. To measure the contents of carbon oxides, it is necessary to use H2 or He as the carrier gas, instead of N2, because the conductivity difference between H2 and carbon oxides is greater and that between N2 and carbon oxides is smaller. For the same reason, to measure the content of H2, it is necessary to use N2 as the carrier gas, instead of He, which has conductivity similar to that of H2. Besides, we need different kinds of GC columns to separate every component in the MTO/DTO effluent, because the boiling points and polarities of these components are greatly different. Therefore, even if the effluent of MTO is analyzed with TCD GC, if the columns are not right and/or
1. INTRODUCTION Methanol/dimethyl ether (DME)-to-olefins (MTO/DTO) conversion over microporous solid acid catalysts to produce light olefins (ethylene and propylene) has attracted intense attention of chemical engineers and chemists.1−4 The SAPO-34 molecular sieve with a well-defined framework and chabazite cages connected via eight-ring windows is considered as the best MTO/DTO catalyst, owing to its highest selectivity to light olefins.5,6 It is generally accepted that the products of DTO/MTO processes are hydrocarbons; therefore, gas chromatography (GC) equipped with a flame ionization detector (FID) rather than a thermal conductivity detector (TCD) is usually used to measure their product distribution for the higher response sensitivity of FID than TCD. Fatemi and co-workers7 reported that CO, CO2, and H2 could be formed in the MTO reaction over SAPO-34. The operation of the FID is based on the detection of carbonaceous ions formed during combustion of organic compounds in a hydrogen flame. The generation of these ions is proportional to the concentration of organic species in the sample gas stream. The TCD senses change in the thermal conductivity of the column effluent and are compared to a reference flow of carrier gas. When an analyte elutes from the column, the effluent thermal conductivity is changed and a detectable signal is produced. Therefore, FID GC is only applicable to detect carbonaceous organic compounds,8 while TCD GC based on the difference in thermal conductivity can be applied to detect almost all kinds of © XXXX American Chemical Society
Received: July 22, 2017 Revised: October 26, 2017 Published: October 31, 2017 A
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Analysis Functions of the GC Columns GC equipment
column
detector
carrier gas
components analyzed in this work
SP 3420 Agilent 6890
Agilent HP-PLOT Q 5A molecular sieve column HP-PLOT Al2O3/S capillary column Porapak Q packed column (1/8 in. × 3 ft) Porapak Q packed column (1/8 in. × 6 ft) 13X molecular sieve column
FID TCD FID
N2 N2 N2 H2 H2 H2
methanol, DME, and C1−C6 hydrocarbons H2 C2−C6 hydrocarbons pre-separation of components with a carbon number less than 3 CO2 CO and CH4
TCD TCD
column (1/8 in. × 10 ft), two Porapak Q packed columns (one column is 1/8 in. in diameter and 3 ft in length, and the other column is 1/8 in. in diameter and 6 ft in length), and two detectors, TCD and FID. The analysis functions of the GC columns used to analyze the products of MTO/DTO are summarized in Table 1. Agilent 6890 GC consisted of three systems for separation and analysis of the product. The first system, including a 5A molecular sieve column and TCD, was used for analyzing H2, employing N2 as the carrier gas. The second system, including HP-PLOT Al2O3/S capillary column and FID, was employed to measure C2−C6 hydrocarbons, using N2 as the carrier gas. The third system, comprising two Porapak Q packed columns (one column is used to pre-separate the components with a carbon number less than 3, and the other column is used to separate CO2), 13X molecular sieve column (separating CO and CH4), and TCD, was used to analyze CO2, CO, and CH4, employing H2 as