Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis
Edited by Ive Hermans, University of Wisconsin–Madison, Madison, WI; received March 7, 2021; accepted November 4, 2021 by Editorial Board Member Tobin J. Marks
Significance
Methanol conversion to hydrocarbons has emerged as a key reaction for synthetic energy carriers and light alkenes. The autocatalytic nature and complex reaction network make a mechanistic understanding very challenging and widely debated. Water is not only part of the overall conversion, it is also frequently used as diluent, influencing, in turn, activity, selectivity, and stability of the catalysts. Water directly and indirectly influences the processes that initiate the C–C formation via adjusting the chemical potential of methanol and dimethyl ether, with the latter being more efficient to generate highly reactive C1 species via hydride transfer. The insight shows paths to optimize the stability of catalysts and to tailor the product distribution for H-ZSM-5–based catalysts.
Abstract
Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H2O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C1 reactive mixture—which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C1 pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C1 intermediates and initiates the formation of first C–C bonds, CH4 is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH4 formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH4 formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process.
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The catalytic conversion of methanol (MeOH) to hydrocarbons (MTH) and its variants to lower olefins (MTO), gasoline (MTG), or aromatic compounds (MTA) has evolved as the major route to produce hydrocarbons via a synthesis gas (1–4). For light alkene synthesis two types of molecular sieve catalysts, the aluminosilicate MFI and the silicoaluminophosphate of CHA, SAPO-34, have been commercialized and provide routes to propene and ethene, respectively. On first sight, the differences in selectivities between two groups of catalysts are mainly related to differences in the pore structures. However, the MTH reaction is a complex autocatalytic reaction with a dual-cycle mechanism involving multiple organic intermediates that can be regarded as cocatalysts (5). Zeolite porosity is only one of the parameters that affect the nature and mobility of the hydrocarbons entrained in the pores, and therefore the interplay of reaction conditions and stabilization and transport of reaction intermediates plays an important role in the final catalyst lifetime and product distribution that can be achieved.
It is a characteristic of the MTO process that the feedstock contacts the catalysts as a mixture of MeOH, dimethyl ether (DME), and water, in ratios determined by the acid-catalyzed reversible dehydration of MeOH. Even when only DME is used, its conversion to hydrocarbons forms water. Using water to dilute hydrocarbons and to promote heat transfer has evolved as common practice in industrial applications, modulating inevitably the interconversion between MeOH and DME. Therefore, the differences in the reactivity between MeOH and DME to both alkenes as well as alkanes and aromatic molecules have been frequently addressed (6–11). The different reactivities of MeOH and DME would be less important if they were equilibrated under reaction conditions. However, reports show unequivocally that partial pressures of MeOH, DME, and water often deviate from equilibrium during MTO catalysis (9, 10).
The conversion of MeOH/DME on zeolites proceeds via several stages (12): 1) an initiation stage, during which the conversion is very low, but the first C–C bonds are formed and the resulting species are gradually converted to a hydrocarbon pool; 2) as soon as a certain concentration of hydrocarbons in the pores is reached (the hydrocarbon pool), methylation begins to dominate and MeOH and DME are rapidly and autocatalytically converted in a dual-cycle mechanism (13, 14); and 3) after complete conversion of methanol and DME, the interconversions between hydrocarbons, mainly light alkenes, alkanes, and aromatics, determine the final product distribution. In a fixed-bed reactor, this zoning leads to the characteristic S-shaped increase of the conversion with contact time, as illustrated in Fig. 1.
Fig. 1.
Understanding the impact of the feed composition (i.e., the relative concentration of MeOH, DME, and H2O) on catalyst lifetime and activity is of fundamental importance to control activity, selectivity, and stability in industrial applications. DME has been reported to be more reactive than MeOH (9, 15). On the other hand, MeOH is generally regarded as more reactive for the reaction paths leading to nonalkene by-products and deactivation (9, 10, 16). Despite these reports, consensus has not been reached on the mechanistic origin of the reactivity differences observed between DME and MeOH. In practice, MeOH, DME, and water always coexist and their partial pressures change as a result not only of methylation reactions but also of the MeOH/DME interconversion. Therefore, a quantitative evaluation of the influence of a single component in the MTO mechanism requires complex experimental strategies. The highly reactive key mechanistic intermediates in the reaction pathways leading to nonalkene by-products and catalyst deactivation add additional challenges (17–19).
The observed effects of water on the overall turnover number capacity of MTH catalysts are compounded with the influence of a set of parameters including temperature, contact time, surface coverage, and acidity and pore size on the rates of autocatalytic reactions and deactivating reactions. In order to obtain the relevant kinetic parameters in the complex MTH reaction network it is necessary to perform measurements at conditions at which these opposing processes are decoupled.
Here, we report a kinetic study at selected MTO reaction conditions during which quasi-equilibration of DME, MeOH, and H2O has been reached. Our experimental study was performed at very low conversions, to ensure that hydrocarbon chemistry and in particular fast methylation reactions are negligible. This allowed us to evaluate quantitatively the reactivity of DME and MeOH in the reaction steps that lead to the initiation of the hydrocarbon pool and the dual cycle. At the typical temperatures used in MTH and also in this work, the fast MeOH-to-DME interconversion takes place mediated by methoxy species (20). The formation of HCHO and equivalent intermediates such as CO is widely accepted as key step in the formation of the first C–C bond-containing products and intermediates (18, 21–23). We provide quantitative evidence of a higher reactivity of DME for the formation of CH4 and HCHO compared to MeOH, and, in good agreement, higher overall rates for the autocatalytic MTH conversion in the absence of water. Our results also demonstrate that methylation rates in the production of olefins by the dual-cycle mechanism mainly depend on the population of methoxy species and, thus, they are negatively affected by increasing concentrations of H2O.
