Effect of Oxygenate Impurities on the Conversion of ... - ACS Publications

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Effect of Oxygenate Impurities on the Conversion of Alcohols to Olefins D. L. S. Nieskens,*,† D. Ferrari,‡ Y. Liu,‡ and S. A. de Putter† †

Dow Benelux B.V., P.O. Box 48, 4530 AA, Terneuzen, The Netherlands The Dow Chemical Company, 2301 North Brazosport Boulevard, Freeport, Texas 77541, United States

‡

S Supporting Information *

ABSTRACT: This Article is focused on the study of the effect of oxygenate impurities like esters and acids in the mixed methanol and ethanol feed stream on the conversion of methanol and ethanol to light olefins. The conversion of such components at 450 °C over ZSM-5 was investigated. Furthermore, the effect on conversion and selectivity of adding yttrium oxide to the catalyst was studied. It was shown that the presence of relatively small amounts of esters in the alcohol feed stream has a significant effect on catalyst lifetime. These esters will decompose on the zeolite surface leading to additional methanol or ethanol (depending on the type of ester) and to a carboxylic acid. The additional methanol or ethanol will react according to the alcohol to olefins mechanism, leading to either an increase or a decrease in the propylene/ethylene (P/E) selectivity ratio, respectively. The acid that is released upon decomposition of the ester is responsible for the decrease in lifetime of the zeolite by adsorption on the zeolite active sites. Addition of metal oxides, specifically yttria, to ZSM-5 limits this decrease in catalyst lifetime. The proposed pathway is by a stronger adsorption of the acid on yttria than on the active site of the zeolite. Removal of the acid by yttria can in some cases (typically when the ester conversion is not 100% without the yttria present) lead to a higher conversion of the ester and thereby indirectly to an effect on the P/E selectivity ratio due to the additional methanol or ethanol released.

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INTRODUCTION

The conversion of methanol and ethanol to olefins is a process that has been researched for many years, but its exact mechanism is still a topic of debate.12−20 A distinction can be made between the catalytic transformation of alcohols to olefins on the strong acid sites of the zeolite and the straightforward dehydration of ethanol into ethylene, which occurs on weak acid sites. The latter sites are known to deactivate much slower as compared to the former. Several excellent review papers have been published on the basic zeolite deactivation mechanism in the methanol to olefins reaction21,22 for either ZSM-5,23,24 SAPO-34,25−28 or Beta29 zeolite. Many details of this deactivation mechanism are still unclear. For HSAPO-34, there is a progressive aging turning the methylbenzenes (belonging to the hydrocarbon pool) into larger aromatic ring systems. These eventually block the transport of reactants and products leading to a drop in conversion. For ZSM-5, the studies show that coke formation on the catalyst is the main reason for the primary catalyst deactivation. The exact nature of this coke (single ring or polycyclic aromatic hydrocarbons, wax, etc.) is still being debated in the literature. Most of the deactivation caused by coke deposition is recoverable by an appropriate treatment in air at elevated temperature. In addition, however, there is a nonrecoverable activity loss due to dealumination. This Article is focused on the effect of oxygenate impurities like esters and acids in the mixed alcohol feed stream. The

Worldwide, due to the continued rise in oil price, significant research efforts are undertaken to deliver technology capable of producing ethylene and propylene from a wide variety of alternative feedstocks, such as biomass, natural gas, and coal.1 To this end, syngas is produced by the gasification of the alternative feedstock. Syngas can be converted to ethylene and/ or propylene via methanol by methanol to olefins (MTO) and methanol to propylene (MTP) technologies or via higher alcohols by dehydration.2 For MTO, both UOP/Hydro and DICP’s (Dalian Institute of Chemical Physics) DMTO (partner with ABB Lummus) are currently available for license and are based on a fluidized bed reactor using a SAPO-34 catalyst.3 For MTP, Lurgi’s MTP process and Tsinghua University’s FMTP are available for license.4,5 Lurgi’s process is based on a fixed bed reactor system using a ZSM-5 catalyst, while the FMTP uses a SAPO-34/SAPO-18 mixed catalyst in a fluidized bed reactor similar to that of MTO.6 Substantial research on the catalytic conversion of syngas to mixed alcohols has been conducted since the beginning of the 20th century.1,7−9 The Dow Chemical Co. developed CoMoS catalysts for mixed alcohols synthesis from syngas in the early 1980s.10,11 Recently, Dow restarted the research of syngas to mixed alcohols and the production of ethylene and propylene via mixed alcohols, that is, methanol and ethanol over zeolite catalysts. It was observed that very small amounts of esters and acids were produced with mixed alcohols during the syngas to mixed alcohols synthesis. It is desirable to know the effect of these oxygenate impurities on, for example, the catalyst lifetime in the conversion of alcohols to olefins. © XXXX American Chemical Society