the carrier gas. The components injected into GC were switched among these systems by the use of four switching valves, among which there were three valves with sample loops of 1 mL for sample quantification. For the detection of H2, to have high TCD response sensitivity, N2 rather than He was used as the carrier gas, because the thermal conductivity difference between H2 and N2 is much larger than that between H2 and He. In addition, because the FID has much higher response sensitivity of hydrocarbons than the TCD, to obtain accurate data of low-concentration hydrocarbons, the FID was used to analyze the hydrocarbons in the MTO/DTO gaseous products. The minimum concentration of hydrocarbon that can be detected by the FID is ∼5 ng/mL (∼4 × 10−6 mL/mL for C2H4). The minimum concentration of carbon oxides that can be detected by the TCD is 2 × 10−5 mL/mL. The analysis errors with Agilent 6890 GC and SP 3420 GC are ∼5 and 2%, respectively. The relative hydrocarbon contents measured with SP 3420 GC are almost identical to those measured with Agilent 6890 GC. A large amount of water is formed during the DTO/MTO processes (>50 wt % H2O in the products for MTO). After the reaction effluent flows out of the reactor, the effluent temperature goes down to room temperature quickly and the water vapor in the effluent condenses, so that the water content is difficult to be accurately measured with GC. Therefore, the water content in the products was calculated by an oxygen molar balance rather than measured by GC analysis. In addition to hydrocarbons, carbonaceous products of the DTO/ MTO reaction also include coke, CO, CO2, formaldehyde, formic acid, etc. Our previous research indicated that formaldehyde and formic acid were formed during the MTO process.22 Because these components are unstable under the reaction conditions, their concentration in the effluent is too low to be detected with TCD GC. Formaldehyde and formic acid also cannot be measured by FID GC because of their insensibility to the FID. Thanks to their very low concentrations in the product, the concentration missing of these components in the product does not induce errors in the product distribution. To make the carbon mole balance, the amount of solid carbon is measured on the basis of the “coke” content of the catalyst, which was determined with a HIR-944B infrared carbon and sulfur analyzer (Wuxi HighSpeed Analyzer Co., China). Both DME and methanol were considered as reactants for the calculation of conversion. DME and methanol conversions were measured on a carbon basis. DME conversion in the DTO reaction and methanol conversion in the MTO reaction were defined as the conversion of oxygenates (DME and methanol). The catalyst lifetime
only one carrier gas is used, some products may be missed in the GC analysis data. In present work, to measure the contents of all possible products formed in the MTO/DTO processes, we used GC equipped with both TCD and FID with carrier gas of H2 and N2 to detect the effluent of MTO/DTO. It was found that, besides the hydrocarbons, most of which were ethylene and propylene, significant amounts of CO, CO2, and H2 were formed on some metal-oxide-modified SAPO-34 catalysts, while the amounts of CO, CO2, and H2 formed on the pure SAPO-34 catalyst were negligible, especially when DME or methanol conversions were near 100%.