Properties of H-ZSM-5 Catalyst Used for the Mechanistic Study
It is known that a low concentration of Brønsted acid sites (BAS) minimizes the probability of a molecule to undergo successive reactions along the diffusion path leading to aromatization and coking (24). Therefore, in order to ensure viable reaction conditions at which the catalyst can operate within the initiation stage (I in Fig. 1), we have chosen to perform our experiments on steamed H-ZSM-5 with an Si/Al ratio of 90, a commercial methanol-to-propene catalyst provided by Clariant AG. The catalyst has 68 ± 4 µmol·g−1 BAS and 37 ± 2 µmol·g−1 Lewis acid sites, determined by adsorption of pyridine. The zeolite sample had 0.14 cm3/g micropore volume, determined by N2 adsorption.
MeOH and DME Interconversion
The acid-catalyzed reversible intermolecular dehydration of MeOH to DME is a primary reaction in MTH. Here, as we start from mixtures of DME and H2O, we express the interconversion as hydrolysis of DME to form MeOH (Reaction [Rxn.] 1):
[Rxn. 1]
As a consequence, under typical MTH conditions, not only MeOH but also DME and H2O are present in the feed. The gas-phase composition is determined by the initial composition of the feed and the MeOH–DME interconversion (Rxn. 1) but also (as the MTH reaction progresses) by the different rate of consumption of MeOH and DME in the formation of hydrocarbons. We have observed that before methylation reactions become dominant the concentrations of MeOH, DME, and H2O remain constant in a relatively broad contact time range (broader for higher concentrations of H2O; SI Appendix, Fig. 1 ). These concentrations of MeOH, DME, and H2O indicate that interconversion between MeOH and DME is equilibrated. We have quantified the approach to equilibrium for different feed compositions as a function of contact time. Because our starting point is dry DME mixed with increasing concentrations of H2O, we define the approach to equilibrium for Rxn. 1, i.e., DME hydrolysis. The deviation of the system from equilibrium can be better examined by the use of the approach-to-equilibrium value ηeq of the reverse of Rxn. 1 (Fig. 2A):
[1]
Fig. 2.
In Eq. 1, MeOH, DME, and H2O are assumed to behave ideally. The MeOH/DME conversion in Fig. 2A follows the characteristic S-shape curve with contact time, with an initiation zone ranging from 0 to 0.05 min·kgcat·moltotal carbon−1 and autocatalysis zone from 0.05 to 0.15 min·kgcat·moltotal carbon−1. Fig. 2A also shows the approach to equilibrium and the partial pressures of MeOH, H2O, and DME measured for a reaction with initial feed of 90 mbar DME and 90 mbar water on H-ZSM-5 (SI Appendix, section 1 for details). It can be seen that MeOH, DME, and H2O quickly reached steady partial pressures at short contact times (ca. 0.025 min·kgcat·moltotal carbon−1), with a calculated pseudoequilibrium constant of PM2/(PDPW) = 0.201 ± 0.004, in which PM, PD, and Pw are the pressure of MeOH, DME, and water, respectively. It should be noted that at very short contact time (0.01 min·kgcat/moltotal carbon in this case), the partial pressures of MeOH, DME, and H2O are already close to the equilibrium composition. However, due to the bimolecular nature of the reaction, a PM deviation of 20% translates into an approach to equilibrium of only ca. 0.5.
On the other hand, after the onset of alkene formation, MeOH and DME are rapidly consumed (and H2O produced) due to the fast autocatalytic conversion either via alkene (olefin pathway) or via arene (aromatics pathway) alkylation. For the case illustrated in Fig. 2, this occurs at contact times above 0.05 min·kgcat/moltotal carbon, as marked by the dashed orange line. Once these reactions become faster than the rates leading to interconversion of MeOH, DME, and H2O the partial pressures of these molecules are no longer equilibrated at the reactor outlet.
In order to obtain kinetic parameters of the rate-limiting reaction in the first C–C bond formation and related reactions during the first stages of the MTH conversion, we establish that the reaction conditions applied should ensure equilibration of MeOH, DME, and H2O. Therefore, for each feed composition we determine the range of contact time at which approach to equilibrium is ca. 1 and, simultaneously, the yield of hydrocarbon products, represented by the first-most-abundant product detected propene, is below 0.02% C. In Fig. 2B, we can see that setting a limitation of the hydrocarbon yields to <0.02% C (black dashed line) is more restrictive than the equilibration of DME and MeOH. In other words, by establishing the hydrocarbon yields upper limit to 0.02% C we conservatively impose reaction conditions at which the equilibration of DME, MeOH, and H2O is ensured in all conditions tested in this work.
CH4 Formation as Primary Product of DME and MeOH on ZSM-5
Besides the interconversion of MeOH and DME via dehydration and hydrolysis, small concentrations of CH4, HCHO, and CO form in the initiation stage of the MTH conversion. In the absence of other hydrocarbon products or intermediates, these C1 molecules are hypothesized to form via hydrogen transfer from MeOH to a methoxy group formed from dissociative adsorption of methanol (25, 26).