Received: February 11, 2014 Revised: June 13, 2014 Accepted: June 17, 2014

A

dx.doi.org/10.1021/ie500595u | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Figure 1. Conversion (a) and selectivity (b−d) for a feed of pure methanol and ethanol and ZSM-5 as catalyst.

(DME). In this way, the reported conversion is the conversion to final products and byproducts as described in eq 1:

conversion of such components and their effect on the lifetime and selectivity of the catalyst under typical reaction conditions and catalyst were investigated. The effect of oxygenate contaminants on ZSM-5 has previously been studied by Minnie et al.30 and De Klerk et al.31 Furthermore, the effect on conversion and selectivity of adding yttrium oxide to the catalyst was studied. Davis et al. already showed that metal oxides are able to dehydrate alcohols.32 ExxonMobil has published several patent applications regarding the extension of the lifetime of SAPO-34 by physically mixing lanthanide and actinide oxides with SAPO34.33−38 Among these oxides, yttrium oxide exhibited the best effect in terms of lifetime extension.

XMeOH + DME = (FMeOH,in − FMeOH,out − 2*FDME,out) /FMeOH,in*100%

(1)

where FX is the molar flow rate of component X. Likewise, the conversion of ethanol is corrected for the reversible dehydration to diethyl ether (DEE). In this way, the reported conversion is the conversion to final products and byproducts as shown in eq 2: XEtOH + DEE = (FEtOH,in − FEtOH,out − 2*FDEE,out)/FEtOH,in

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*100%

EXPERIMENTAL SECTION Test Unit Details. The reaction is carried out in a continuous flow micro reactor system at ambient pressure. The reactor consists of a U-shaped quartz reactor with an i.d. of 4 mm, capable of holding a maximum of approximately 300 mg of catalyst. A typical amount of catalyst (50 mg) with a height of about 1/2 cm is placed between two plugs of quartz wool. The reactor can be heated to approximately 800 °C. The setup is equipped with an online Agilent 7890 gas chromatograph (GC) equipped with an HP-1 apolar column used for the separation of hydrocarbons and a Shin Carbon column for the analysis of CO2, O2, H2, N2, and CH4. Two detectors are used in series: (1) a thermal conductivity detector (TCD), which can analyze all types of gases, particularly useful for the analysis of CO2 and other permanent gases; and (2) a flame ionization detector (FID) used for the quantification of all hydrocarbons from C1 to C8, including aromatics (BTX) and organic oxygenates. Definitions. Conversion. The conversion of methanol is corrected for the reversible dehydration to dimethyl ether

(2)

The conversion of the ester is straightforwardly calculated as in eq 3: XEster = (FEster,in − FEster,out)/FEster,in*100%

(3)

Selectivity. The carbon-based selectivity of component X is defined as in eq 4: SX = CFX,out /CFall products*100%

(4)

where CFX,out is the carbon-based mole flow of component X as detected in the reactor outlet, and CFall products is the sum of the carbon-based mole flows of all products (components are labeled products when their outlet flow is higher than their inlet flow). Also, in these calculations, DME and DEE are not included in the sum of the carbon-based mole flows of all products because they are not considered as final products. In some charts, the P/E (propylene/ethylene) selectivity ratio is plotted, which is an industrially relevant measure of catalyst performance. Carbon Balance. The carbon balance (the ratio between the amount of carbon detected in the outlet of the reactor and the B



Article

Figure 2. Conversion (a) and selectivity (b−d) for a feed of pure methanol and ethanol and ZSM-5 + 20 wt % Y2O3 (the “g cat” on the x-axis refers only to the amount of ZSM-5, not to the total amount of yttria + ZSM-5).

Figure 3. Conversion (a) and selectivity (b−d) for a feed of methanol and ethanol + methyl-acetate and ZSM-5 as catalyst.