2. EXPERIMENTAL SECTION 2.1. Catalyst Materials Used for MTO/DTO. SAPO-34 molecular sieve used in MTO/DTO was synthesized via the method of hydrothermal crystallization using triethylamine (TEA) and tetraethylamonium hydroxide (TEAOH) as the template. The molar composition of synthesis gel was 1.0 Al2O3/1.0 P2O5/0.2 SiO2/2.6 TEA/0.4 TEAOH/50 H2O. The detailed preparation procedures were described in the literature.21 Metal oxides, such as La2O3, MgO, MoO3, and Fe2O3, were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further treatment. The purity of the metal oxides was analytical reagent (AR). 2.2. MTO and DTO Reactions and Analysis of the Reaction Products. SAPO-34 and metal-oxide-modified SAPO-34 were used as the MTO and DTO catalysts. The metal oxides are those widely used to modify SAPO-34 in DTO or MTO, i.e., La2O3, MoO3, MgO, and Fe2O3. SAPO-34 or a mixture of SAPO-34 and metal oxide in a 1:1 mass ratio were individually pelletized, then crushed, and screened to particles of 0.45−0.90 mm in diameter. The DTO catalytic performance testing of the catalysts was carried out in a tubular quartz reactor with an inner diameter of 8.0 mm, under the conditions of a temperature of 470 °C, a gas hourly space velocity (GHSV) of 875 mL gcat−1 h−1, and atmospheric pressure. DME was fed at the top of the reactor. MTO catalytic performance testing was performed in a similar way; however, a 95 wt % methanol−water solution was fed at the top of the reactor with a tranquil flow pump and mixed with a nitrogen diluent (N2/methanol = 5:1 mol ratio), and the weight hourly space velocity (WHSV) of methanol was 2.5 h−1 (WHSV = methanol mass flow rate per mass of SAPO-34). The effluent of DTO or MTO was analyzed via two GC equipment. Methanol, DME, and hydrocarbons in the effluent were measured by GC (SP 3420, Beifen, China) equipped with a FID and an Agilent HPPLOT Q capillary column (30 m × 0.53 mm), employing N2 as the carrier gas. The gas sample size injected into SP 3420 GC was 1 mL. Because SP 3420 GC cannot be used to analyze inorganic gases, H2, CO, and CO2, another GC (Agilent 6890) was employed to analyze these inorganic gases as well as hydrocarbons. Because the components of H2, CO, CO2, C1−C6 hydrocarbons, and possibly N2 and O2 in the effluent of the MTO/DTO reactor have greatly different properties, any one GC column is unable to separate all of the components. Therefore, different kind types of GC columns are needed for the analysis of gaseous products. Agilent 6890 was equipped with a 5A molecular sieve column (1/8 in. × 6 ft), HP-PLOT Al2O3/S capillary column (0.53 mm × 50 m), 13X molecular sieve B
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. DME conversion and product distribution in the effluent (H2O is excluded) on SAPO-34: (A) DME conversion measured by FID GC alone, (B) product distribution measured by FID GC alone, (C) DME conversion measured by TCD + FID GC, and (D) product distribution measured by TCD + FID GC (reaction conditions: 470 °C and GHSV for DME of 875 mL gcat−1 h−1).
Figure 2. DME conversion and product distribution in the effluent (H2O is excluded) on the SAPO-34/La2O3 catalyst: (A) DME conversion measured by FID GC alone, (B) product distribution measured by FID GC alone, (C) DME conversion measured by TCD + FID GC, and (D) product distribution measured by TCD + FID GC (reaction conditions: 470 °C, GHSV for DME of 875 mL gcat−1 h−1, and SAPO-34/La2O3 with 1:1 mixed in mass).
C
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels was defined as the time when DME or methanol conversion fell to nearly 0%.
of reactant reacted. On the basis of TCD + FID GC product analysis, assuming that coke on the catalyst does not contain any oxygen element, we calculated the yield of H2O on different catalysts from the oxygen balance, and the results are shown in Table 2. It was found that the yields of H2O were less