Regardless of the mechanism, an overall reaction of methanol to formaldehyde and methane should lead to equimolar quantities of both products:
[Rxn. 2]
Formation of CH4 and formaldehyde from methanol as in Rxn. 2 is a crucial step in the methane–formaldehyde mechanism proposed by Tajima et al. (27), where a C–C bond is subsequently formed between CH4 and formaldehyde. Hydride transfer from DME or methanol to a methoxy group is also the first step in the formation of CO, which can be then subsequently methylated, which has been shown to be more favorable than the methane–formaldehyde mechanism (28). Even though it occurs at very low rates, the formation of HCHO and CO initiated by Rxn. 2 is widely accepted as a key step for the formation of the first alkenes that are subsequently transformed to higher hydrocarbons (hydrocarbon pool) (18, 21–23).
Previous reports (29) had concluded that only MeOH may lead to formaldehyde under MTO conditions. However, our experiments with dry DME reacting on H-ZSM-5 at very short contact times (up to 0.007 min·kgcat·moltotal carbon−1) and low conversions (<0.3 C%) also lead to CH4, HCHO, CO, and CO2 (Fig. 3). Formation of H2 was also observed but could not be quantified. Under these (dry) conditions, the presence of MeOH is ruled out. It can be seen that CH4, HCHO, and CO are the main gaseous products from DME. In a blank test (data point at W/F = 0) equimolar quantities of CH4 and HCHO were formed (ca. 0.01%C), indicating a small contribution from thermal decomposition. CH4 is clearly a primary product of DME and we propose its formation via decomposition of DME with concomitant formation of one molecule of HCHO (Rxn. 3). This decomposition (Rxn. 3) has been mentioned in two earlier contributions (15, 28):
[Rxn. 3]
Fig. 3.
As shown in Fig. 3, a very small amount of C2 and C3 olefins appeared in the DME reaction on ZSM-5 at ∼0.005 min·kgcat·moltotal carbon−1. These short alkenes are the main C–C bond containing products under those conditions. The methane yield increased linearly from the beginning of the reaction, until the formation rate of olefins started to increase significantly (from 0.007 min kgcat·moltotal carbon−1 on). This points to a constant formation rate of methane in the initiation period.
Based on Rxns. 2 and 3, HCHO should be formed in equimolar amounts to CH4. However, measured HCHO formation rates were lower than those to CH4. This difference is attributed to the high reactivity of HCHO, which leads to its rapid consumption in consecutive chemistries, such as the decomposition to CO or the condensation with surface species to build the hydrocarbon pool (18, 19, 23, 28, 30, 31). It should be noted that for the gas products detected in the conditions of Fig. 3 the C and O balance are closed and, therefore, the accumulation of hydrocarbons or oxygenates strongly adsorbed on the zeolite surface can be neglected.
In contrast to the high reactivity of HCHO in secondary reactions, CH4 is stable and inert on H-ZSM-5 under the applied MTO conditions. Even cofeeding 167 mbar CH4 with DME did not promote its reaction (SI Appendix, Table 2 ) and therefore it can be considered to be an end-product. At these low conversions, one can exclude the contribution of reaction pathways other than Rxns. 2 and 3 to CH4 formation. Especially, formation of CH4 by cracking of hydrocarbons from the emerging hydrocarbon pool and from coke deposits that have been reported at higher conversions (32, 33) is negligible under the reaction conditions discussed here. This is shown by the fact that catalyst tested under low conversions led to the same CH4 yield even after being exposed to space velocities where the hydrocarbon pool was allowed to fully develop and the catalyst even converted 20% of DME (SI Appendix, Fig. 2 ). On the other hand, because CH4 was the only alkane observed under the studied conditions, we rule out any significant contribution to HCHO formation from the methanol-induced hydrogen transfer (34) to larger carbenium ions.
The evolution of the methane yield with contact time in the initiation stage of MTO (region I in Fig. 1) was monitored for different feed compositions. In all cases, a linear increase of the CH4 yield with contact time was observed with the slope representing the CH4 formation rate (SI Appendix, section 2 and Fig. 3 ). This indicates that CH4 is a primary and stable product of MeOH and DME on H-ZSM-5.
Based on all experimental evidence, we conclude that the CH4 formation rate at low conversions is a suitable quantitative descriptor of the formation rate of HCHO from DME/MeOH feeds, because all the possible CH4 formation pathways entail the formation of equimolar quantities of HCHO (Rxns. 2 and 3).
Kinetics of CH4 Formation, as a Proxy for HCHO Formation, in the Initiation Stage of MTH Conversion
Determining the reaction kinetics for CH4 formation as a function of the chemical potential of the three feed components (DME, methanol, and water) is the basis for further understanding of the reaction mechanism of oxygenates in the initiation stage. However, due to the fast interconversion between MeOH, DME, and water (Rxn. 1), it is not possible to independently vary their partial pressures. In order to ensure a stable and well-defined feed composition for the kinetic study of methane formation, we only use data in the contact time range, for which quasi-equilibration of Rxn. 1 is achieved (i.e., between 0.020 and 0.055 min·kgcat·moltotal carbon−1 in Fig. 2). It should be noted that such a contact time range varies with H2O content in the feed and must be adjusted for each condition (SI Appendix, section 1 ). In the selected contact time region, the partial pressure of one of the three feed components can be kept constant by adjusting the overall flow with an inert gas (N2), while the other two vary accordingly to their equilibrium.