°C, followed by calcination at this temperature for 4 h. Afterward, it was pelletized, crushed, and sieved to a 300−600 μm fraction. This catalyst was used as is, or mixed with an additional 20 wt % Y2O3 having the same mesh size. Process Conditions. The process conditions were fixed at T = 450 °C, p = atmospheric, liquid flow = 1.06 g/h, N2 flow = 94 mL/min. This results in a gas-phase nitrogen concentration

amount of carbon entering the reactor) is not given explicitly for each run, but its typical value was between 95% and 105%. Catalyst. The catalyst was prepared and supplied by SüdChemie (currently Clariant International Ltd.).39 The zeolite used was a ZSM-5. The catalyst was used in its zeolite powder form, so not extruded with a binder. The powder was pretreated by heating with a ramp rate of 2 K/min to 550 C



Article

Figure 4. Conversion (a) and selectivity (b−d) for a feed of methanol and ethanol + methyl-acetate with 20 wt % Y2O3 mixed through the ZSM-5 catalyst (the “g cat” on the x-axis refers only to the amount of ZSM-5, not to the total amount of yttria + ZSM-5).

selectivities, while at the same time the conversion of ethanol (dehydration) remains high, leading to a high ethylene selectivity. For the next experiment, a mix was made of the standard amount of catalyst and an additional 20 wt % Y2O3. This mixture was tested under the same set of process conditions. The result is displayed in Figure 2. It is apparent from the figure that the selectivities were barely affected. There was, however, a significant impact on the catalyst lifetime: it increased to 1998 g of alcohol fed/gram catalyst, which represents a 30% increase. Note that the feed consists only of methanol and ethanol for this experiment. It is hypothesized that there are either oxygenate impurities in the feed or produced during the alcohols to olefins reaction that are converted or otherwise removed by the yttria, leading to an increased lifetime. In the next experiment, methyl-acetate was added to the alcohol feed. The amount added was such that the ester/ methanol carbon mole ratio was 0.20. This addition had a significant effect on the catalyst lifetime: it was reduced by a factor of 6 (see Figure 3). The initial alcohol conversion and the selectivity versus conversion profiles, however, remained the same. Therefore, also the P/E selectivity ratio remained constant. The next step was to evaluate the effect of adding yttrium oxide to the catalyst using a feed, which has the ester added. For this purpose, the catalyst was again mixed with 20 wt % Y2O3. This addition proved to have a positive effect on catalyst lifetime: it doubled as compared to the case without yttria, although it still was a factor of 3 lower as compared to the case without ester in the feed; see Figure 4. In terms of selectivity, there were some marked changes. The ethylene selectivity dropped significantly, while the propylene selectivity increased,

of approximately 90 mol %. The standard amount of ZSM-5 catalyst loaded was 50 mg. When 20 wt % Y2O3 was added to the catalyst, a mix was made of 10 mg of Y2O3 and 50 mg of ZSM-5. The catalyst was run until the conversion of methanol dropped below at least 30%. The liquid feed consisted of a mix of methanol, ethanol, and optionally an ester. The weight % ratio of ethanol to methanol was kept constant at 1:1, which is equivalent to an EtOH/MeOH carbon mole ratio of 1.39. Likewise, the ester/methanol carbon mole ratio was kept constant at 0.20.

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RESULTS AND DISCUSSION First, an experiment was done using only the zeolite as catalyst and a feed that consisted of only methanol and ethanol, so without any additional ester present in the feed. The results are displayed in Figure 1. The initial methanol conversion is 88%, and the ethanol conversion is 100%. The lifetime is defined as the amount of alcohol fed per gram of catalyst until the methanol conversion drops below an arbitrarily set value of 30%. For this experiment, the lifetime of the catalyst is determined to be 1543 g of alcohol/gram of catalyst. The ethanol conversion remains 100% throughout the whole lifetime. The main product is ethylene, followed by propylene. The most abundant byproducts are the C4 olefins, followed by C5 and C6 hydrocarbons. When the catalyst deactivates for the conversion of methanol, it still converts 100% of ethanol. The main product in this case is ethylene. A different catalytic route is involved in the conversion of methanol as compared to the dehydration of ethanol to ethylene. As mentioned in the Introduction, the methanol to olefins reaction requires strong acid sites of the zeolite, while the dehydration of ethanol into ethylene occurs on weak acid sites. The results thus suggest that with time on stream the strong acidity of the zeolite is lost, leading to a drop in methanol conversion and C3(+) D



Article

leading to a higher P/E selectivity ratio as compared to a feed of pure methanol and ethanol without methyl-acetate. These experiments were repeated for other esters: methylpropionate, ethyl-acetate, ethyl-propionate, and methyl-formate. Their performance in terms of conversion, selectivity, and lifetime were evaluated with and without yttria mixed through the catalyst. The results have been summarized in Figure 5. The full data set is given in Table S1 in the Supporting Information.