3. RESULTS AND DISCUSSION Figure 1 shows DME conversion and product distribution (H2O is excluded) in the effluent on SAPO-34 in the DTO reaction. DME conversion and product distribution, shown in panels A and B of Figure 1, were determined by FID GC (SP 3420) alone as the previous researchers usually did, while those shown in panels C and D of Figure 1 were determined by TCD + FID GC (Agilent 6890 + SP 3420). Bar graphs shown in Figure 1 represent the normalized mole percentage from the gas chromatographs. When DME conversion is close to 100%, DME conversion measured by FID GC alone is similar to that measured by TCD + FID GC. In the initial stage of DTO, ∼8 mol % H2 and a trace amount of CO2 and CO were detected by TCD + FID GC. With the time on stream passing, the H2 content reduces, while the CO content increases. Anyway, because the non-hydrocarbon components (H2, CO2, and CO) are much less than hydrocarbons, the effluent detection with FID GC alone would not result in a large error for the product distribution. When DME conversion is less than 100%, DME conversion and product distribution obtained with FID GC alone were significantly different from those obtained with TCD + FID GC. When the DME conversion was ∼22%, a significant amount of CO (∼11 mol %) was detected. Thus, the DME conversion measured by FID GC alone (lack of CO content) is lower than that measured by TCD + FID GC. As shown in panels B and D of Figure 1, when the conversion is ∼22%, the true selectivity to ethylene plus propylene is ∼30% (obtained with TCD + FID GC), while the obtained selectivity to ethylene plus propylene is ∼40 mol % if measured with FID GC alone. Figure 2 shows DME conversion and product distribution in the effluent (H2O is excluded) of the DTO reaction on La2O3modified SAPO-34 (SAPO-34/La2O3 catalyst). DME conversions determined with TCD + FID GC are higher than those determined with FID GC alone. As shown in Figure 2D, in the initial stage of rgw DTO reaction, the selectivity to H2, CO2, and CO on SAPO-34/La2O3 is ∼15 mol % and then increases up to more than 40 mol % with the evolution of time on stream. The average selectivity to ethylene plus propylene, measured with TCD + FID GC, is ∼63 mol % on SAPO-34/ La2O3. However, if the product composition is measured with the FID GC alone, the average selectivity to ethylene plus propylene on SAPO-34/La2O3 is ∼80 mol %. Besides, we also measured DME conversion and product distribution in the DTO reaction over MgO-, MoO3-, or Fe2O3modified SAPO-34 as well as those in the MTO reaction over SAPO-34 and MgO-modified SAPO-34, and these results are shown in Figures S1−S5 of the Supporting Information. It was also found that DME (or methanol) conversions over the metal-oxide-modified SAPO-34 determined with TCD + FID GC were higher than those determined with FID GC alone and more than 15 mol % selectivity to H2, CO2, and CO was detected by TCD + FID GC. As a consequence, if FID GC alone was used to detect the effluent composition of DTO on metal-oxide-modified SAPO-34, large errors would occur in composition analysis of products, and hence, mistakes would be made in the catalytic performance evaluation and reaction mechanism establishment. According to the stoichiometry of DTO and MTO, the H2O yield should be 1 mol of H2O/mol
Table 2. Calculated Yield of H2O on Different Catalysts over the Lifetime of the Catalyst catalyst SAPO-34 (DTO) SAPO-34/La2O3, 1:1 (DTO) SAPO-34/MgO, 1:1 (DTO) SAPO-34/MoO3, 1:1 (DTO) SAPO-34/Fe2O3, 1:1 (DTO) SAPO-34 (MTO) SAPO-34/MgO, 1:1 (MTO)
calculated yield of H2O (molH2O/molcarbon in reactants) 0.9768 0.8958 0.8724 0.4284 0.2320 0.9961 0.8656
than 1 mol of H2O/mol of reactant reacted, owing to the formation of carbon oxides. The more the carbon oxides formed, the less the H2O yield. The carbon mole balance, i.e., the carbon-based product distribution, was calculated with TCD + FID GC analysis data and the coke content on the DTO/MTO catalysts, and the results are summarized in Table 3. CO, CO2, and H2 are formed during the MTO or DTO processes. When the plain acid molecular sieve SAPO-34 is used as the catalyst, the amounts of CO, CO2, and H2 formed are very low in comparison to other hydrocarbon products; however, when using metal-oxide-modified acid molecular sieve SAPO-34, significant