The reaction with dry DME is the simplest case. In the absence of H2O, hydrolysis of DME to MeOH (Rxn. 1) is excluded and the formation rate of methane is only related to reactions from DME. The CH4 formation rate increased proportionally with increasing DME pressure (Fig. 4, black line), indicating a first-order reaction with respect to DME (SI Appendix, Fig. 4 ). This indicates that a surface reaction exists in which DME is converted to CH4 and HCHO (Rxn. 3).
Fig. 4.
Next, we cofed DME and H2O in a N2 flow under reaction conditions similar to the experiments in absence of water. Under these conditions MeOH, DME, and H2O quasi-equilibrate at very short contact times. Based on this quasi-equilibration, we adjusted the DME and H2O partial pressures in the feed in order to reach constant MeOH partial pressure for all measurements in Fig. 4. The measured CH4 formation rate in this contact time range (Fig. 4, blue line and SI Appendix, Table 3 ) was significantly lower than the rate obtained with dry DME. The reaction order of ∼1 in DME was preserved, despite the presence of different concentrations of H2O (SI Appendix, Fig. 5A ). With constant DME pressure, the methane formation rate decreased with increasing MeOH and H2O partial pressures, showing a negative reaction order of −0.59 for MeOH (SI Appendix, Fig. 5B ).
We hypothesize that at low conversions only two possible reaction pathways of DME and MeOH exist that could lead to CH4: hydrogen transfer from DME to surface methoxy species (SMS), as shown in Fig. 5, Pathway 1 and hydrogen transfer from MeOH to SMS as shown in Fig. 5, Pathway 2. The direct decomposition of DME on BAS to form CH4 and HCHO is concluded to be unlikely, as will be discussed below. Other routes leading to CH4, such as cracking or hydrogen transfer from entrained hydrocarbons or coke, can be ruled out under the current reaction conditions.
Fig. 5.
Previous work has shown that the formation of CH4 and HCHO from DME via Pathway 1 is energetically feasible on H-SSZ-13 (28). In order to shed light on the reaction of methanol and DME with BAS and with SMS on ZSM-5, we turned to computations of the mechanistic pathways, focusing on Scheme 1 for the T12 site of H-ZSM-5 using periodic density functional theory calculations employing the Perdew–Burke–Ernzerhof (PBE) functional with dispersion corrections (D3) in conjunction with highly accurate ab initio calculations on cluster models (see Methods). Free energies were computed at 748 K and 1 bar reference pressure. As shown in Fig. 5, the reaction of MeOH with an SMS leading to the formation of formaldehyde and methane has a free energy barrier of 222 kJ/mol. For DME, the analogous formation of formaldehyde and methane via reaction at an SMS has a lower barrier of 212 kJ/mol. We have additionally investigated H2 formation both from methanol and DME (SI Appendix, Fig. 10 ) and found the corresponding barriers to be 232 and 254 kJ/mol, respectively, thus being significantly higher than for CH4 formation. Our calculations suggest that the direct decomposition of DME on BAS to form CH4 and CHOH without mediation of methoxy species is not favorable. Attempts to locate transition states for this mechanism instead lead to different reactions, such as the production of H2.
Scheme 1.
For the hydrogen transfer in Pathways 1 and 2, we propose the reaction network linked by surface methoxy species in Scheme 1, including the formation of methoxy and CH4 from DME and MeOH in the initiation zone, and the concomitant generation of key intermediates for the formation of C–C products (i.e., formaldehyde). The reactions (I) and (II) in Scheme 1 are proposed to be elementary steps in the interconversion of DME and MeOH (Rxn. 1) on H-ZSM5 at 748 K, with methoxy species as main surface intermediates (35). We only considered CH4 formation via surface methoxy species and not via adsorbed methanol, because the latter type of mechanism is known to become unfavorable at high temperatures due to the associated entropic loss. For example, Jones and Iglesia have predicted the cross-over in methanol dehydration rates from direct to stepwise mechanism to occur at 503 K for 0.1 bar of methanol (20).
As mentioned above, the gas phase composition of MeOH, DME, and H2O is equilibrated in most of the initiation stage (Fig. 2 and SI Appendix, section 1 ). Therefore, at the reaction conditions used here (T = 748 K, conversions <0.8 C%) it is hypothesized that the transformation of BAS into methoxy species is determined by the reaction pseudoequilibria I and II in Scheme 1 (20).
According to Scheme 1, the reaction of SMS with methanol leads either reversibly to DME (Reaction II) or quasi-irreversibly to formaldehyde and methane via hydrogen transfer (Reaction III). It should be noted that the reaction of SMS with H2O forms methanol and restores a BAS (Scheme 2, Reaction I). Thus, the water partial pressure influences not only the partial pressure of MeOH and DME by Rxn. 1 but also the available concentration of reactive surface methoxy species, which are relevant for Pathways 1 and 2 in Fig. 5.