Table 1. Expected Change in P/E Selectivity Ratio Based on the Decomposition Products of the Respective Esters

expected change in P/E ratio

methyl‐propionate → methanol + propionic acid

(6)

ethyl‐acetate → ethanol + acetic acid

(7)

ethyl‐propionate → ethanol + propionic acid

(8)

methyl‐formate → methanol + formic acid

(9)

ethylacetate

ethylpropionate

methylformate

↑

↑

↓

↓

↑

Figure 6. Ester conversion as a function of methanol conversion without (open symbols) and with (closed symbols) 20 wt % yttria mixed through the catalyst.

dependent on the methanol conversion level, type of ester, and the presence of yttria. Some esters, like methyl-formate and ethyl-propionate, give 100% conversion with or without yttria being present. From Figure 5 it is clear that adding yttria therefore has no effect on the P/E ratio for these esters. Ethylacetate conversion is also not affected by the addition of yttria, although its conversion is not 100% but slightly below. Also, this ester shows a slightly decreasing conversion trend with increasing methanol conversion, which is opposite to the other esters tested. The reason for this behavior is unclear. Other esters, like methyl-acetate and methyl-propionate, do show a conversion significantly lower than 100% when yttria is not present. Adding yttria increases the conversion of these esters as shown in Figure 6. This immediately has an effect on the selectivities as well. From Figure 5 it is clear that adding yttria now does have an effect on the P/E ratio for these esters. The P/E ratio increases due to the extra methanol that is released and converted into more propylene than ethylene. It is hypothesized that the same reason underlies both the observation that yttria increases the conversion of some esters (when they are not fully converted without yttria) as well as extends the lifetime of the zeolite. The underlying reason is proposed to be related to the acid formed upon dissociation of the ester. Yttria is believed to either convert or otherwise remove the acid from the gas stream.33−35,38 Removal of this acid will drive reactions 5−9 to completion, leading to a higher conversion of the ester. Additional experiments were performed to further investigate the effect of an acid on the lifetime of the zeolite in the alcohols to olefins reaction. It is known from the literature that acids can adsorb on the zeolite surface, thereby poisoning the active sites leading to a decrease in lifetime of the zeolite.40 The acid not only adsorbs on the acid sites of the zeolite and blocks them,

Dissociation of the ester will lead to the corresponding alcohol and acid, resulting in the following list of possible reactions 5−9 and their reaction products: (5)

methylpropionate

conversion of the ester is increased by the presence of yttria (Figure 6). The conversion of the esters proved to be

Figure 5. Effect of adding an ester to the alcohol feed and optionally yttria to the catalyst mixture. The left y-axis shows the lifetime, while the right y-axis shows the propylene/ethylene selectivity ratio. The horizontal dotted line highlights the resulting P/E ratio of a feed without ester (the “g cat” on the y-axis refers only to the amount of ZSM-5, not to the total amount of yttria + ZSM-5).

methyl‐acetate → methanol + acetic acid

methylacetate

The additional methanol and ethanol formed via the decomposition of the ester will participate in the alcohols to olefins reaction and influence the selectivities. The ethylene and propylene selectivities of a feed consisting of pure methanol or ethanol are known from other processes. The additional ethanol will almost completely be dehydrated into additional ethylene, thereby lowering the P/E selectivity ratio of the standard feed (which is a 1:1 weight ratio mix of methanol and ethanol). The additional methanol will be converted into a mixture of propylene and higher hydrocarbons with relatively low ethylene selectivity (following the typical methanol to olefins (MTO) chemistry). This will thus lead to an increase of the P/E selectivity ratio as compared to the standard feed without ester. Hence, it can be predicted in which direction the P/E selectivity ratio will move when these esters are co-fed and dissociated; see Table 1. This is fully in line with the results as displayed in Figure 5. However, the trend is most clear when yttria is present in the catalyst mixture. The most probable explanation is that the E