amounts of CO, CO2, and H2 are formed during MTO or DTO. Because the CO, CO2, and H2 components have no FID response, if FID GC alone is used to analyze the product distribution, these non-hydrocarbon components would be totally missed, resulting in big errors in the product analysis. For example, in the case of SAPO-34/MoO3 as the catalyst, if FID GC alone was used to analyze the effluent composition, the error in the obtained mole balance could be ∼19%. Reliable mole balance (material balance) is a prerequisite for the reaction heat (energy balance) calculations. We calculate reaction heat (enthalpy change), and the results are shown in Table 4. The reaction stoichiometry in Table 4 is determined from the FID + TCD GC analysis data and the carbon content of the activated catalyst. Haw and co-workers reported that “coke” deposited on SAPO-34 molecular sieves consisted essentially of methylnaphthalene and a small amount of methylbenzene.23 Herein, methylnaphthalene is assumed to be coke species. For comparison, the reaction heat of “ideal” DTO/MTO reactions (products involve only ethylene, propylene, and water) was also calculated and listed in Table 4. As shown in Table 4, when SAPO-34 is used as the catalyst of DTO/MTO, the reaction heat is higher than that of ideal DTO/MTO. When metal-oxide-modified SAPO-34 was used as the catalyst of DTO/MTO, the reaction heat decreases significantly, owing to the formation of CO2, CO, and H2. If FID GC alone was used to analyze the product distribution, because some product components (H2, CO, and CO2) were missed, we cannot obtain the chemical reaction D
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. MTO/DTO Carbon Mole Balance (i.e., Carbon-Based Product Distribution) product distributiona (on the basis of carbon mol %)
catalyst
conversion (%)a
CH4
C2H4
C2H6
C3H6
C3H8
C4
C5
C6+
CO2
CO
coke
selectivity to carbon oxides by TCD + FID GCa (mol %)
100 100
5.25 4.18
44.52 43.62
0.92 0.61
31.48 33.27
0.72 0.49
6.54 7.97
1.67 2.28
0.29 0.51
0.00 1.47
0.60 2.23
8.01 3.36
0.60 3.70
130 340
100
5.41
35.05
0.67
36.52
0.59
9.34
2.57
0.52
1.36
4.62
3.36
5.98
340
100
23.83
23.42
1.48
15.27
0.52
4.27
1.26
0.27
12.96
6.29
10.42
19.25
100
100 100
4.53 4.55
53.84 48.05
0.39 0.34
25.76 27.18
0.19 0.20
7.31 7.48
3.72 3.69
0.00 0.00
0.00 1.50
0.00 4.38
4.25 2.64
0 5.88
245 395
SAPO-34 (DTO) SAPO-34/La2O3, 1:1 (DTO) SAPO-34/MgO, 1:1 (DTO) SAPO-34/MoO3, 1:1 (DTO) SAPO-34 (MTO) SAPO-34/MgO, 1:1 (MTO) a
catalyst lifetime (min)
Conversion, product distributions, and selectivity to carbon oxides determined by TCD + FID GC were obtained at 1/2 the catalyst lifetime.
observed a higher selectivity to ethylene on the modified catalysts relative to Ni-SAPO-34 and thought that the decrease in the number of acid sites on the external surface of the modified catalysts may be the cause of the observation.24 Sedighi et al. studied the effect of metal (Fe, Co, Ni, La, and Ce) on the MTO catalytic performance of SAPO-34, and the product analysis was also performed on FID GC only.25 On the basis of their FID GC analysis data, they drew a conclusion that metal-modified SAPO-34 favored the ethylene and propylene products, which is attributed to the decrease in acidity of catalysts as a result of the incorporation of metals.25 Behbahani and co-workers investigated the product distribution of the methanol-to-gasoline (MTG) reaction over Zn-modified ZSM5.28 From analysis of gas- and liquid-phase organic products using FID GC, they found that Zn-modified ZSM-5 enhanced the production of C5+ hydrocarbon and xylene and concluded that this enhancement was a consequence of the synergetic effect between Zn species and acid sites of ZSM-5.28 Significant amounts of H2, CO, and CO2 may be formed in these research works, and these components were totally missed during the FID GC analysis. If TCD + FID GC had been used to analyze the product distribution, different conclusions should have been made. Different from these previous studies, our research group employed TCD + FID GC to analyze MTO products and found that a lot of non-hydrocarbon components, H2, CO, and CO2 were formed during the MTO process over MgO (or MoO3)-modified SAPO-34.22 According to the analysis results of TCD + FID GC, we have not only obtained accurate product distribution but also proposed a new mechanism coupling acid catalysis on SAPO-34 with methanol oligomerization on MgO.22