In the proposed reaction pathway of Scheme 1, the hydrogen transfer from DME or MeOH to surface methoxy species is the rate-determining step (reactions III and IV) for CH4 formation. The consumption of MeOH, DME, or methoxy species via buildup of a hydrocarbon pool or via methylation of C–C-containing intermediates is hypothesized to be negligible under these reaction conditions. We base this assumption on 1) the very small amounts of C–C products detected (upper limit set at 0.02% C propene for the kinetic study), 2) the short time on stream (TOS) at which data were collected (10 to 20 min), and 3) the closed C balance of products at these low conversions (Fig. 3).
According to the adsorption entropy and enthalpy values reported by us (36), Pope (37) and Piccini et al. (38) calculated that the coverage of BAS by H2O (θW), MeOH (θM), and DME (θD) can be regarded as negligible at 748 K, i.e., θW ≈ θM ≈ θD ≈ 0 (SI Appendix, section 3.1 ). As mentioned above, also the influence of hydrocarbon buildup can be disregarded under the low conversions applied here. Thus, we hypothesize that in the MTH initiation stage the only surface species to be considered are methoxy groups at bridging lattice oxygen. Under this assumption, the coverage is solely affected by the four reactions shown in Scheme 1.
In the initiation stage, the methane formation rate rCH4 is contributed by Reactions III and IV in Scheme 1, with each rate being proportional to surface methoxy coverage (θCH3) and pressure of MeOH (PM) or DME (PD):
[2a]
[2b]
Here, kD and kM refer to the reaction rate constants for the methane formation from DME and MeOH, respectively, Reactions IV and III in Scheme 1. The total methane rate is the sum of the two and can be expressed as Eq. 2, in which PW is the pressure of water (SI Appendix, sections 1 and 3 for details):
[3]
For the special case of the reaction of dry DME at low conversion levels (PW → 0 and PM →0), rCH4 in Eq. 3 is approximated to (SI Appendix, section 3.2 )
[4]
This approximation is in good agreement with the observed proportional increase of the methane rate with DME pressure (reaction order 1) in Fig. 4. Fitting the data with Eq. 4 gives a value of kD of 2.1 ± 0.2 mol·min−1·kgcat−1·bar−1.
To determine the values of kM and KH2O, Eq. 3 is rearranged into
[5]
With the rates at different MeOH, DME, and H2O pressure (SI Appendix, Table 3 ), experimental data for PM·(rCH4-kD·PD), PM2 and rCH4·PW can be displayed as a surface in a three-dimensional plot (SI Appendix, Fig. 7 ). The excellent fit of the data to Eq. 4 with R = 0.96 validates the proposal of reaction network in Scheme 1 and the approximations taken for the kinetic analysis of the initiation stage. The regression led to a reaction rate constant kM of 0.15 ± 0.15 mol·min−1·kgcat−1 bar−1 and pseudoequilibrium constants KH2O of 0.37 ± 0.08 (SI Appendix, Table 4 ). The rate constant kD is one order of magnitude higher than kM. Therefore, we conclude that the main route of CH4 (and HCHO) formation in the initiation stage of methanol conversion to hydrocarbons is the hydrogen transfer from DME to SMS (Pathway 1). From our experimental data fitting to Eq. 5 we obtain ΔG°‡ values for MeOH and DME at 748 K of 210 kJ/mol and 193 kJ/mol, respectively. This barrier difference of 17 kJ/mol is in good agreement with the 10 kJ/mol difference predicted by our theoretical calculations.
Consequences of Hydrogen Transfer Rates for the Formation of the Hydrocarbon Pool
Fig. 6 shows the conversion of DME/MeOH under different DME and water cofeeding compositions. Because the rate of hydrogen transfer from DME and MeOH to methoxy groups is proposed to be the rate-determining step for the overall Rxns. 2 and 3, the rate of formation of CH4 via reactions III and IV in Scheme 1 is set equal to the rate of formation of formaldehyde. It should be emphasized again that HCHO is considered a key intermediate in the initiation reactions for the formation of hydrocarbons (18, 21–23). Then, according to our mechanistic proposal, at lower partial pressures of H2O a faster formation of the hydrocarbon pool is expected—via reaction of DME to form HCHO—reducing the contact time required to reach the onset of olefin formation. In good agreement, Fig. 6 shows that the presence of increasing concentrations of H2O in the feed increases the contact time necessary to trigger the formation of hydrocarbons.
Fig. 6.
The initial formation rate of the hydrocarbon pool cannot be directly measured in the kinetic experiments, but an indirect quantification is provided by examining the minimum contact time necessary to initiate the autocatalytic MTO reaction. As a criterion for the transition from initiation to methylation, we select the contact time at which 0.02 C % of propene is generated as indicator, and we name it the “critical contact time.” This indicator is chosen because at these low conversions C3= and C2= are the main C–C-containing products and only traces of higher alkenes or aromatics are detected. On the other hand, the MeOH/DME conversion curve rapidly increases at C3= yields above 0.05% C.
Fig. 7 shows that the critical contact time increased with increasing water content in the feed. For the highest tested H2O partial pressure (730 mbar), the critical contact time was about four times larger than for dry DME. The critical contact time correlates excellently with the methane formation rates during the initiation period, regardless of the H2O content of the feed (Fig. 7A). Thus, the critical contact time is concluded to be inversely proportional to the C1 hydrogen transfer rate. A pseudorate of formation of the hydrocarbon pool can, thus, be derived from the critical contact time values (Eq. 6):
[6]
Fig. 7.