Article

Figure 7. Methanol and acetic acid conversion as a function of the amount of alcohol fed without (open symbols) and with (closed symbols) 20 wt % yttria mixed through the catalyst (the “g cat” on the x-axis refers only to the amount of ZSM-5, not to the total amount of yttria + ZSM-5).

carbonyl interconversion pathway31 or be reflective of the stability of the methyl-formate versus the heavier methylacetate and methyl-propionate.46 More in depth studies would be necessary to fully understand the acid-zeolite and acid-yttria interaction mechanism.

but it can also be transformed (via an intermediate ketone) into hydrocarbons.41 There is also literature available on the coking ability of acetaldehyde by leading to a coke precursor.42,43 To further investigate, the standard alcohols to olefins test was performed with addition of acetic acid (0.13 acid/MeOH carbon mole ratio) to the methanol + ethanol (1:1 weight ratio) feed stream. This test was performed again with only the zeolite as catalyst or with a mix of the zeolite and 20 wt % yttria (Figure 7). The decrease in lifetime is clear when comparing Figure 7 to Figure 1. The results also clearly show the beneficial effect on the zeolite lifetime of the yttria addition. Furthermore, adding yttria increases the acetic acid conversion. If more acetic acid is converted or otherwise removed by yttria, then the lifetime of the zeolite will increase. This leads to the question of how the acid interacts with yttria, a basic oxide. There is very limited literature available44 on the conversion of acetic acid on ZSM-5, and (to the best of our knowledge) no literature on the conversion of acetic acid on yttria. One possible pathway for the removal of the acid by yttria is via the decarboxylation of the acid. There is, however, no chromatographic evidence for this mechanism as the selectivities to CO2 and alkanes (CH4 and C2H6) remain constant; compare, for example, the CO2 selectivity in Figure 3 to that in Figure 4. A second pathway could be simply a strong adsorption of the acid on yttrium oxide, at least stronger than the adsorption on the active site of the zeolite. It is known from the literature that carboxylic acids will adsorb on metal oxide surfaces and that the strength of the bond depends on the specific combination of acid and metal oxide.45 Binding of the acid to the yttrium oxide surface will lead to (1) a higher observed “conversion” of the acid, (2) a higher lifetime of the zeolite because less of the active sites will be poisoned by the acid, and (3) a higher conversion of the ester because one of the decomposition products (the acid) is removed. The acid “conversion”, which is observed when no yttria is present (see Figure 7, open symbols), would then simply be adsorption of the acid on the active sites of the zeolite, thereby blocking these active sites and leading to a fast deactivation of the zeolite for the alcohols to olefins chemistry. It is not fully clear why different esters affect the catalyst lifetime to a different extent (Figure 5). The methyl-formate ester clearly has a very different impact than the acetate and propionate esters. It may either be reflective of the acid-

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CONCLUSIONS It was shown that the presence of relatively small amounts of esters in the alcohol feed stream has a significant effect on catalyst lifetime. These esters will decompose on the zeolite surface, leading to additional methanol or ethanol (depending on the type of ester) and to a carboxylic acid. This additional methanol or ethanol reacts according to the alcohols to olefins mechanism, leading to a change in the propylene/ethylene selectivity ratio. The acid released upon decomposition of the ester is responsible for the decrease in lifetime of the zeolite by adsorption on the zeolite active sites. Addition of metal oxides, specifically yttria, to ZSM-5 limits this decrease in catalyst lifetime. The proposed pathway is by a stronger adsorption of the acid on yttria than on the active site of the zeolite. Removal of the acid by yttria can in some cases (typically when the ester conversion is not 100% without the yttria present) lead to a higher conversion of the ester and thereby indirectly to an effect on the P/E selectivity ratio due to the additional methanol or ethanol being released.

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ASSOCIATED CONTENT

S Supporting Information *

Full data set. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +31-(0)115-67 1539. Fax: +31-(0)115-67 4001. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Götz Burgfels, Manfred Frauenrath, and Dr. Jens Freiding from Clariant International Ltd. are kindly acknowledged for their cooperation in this research and for providing the catalyst. F



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Article

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