Table 4. Reaction Enthalpy Changes of DTO/MTO catalyst a
ideal DTO ideal MTOb SAPO-34 (DTO)c SAPO-34/La2O3, 1:1 (DTO)d SAPO-34 (MTO)e SAPO-34/MgO, 1:1 (MTO)f
reaction enthalpy change (kJ/mol) −11.36 −18.22 −16.28 −11.14 −19.76 −15.30
a
The ideal DTO reaction included only ethylene, propylene, and water, and 50 mol of DME was converted to 66.6 mol of ethylene, 33.4 mol of propylene, and 50 mol of water. Reaction equation: 50CH3OCH3 → 33.3C2H4 + 11.13C3H6 + 50H2O(g). bThe ideal MTO reaction included only ethylene, propylene, and water, and 100 mol of methanol was converted to 66.6 mol of ethylene, 33.4 mol of propylene, and 100 mol of water. Reaction equation: 100CH4O → 33.3C 2 H 4 + 11.13C 3 H 6 + 100H 2 O(g). c Reaction equation: 50CH3OCH3 → 3.88CH4 + 21.55C2H4 + 0.38C2H6 + 9.37C3H6 + 0.22C3H8 + 1.61C4H6 + 0.54C5H8 + 0.98C6H12 + 0.45CO + 0.73C 11 H 10 + 2.92H 2 + 49.55H 2 O(g). d Reaction equation: 50CH3OCH3 → 3.37CH4 + 22.19C2H4 + 0.29C2H6 + 10.9C3H6 + 0.17C3H8 + 2.10C4H6 + 0.47C5H8 + 0.23C6H12 + 1.18CO2 + 1.79CO + 0.31C11H10 + 7.70H2 + 45.84H2O(g). eReaction equation: 100 CH4O → 4.67CH4 + 31.32C2H4 + 0.33C2H6 + 6.48C3H6 + 0.10C3H8 + 0.68C4H6 + 1.07C5H6 + 0.39C11H10 + 0.04H2 + 100H2O(g). f Reaction equation: 100CH4O → 4.14CH4 + 25.20C2H4 + 0.18C2H6 + 9.56C3H6 + 0.09C3H8 + 0.74C4H6 + 1.05C5H8 + 1.36CO2 + 3.98CO + 0.24C11H10 + 10.88H2 + 93.29H2O(g).
equation with right stoichiometry coefficients. Therefore, it is impossible to calculate the reaction heat. Correct measurements of the product distribution are essential to not only the evaluations of the conversion of the reactant, product yields, mass balance, reaction heat, and catalytic performance of catalysts used but also the correct practical application judgment of the reaction and establishment of reaction mechanisms and reaction kinetics. If metaloxide-modified acid molecular sieve is used as the catalyst of methanol or DME conversion, a lot of hydrogen and carbon dioxide are formed and it is necessary to use FID + TCD GC to analyze the gaseous products. A lot of previous studies employed FID GC alone to analyze the gaseous product composition of methanol conversion to olefins, aromatics, or gasoline over metal-oxide- or metalmodified acid molecular sieves SAPO-34 or ZSM-5.24−27 Kang and Inui investigated the MTO catalytic performance of basic alkaline- or alkaline-earth-metal-oxide-modified Ni-SAPO-34 through analyzing the gaseous products by FID GC only. They
4. CONCLUSION It is generally accepted that the products of methanol or DME conversion to olefins, gasoline, or aromatics are hydrocarbons, and FID GC is usually used to measure their product distribution as a result of the higher FID response sensitivity of hydrocarbons. We found that some FID non-response components of CO, CO2, and H2 were formed during the methanol/DME conversion processes. When plain acid molecular sieves are used, the catalyst of methanol (or DME) conversion, the amounts of CO, CO2, and H2 formed are small and the use of FID GC alone to analyze the product composition would not induce a big error. In the case of metal-oxide-modified acid molecular sieves as the catalyst of E
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