This pseudo rate of hydrocarbon pool formation is directly proportional to the CH4 formation rate (Fig. 7B). Based on our own results and literature, CO and HCHO are involved in several pathways to start forming the hydrocarbon pool (22, 23, 27, 28). The correlation depicted in Fig. 7B supports these proposals, suggesting that the kinetically relevant step in the formation of the first C–C products is identical to the rate of CH4 formation, which is stoichiometrically linked to HCHO (and CO). Thus, one can conclude that the rate-determining step of the hydrocarbon pool formation is proportional to the formation rate of HCHO (and CO), i.e., the hydrogen transfer between DME or MeOH and surface methoxy species to generate the C1 reaction intermediates. It should be noted that our data cannot rule out the coexistence of other proposed mechanisms for formation of the first C–C bond such as those mediated by carbene species (23, 39). However, the correlation between CH4 formation and C–C bond formation is a strong indication that the formaldehyde-mediated mechanism dominates the MTH kinetics in the initiation stage.
Effects of H2O Partial Pressure on Methylation and the Dual-Cycle Mechanism
Experimentally, the absolute methylation rates observed for MeOH feeds are lower than for DME feeds (9, 15). Accordingly, in Fig. 6 we observed a decrease in the slope of the conversion curve (in the 10 to 90% conversion range) when the H2O content increased. In this conversion range (methylation stage, II in Fig. 1), as soon as a critical concentration of alkenes is formed, the continuous propagation of the dual (olefin and aromatics) cycle sets in. In the dual cycle, DME and MeOH are rapidly consumed via methylation of alkenes and arenes in the olefin and aromatic cycle, respectively. The rates of these methylation reactions are orders of magnitude higher than the reactions initiating the hydrocarbon pool, which results in the S-shape of the MeOH/DME-conversion curves in Figs. 2A and 6. The methylation of an alkene (e.g., butene) or an aromatic ring with MeOH generates one H2O molecule per MeOH consumed (Rxn. 4). Conversely, methylation via DME stoichiometrically generates a molecule of MeOH (Rxn. 5):
[Rxn. 4]
[Rxn. 5]
As soon as the dual cycle propagates, the concentration of hydrocarbons in the zeolite pores increases and the methylation Rxns. 4 and 5 become faster than the interconversion of DME and MeOH (Rxn. 1). Under such conditions, it is not possible to apply conventional kinetic analysis to calculate a net methylation rate from experimental data. However, we note that examining the evolution of DME/MeOH/H2O partial pressures in the first stages of methylation, the initial consumption rate of DME is much larger than that of MeOH, the latter being close to zero (Fig. 2B). Given that methylation with DME as in Rxn. 5 generates 1 mol of MeOH per mole of reacted DME, the relatively constant MeOH concentration to conversions even above 50% (Fig. 2B) indicates that Rxns. 4 and 5 have rates in the same order of magnitude. We conclude that methylation rates in the 10 to 50% region are dependent on the concentration of surface methoxy species, regardless of the nature of the oxygenate in the gas phase.
In addition to methylation Rxns. 4 and 5, the formation of hydrogen transfer products (aromatics and alkanes) becomes also significant in the educt stream already in the conversion range of 1 to 10%. Previous studies have shown that at low oxygenate conversions the dominant pathway for the formation of aromatics and alkanes in ZSM-5 is the hydrogen transfer from methanol to an alkene molecule (so-called methanol-induced hydrogen transfer [MIHT], Scheme 2) (34).
Scheme 2.
In this mechanism, hydrogen transfer from MeOH to an olefin generates highly reactive HCHO and an alkane. The subsequent reaction of HCHO with olefins generates aromatics via the Prins reaction (18). At medium conversions (stage II in Fig. 1) it is not possible to differentiate between the net contribution of DME and MeOH to this type of hydride transfer. While evaluating the rate constants is not possible due to the unknown concentration of hydrocarbons, the overall alkane formation rate in the different stages of MTO conversion for different DME/H2O feeds can be determined (SI Appendix, Fig. 9 ). In agreement with the lower hydride transfer rate of MeOH compared to DME in their reaction with methoxy species (stage I), C1–4 alkane formation rates at partial MTO conversions via hydride transfer of oxygenates to alkenes (stage II) also decrease with the H2O partial pressure, i.e., with increasing concentration of MeOH in the feed. Once full conversion is achieved the rate of formation of alkanes is similar regardless the concentration of water in the feed.
Fig. 8 compares the effect of feed H2O content in the methylation rates, calculated as the combined consumption of methanol and DME, and in the MIHT rates calculated as formation rate of aromatics and C1–4 alkanes. Stepwise methylation is accepted as the dominant mechanism for alkene formation at these low conversions (40, 41). In this mechanism, methoxy species are the key surface intermediates and methylations occur via nucleophilic attack of an alkene or an aromatic ring on a surface methoxy species. The presence of water reduces the surface concentration of methoxy groups on BAS (Scheme 1, I) and, hence, decreases the concentration of potential transition states and lowers the observed overall methylation rate.
Fig. 8.