(5) Dai, W.; Wu, G.; Li, L.; Guan, N.; Hunger, M. Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts. ACS Catal. 2013, 3 (4), 588−596. (6) Li, Y.; Zhang, M.; Wang, D.; Wei, F.; Wang, Y. Differences in the methanol-to-olefins reaction catalyzed by SAPO-34 with dimethyl ether as reactant. J. Catal. 2014, 311, 281−287. (7) Najafabadi, A. T.; Fatemi, S.; Sohrabi, M.; Salmasi, M. Kinetic modeling and optimization of the operating condition of MTO process on SAPO-34 catalyst. J. Ind. Eng. Chem. 2012, 18 (1), 29−37. (8) Holm, T. Mechanism of the flame ionization detector II. Isotope effects and heteroatom effects. J. Chromatogr. A 1997, 782 (1), 81−86. (9) Tadesse, K.; Smith, A.; Brydon, W. G.; Eastwood, M. Gas chormatographic technique for combined measurement of hydrogen and methane using thermal conductivity detector. J. Chromatogr. A 1979, 171, 416−418. (10) Jalali-Heravi, M.; Fatemi, M. Prediction of thermal conductivity detection response factors using an artificial neural network. J. Chromatogr. A 2000, 897 (1), 227−235. (11) Salmasi, M.; Fatemi, S.; Taheri Najafabadi, A. Improvement of light olefins selectivity and catalyst lifetime in MTO reaction; using Ni and Mg-modified SAPO-34 synthesized by combination of two templates. J. Ind. Eng. Chem. 2011, 17 (4), 755−761. (12) Tian, S.; Ji, S.; Lü, D.; Bai, B.; Sun, Q. Preparation of modified Ce-SAPO-34 catalysts and their catalytic performances of methanol to olefins. J. Energy Chem. 2013, 22 (4), 605−609. (13) Á lvaro-Muñoz, T.; Márquez-Á lvarez, C.; Sastre, E. Aluminium chloride: A new aluminium source to prepare SAPO-34 catalysts with enhanced stability in the MTO process. Appl. Catal., A 2014, 472, 72− 79. (14) Pajaie, H. S.; Taghizadeh, M. Optimization of nano-sized SAPO34 synthesis in methanol-to-olefin reaction by response surface methodology. J. Ind. Eng. Chem. 2015, 24, 59−70. (15) Ren, S.; Liu, G.; Wu, X.; Chen, X.; Wu, M.; Zeng, G.; Liu, Z.; Sun, Y. Enhanced MTO performance over acid treated hierarchical SAPO-34. Chin. J. Catal. 2017, 38 (1), 123−130. (16) Cai, G.; Liu, Z.; Shi, R.; He, C.; Yang, L.; Sun, C.; Chang, Y. Light Alkenes from Syngas via Dimethy Ether. Appl. Catal., A 1995, 125, 29−38. (17) Wu, L.; Hensen, E. J. M. Comparison of mesoporous SSZ-13 and SAPO-34 zeolite catalysts for the methanol-to-olefins reaction. Catal. Today 2014, 235 (0), 160−168. (18) Wu, X.; Abraha, M. G.; Anthony, R. G. Methanol conversion on SAPO-34: Reaction condition for fixed-bed reactor. Appl. Catal., A 2004, 260 (1), 63−69. (19) Dahl, I. M.; Kolboe, S. On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34: I. Isotopic labeling studies of the co-reaction of ethene and methanol. J. Catal. 1994, 149 (2), 458−464. (20) Stöcker, M. Methanol-to-hydrocarbons: Catalytic materials and their behavior. Microporous Mesoporous Mater. 1999, 29 (1), 3−48. (21) Wang, Y.; Chen, S.-L.; Jiang, Y.-J.; Cao, Y.-Q.; Chen, F.; Chang, W.-K.; Gao, Y.-L. Influence of template content on selective synthesis of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves used for methanol-to-olefins process. RSC Adv. 2016, 6 (107), 104985−104994. (22) Wang, Y.; Chen, S.-L.; Gao, Y.-L.; Cao, Y.-Q.; Zhang, Q.; Chang, W.-K.; Benziger, J. B. Enhanced Methanol-to-Olefin Catalysis by Physical Mixtures of SAPO-34 Molecular Sieve and MgO. ACS Catal. 2017, 7 (9), 5572−5584. (23) Fu, H.; Song, W.; Haw, J. F. Polycyclic aromatics formation in HSAPO-34 during methanol-to-olefin catalysis ex situ characterization after cryogenic grinding. Catal. Lett. 2001, 76 (1/2), 89−94. (24) Kang, M.; Inui, T. Effects of decrease in number of acid sites located on the external surface of Ni-SAPO-34 crystalline catalyst by the mechanochemical method. Catal. Lett. 1998, 53 (3−4), 171−176. (25) Sedighi, M.; Ghasemi, M.; Sadeqzadeh, M.; Hadi, M. Thorough study of the effect of metal-incorporated SAPO-34 molecular sieves on catalytic performances in MTO process. Powder Technol. 2016, 291, 131−139.
methanol or DME conversion, significant amounts of CO, CO2, and H2 could be formed. Because the CO, CO2, and H2 components have no FID response, if FID GC was used to analyze the product distribution, wrong conclusions on the product distribution, catalytic performance evaluation of catalysts, mass balance, reaction heat, and even reaction mechanisms and reaction kinetics could be made. To obtain the correct and accurate product distribution, it is necessary to use TCD + FID GC equipped with different kinds of GC columns to analyze the product composition with different carrier gases. FID should be used to analyze hydrocarbons. TCD should be used to analyze non-hydrocarbons, with N2 and H2 (or He) being used as the carrier gas to analyze H2 and carbon oxides, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02145. Thermal conductivity of different substances (Table S1), DME conversion and product distribution (H2O is excluded) on the SAPO-34/MgO catalyst (Figure S1), DME conversion and product distribution (H2O is excluded) on the SAPO-34/MoO3 catalyst (Figure S2), DME conversion and product distribution (H2O is excluded) on the SAPO-34/Fe2O3 catalysts (Figure S3), methanol conversion and product distribution in the gaseous products (H2O is excluded) of MTO using SAPO-34 as the catalyst (Figure S4), and methanol conversion and product distribution in the gaseous products (H2O is excluded) of MTO using SAPO-34/ MgO (1:1 mass mixture) as the catalyst (Figure S5) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 86-10-89733396. Fax: 86-10-69724721. E-mail:
[email protected]. ORCID
Sheng-Li Chen: 0000-0003-4718-7231 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research work was supported by the National Natural Science Foundation of China (Grant 91534120). REFERENCES
(1) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. The mechanism of methanol to hydrocarbon catalysis. Acc. Chem. Res. 2003, 36, 317−326. (2) Baek, S.-C.; Lee, Y.-J.; Jun, K.-W.; Hong, S. B. Influence of catalytic functionalities of zeolites on product selectivities in methanol conversion. Energy Fuels 2009, 23 (2), 593−598. (3) Song, W.; Wei, Y.; Liu, Z. Chemistry of the Methanol to Olefin Conversion. In Zeolites in Sustainable Chemistry; Xiao, F.-S., Meng, X., Eds.; Springer: Berlin, Germany, 2016; Green Chemistry and Sustainable Technology, pp 299−346, DOI: 10.1007/978-3-66247395-5_9. (4) Gao, Y.; Chen, S.-L.; Wei, Y.; Wang, Y.; Sun, W.; Cao, Y.; Zeng, P. Kinetics of coke formation in the dimethyl ether-to-olefins process over SAPO-34 catalyst. Chem. Eng. J. 2017, 326, 528−539. F
DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels (26) Lu, J.; Wang, X.; Li, H. Catalytic conversion of methanol to olefins over rare earth (La, Y) modified SAPO-34. React. Kinet. Catal. Lett. 2009, 97 (2), 255−261. (27) Kim, H.-S.; Lee, S.-G.; Kim, Y.-H.; Lee, D.-H.; Lee, J.-B.; Park, C.-S. Improvement of Lifetime Using Transition Metal-Incorporated SAPO-34 Catalysts in Conversion of Dimethyl Ether to Light Olefins. J. Nanomater. 2013, 2013, 1−9. (28) Fattahi, M.; Behbahani, R. M.; Hamoule, T. Synthesis promotion and product distribution for HZSM-5 and modified Zn/ HZSM-5 catalysts for MTG process. Fuel 2016, 181, 248−258.
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DOI: 10.1021/acs.energyfuels.7b02145 Energy Fuels XXXX, XXX, XXX−XXX