In Fig. 8 it can be seen that as the water partial pressure increased the rate of formation of aromatics and alkanes (represented as MIHT rate) decreased. As shown in Scheme 2, the rate-determining step of the MIHT pathway have been proposed to be the formaldehyde formation on LAS and does not involve SMS or BAS (34). Although this mechanism was originally proposed for MeOH as reactant, there is no evidence against a reaction starting from DME under H2O-poor conditions. Therefore, we hypothesize that the negative effect of H2O on this rate is related to an overall higher reactivity of DME for hydrogen transfer.
The presence of water is known to impact the product selectivity of MTH (6, 7, 9). In Fig. 8, it is also shown how the MIHT-to-methylation-rate ratio changes with H2O partial pressure. Although both rates decreased in presence of H2O, the effect was stronger for the methylation pathway, causing the MIHT/methylation ratio to increase. As a consequence, higher selectivities to alkanes and aromatics can be expected at elevated H2O partial pressures. This is in good agreement with other studies that have reported lower alkane and aromatic yields, when DME was used as reactant instead of MeOH (9). Consequently, the overall catalyst lifetime is expected to decrease with increasing H2O content (9, 10, 16). Although this effect was already observed and described in the abundant MTH literature, we demonstrate here that its cause is not related to a lower reactivity of DME to hydrogen transfer reactions. On the contrary, the negative impact of H2O (and with it the higher concentration of methanol) on the individual rates of C–C bond formation, methylation, and formation of alkanes and aromatics is the combined result of the concentration of surface methoxy species—regulated by the presence of H2O and the hydrolysis of SMS to restore BAS—and the higher reactivity of DME for hydrogen transfer reactions, i.e., about one order of magnitude for the measurable case of hydrogen transfer to methoxy species.
Conclusions
The experiments and theory reported here provide a mechanistic understanding of the effect of H2O on rates and selectivities of zeolites such as ZSM-5 for methanol conversion to hydrocarbons. Reaction conditions leading to very low conversions allow us to eliminate the interference of the fast and mechanistically dominant methylation of hydrocarbons and of associated subsequent reactions. In this way, kinetic information of the reactivity of oxygenates on zeolites that was hitherto not available becomes experimentally accessible. In the initiation stage of the autocatalytic conversion of MTH, both DME and MeOH are found to react to C1 intermediates that trigger the C–C bond formation on ZSM-5. Among the different possible reaction pathways, kinetic analysis and ab initio calculations show that the dominant reaction is the hydride transfer from DME to methoxy groups formed on zeolite BAS to produce HCHO and CH4. This hydride transfer step is one order of magnitude faster than hydride transfer from methanol, i.e., dimethyl ether is the stronger hydrogen donor to methoxy species than MeOH. The strictly linear correlation between the rate of hydrocarbon pool formation and of methane formation allows us to conclude that the hydride transfer step leading to formaldehyde and CO is rate-determining for the initial C–C bond formation that starts the autocatalytic MTO conversion.
Once a threshold concentration of species in the hydrocarbon pool is formed, the methylation reactions become dominant. In the low contact time regime (hydrocarbon interconversions negligible), however, the net methylation rate does not depend on the partial pressure of MeOH or DME but on the population of surface methoxy intermediates. Because the overall rate of methylation decreased sharply with increasing H2O partial pressures, we conclude that H2O reduces the coverage of methoxy species via hydrolysis, restoring BAS and reducing both the methylation rate and the formation rate of C1 species, by reducing the concentration of potential sites for hydride transfer.
The rate of formation of nonolefinic by-products, i.e., alkanes and aromatics, also decreases with increasing water partial pressure for medium conversions. We hypothesize that the alkane and aromatics formation rates decrease with H2O content because of the higher reactivity of DME in comparison to MeOH for hydrogen transfer to alkenes. Due to the lesser effect of H2O on the hydrogen transfer rates, the methylation/hydrogen transfer rates ratio decreases with H2O partial pressure and therefore the selectivity toward aromatics and alkanes increases.
A better understanding of these elementary steps involved in the MTH reaction and their dependence on the feed composition is fundamental to improve catalyst properties and to guide operations of the process. Here it is shown that the different reactivity of DME and MeOH in hydrogen transfer reactions and the concentration of surface methoxy species as main active species in H-ZSM-5 in presence of oxygenates determine the catalytic activity. Hydride transfer of DME and MeOH determines the formation rate of formaldehyde and in consequence its chemical potential, acting as key intermediate in both activation and deactivation during MTH conversion in zeolites. The quantitative measure of hydrogen transfer kinetics of oxygenates and its relationship with Brønsted and Lewis acid sites and surface species serves as an additional tool to adjust product distribution and predict the lifetime of a MTH catalyst based on the H2O content of the feed.
Methods
Materials.
H-ZSM-5 catalyst has an Si/Al ratio of 90 and was synthesized according to the procedure described by Ong et al. (42). Further details on the preparation are provided in SI Appendix . Methanol (≥99.9%) and DME (≥99.9%) were supplied by Sigma-Aldrich. The introduced deionized water was further purified by an EASYpure II RF-Compact-Waterfilter by Barnstead.
Computational Methods.
A periodic model of H-ZSM-5 was employed with an Al/Si ratio of 1/95, where one Al atom per unit cell is located in the T12 position. The lattice constants were taken from previous work (43) and were kept fixed in all calculations in this work. Periodic PBE-D3 calculations (44, 45) were performed with the projector augmented wave (PAW) method as implemented in the VASP program package (46, 47) in version 5.4.1, appropriate PAW potentials and an energy cutoff of 400 eV for the plane wave basis set. Free energies at 748 K and 1 bar reference pressure were computed using the harmonic approximation and the free translator/rigid rotator approximation for gas phase molecules. The accuracy of the obtained energies was further improved by performing single-point energy calculations with the ORCA program (48) on cluster models using DLPNO-CCSD(T)/cc-pVDZ (49, 50) and complete basis set extrapolated (CBS) DLPNO-MP2 calculations. The CBS extrapolation is based on the cc-pVXZ basis sets and is done separately for Hatrtree–Fock using three points (X = D, T, Q) and the exponential formula (51) and for the correlation part using two points (X = D, T) and the l−3 formula (52). More detailed information, all computed structures, corresponding total energies, and free energy corrections are provided in SI Appendix .
Catalytic Testing.
All catalytic results were obtained using a fixed-bed quartz reactor with an internal diameter of 6 mm at 748 K and ambient pressure. The catalyst particles (200 to 280 μm) were homogeneously diluted with silicon carbide (ESK-SiC) in the range of 355 to 500 μm to ensure temperature uniformity in the catalytic bed. Catalysts were activated at 748 K for 1 h under N2 atmosphere before the reaction. The contact time over the catalyst bed was varied via changing the catalyst loading, while the overall volumetric flow was kept constant. Each data point was measured on a fresh catalyst loading 10 min after the respective feed conditions were introduced. Internal diffusion limitation was ruled out by changing particle size and external diffusion limitation was ruled out by measuring at different total flows at similar contact times.
Methanol and water vapor at partial pressures up to 180 mbar were fed by passing N2 through saturators which were thermo-stated by a VWR circulation thermostat at temperatures up to 333 K. Higher partial pressures were reached by addition of the respective fluid by a Shimadzu HPLC-pump combine with an aDROP direct evaporator (µSteam). DME was introduced by a Bronkhorst MFC and diluted with N2.
The reactor effluents were transferred via a heated line into a gas chromatograph (HP 5890) equipped with an HP-PLOTQ capillary column and a flame ionization detector. The product distributions are given on a carbon basis. For the determination and quantification of CO, CO2, and H2 in the reaction products, we used a QMG 220 M1, PrismaPlus (Pfeiffer Vacuum) online mass spectroscopy with a continuous secondary electron multiplier (C-SEM).
Formaldehyde was quantified by condensing the reaction effluent into two bottles at 275 K for 30 min TOS with subsequent stoichiometric Hantzsch reaction as described by Nash (53) and depicted in Rxn. 6:
[Rxn. 6]
A Varian Cary 50 ultraviolet-visible spectrophotometer was used to determine the formaldehyde concentration via the absorption of the formed 3,5-diacetyl-1,4-dihydro-2,6-lutidine. In order to compare the resulting values with the yields of other products determined by GC, it was calculated an average value of formaldehyde yield for 30-min periods.
Adsorption of DME.
The adsorption of DME on H-MFI(Si/Al 45) was measured in a Seteram TG-DSC 111 calorimeter connected to a high-vacuum system. About 20 mg of H-MFI zeolite was loaded in a quartz sample holder and activated at 723 K for 1 h under vacuum (P < 10−4 mbar) with a heating rate of 5 K⋅min−1. Afterward, the sample was cooled to 373 K and DME was introduced into the system via a dosing valve. Via carefully opening and closing the dosing valve, the pressure of DME was increased slowly in small pressure steps from 5·10−3 to 0.4 mbar and kept at each desired pressure. The DME uptake was determined by the increase of sample weight, and the released heat was obtained by integration of the heat flux signal.
Data Availability
All study data are included in the article and/or SI Appendix .
Acknowledgments
We acknowledge the support of the Bavarian Ministry of Economic Affairs and Media, Energy and Technology and Clariant Produkte (Deutschland) GmbH. F.M.K. thanks Lei Tao, Laura Tebcharani, Moritz Eder, Markus Pschenitza, and Greta Zambo for their helpful assistance in the experimental work. P.N.P. and F.S. acknowledge support by the state of Baden-Württenberg through bwHPC (bwunicluster and JUSTUS, RV bw17D011), and financial support from the Helmholtz Association is also gratefully acknowledged.
Supporting Information
Materials/Methods, Supplementary Text, Tables, Figures, and/or References
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Copyright © 2022 the Author(s). Published by PNAS. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
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Received: March 7, 2021
Accepted: November 4, 2021
Published online: January 19, 2022
Published in issue: January 25, 2022
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Acknowledgments
We acknowledge the support of the Bavarian Ministry of Economic Affairs and Media, Energy and Technology and Clariant Produkte (Deutschland) GmbH. F.M.K. thanks Lei Tao, Laura Tebcharani, Moritz Eder, Markus Pschenitza, and Greta Zambo for their helpful assistance in the experimental work. P.N.P. and F.S. acknowledge support by the state of Baden-Württenberg through bwHPC (bwunicluster and JUSTUS, RV bw17D011), and financial support from the Helmholtz Association is also gratefully acknowledged.
Notes
This article is a PNAS Direct Submission. I.H. is a guest editor invited by the Editorial Board.
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Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis, Proc. Natl. Acad. Sci. U.S.A.
119 (4) e2103840119,
https://doi.org/10.1073/pnas.2103840119
(2022).
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