Conversion of Synthesis Gas to Light Olefins: Impact of Hydrogenation

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Conversion of Synthesis Gas to Light Olefins: Impact of Hydrogenation Activity of Methanol Synthesis Catalyst on the Hybrid Process Selectivity over Cr−Zn and Cu−Zn with SAPO-34 Alexey V. Kirilin,*,† Joseph F. Dewilde,‡ Vera Santos,† Adam Chojecki,† Kinga Scieranka,† and Andrzej Malek‡ †

Dow Benelux B.V., Herbert H. Dowweg 5, Building 443 (BBB), 4252 NM, Hoek, Netherlands The Dow Chemical Company, Building 1776, Midland, Michigan 48674, United States

‡

S Supporting Information *

ABSTRACT: Direct conversion of syngas to hydrocarbons occurs over hybrid catalyst mixtures containing methanol synthesis and microporous acid components. In particular, both copper and zinc oxide-based as well as chromium- and zinc-based catalysts are active for methanol synthesis and can be used in the hybrid catalyst process. The choice of methanol synthesis catalyst alters product selectivity and distribution. In particular, reaction products of the Cu−Zn/SAPO-34 system include only saturated hydrocarbons, while the Cr−Zn/SAPO-34 catalyst enables light olefin production directly from syngas. Hydrogenation properties of the methanol synthesis catalyst influence the C3/C2 yield ratios in the hydrocarbon products. We analyze the observed differences of selectivity with respect to olefin hydrogenation activities of the methanol synthesis components and their interaction with SAPO-34 for methanol-to-olefins conversion. A simplified kinetic model for the hybrid system is proposed to describe the observed selectivity patterns.

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INTRODUCTION Light olefins (ethylene and propylene) serve as industrially vital feedstocks for production of plastics, functional materials, and as platform chemicals for production of other derivatives (ethylene oxide, acids, aldehydes, amines). Most industrially produced light olefins originate from steam cracking of hydrocarbons such as naphtha, gas oil, or natural gas condensates. This noncatalytic thermal process operates at high temperatures of around 850 °C. Fluctuations of crude oil price and the corresponding high volatility of the petrochemicals market have encouraged researchers worldwide to search for alternative feedstocks for light olefin production, including coal, natural gas, and even biomass. The use of alternative feedstocks also decreases oil dependence and facilitates new technologies for production of olefins. One attractive option originating from utilization of alternative feedstocks is synthesis gas − a mixture of hydrogen and carbon monoxide. Synthesis gas can be produced by coal/ biomass gasification or by the steam reforming of natural gas. Numerous literature reports describe synthesis gas to olefin conversion via different routes. The majority of these reports are based on Fischer−Tropsch (FT) and Methanol to Olefins (MTO) chemistries. Both approaches are briefly discussed below. I. Direct Catalytic Conversion of Synthesis Gas to Olefins via the Fischer−Tropsch Reaction (FTO). The © XXXX American Chemical Society

Fischer−Tropsch (FT) reaction for direct conversion of synthesis gas to olefins has been a subject of research for decades. In this process, carbon monoxide undergoes hydrogenation in the presence of Fe (high temperature FT) or Co (low temperature FT) catalysts with a subsequent carbon chain growth. The reaction results in the formation of a broad range of hydrocarbons. The target range of products (C2−C4 fraction) is limited by the Anderson-Schulz−Flory (ASF) distribution. Formation of C5+ and methane as well as coproduction of paraffins are among the key technical challenges. Mainly, Fe-based catalysts are applied for the FTO process. A recent report by Zhong et al., however, demonstrated successful use of cobalt nanoprisms for low temperature conversion of syngas to olefins via FT.1 The addition of promoters can improve the selectivity of Fe catalysts in FTO. For instance, Torres Galvis et al. reported improved activity and selectivity to olefins for supported Fe catalysts promoted with alkali and SO4.2 Other promoters, such as manganese and copper, were also reported to lower methane Special Issue: Tapio Salmi Festschrift Received: June 12, 2017 Revised: September 6, 2017 Accepted: September 8, 2017

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DOI: 10.1021/acs.iecr.7b02401 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research formation and extend catalyst lifetime.3 Despite the addition of promoters, catalyst activity, selectivity, and stability with time can be significantly influenced by the deposition of carbon and loss of Fe surface area.4 Koeken et al. described the effect of process conditions to minimize carbon deposition and improve catalyst performance.5 In the FTO process, despite the fact that high catalytic activity and conversions (>90%) can be achieved, selectivity to target C2−C4 olefins, formation of methane, and catalyst deactivation remain challenging. II. Production of Olefin from Synthesis Gas via the Methanol-to-Olefins Process (MTO). This technology has attracted considerable attention, mainly due to commercial deployments in China. Synthesis gas is a major source for methanol production. Methanol, in turn, can be directly converted to olefins via a one-step catalytic process using a zeolite catalyst SAPO-34. Reactions, typically performed at 350−450 °C, result in high olefin yields and low methane make. Coke formation rapidly deactivates the catalyst, although this is countered by continuous regeneration in a fluidized process. The need to have economical access to methanol can be a drawback. The use of methanol derived from a variety of sources, however, also allows for easier utilization of remote feedstocks. The mechanisms of MTO reaction propagation and deactivation over acidic zeolite and zeotype materials have been extensively studied and summarized thoroughly in a variety of reviews in the academic literature.6−9 Therefore, we hope to provide a brief summary of the chemistry and encourage the readers to reference the cited papers for more detail.6−9 Transient kinetic6,10 and carbon isotope labeling11,12 experiments have led the academic community to describe MTO using an autocatalytic “dual-cycle hydrocarbon pool” mechanism. In this mechanism, two major chemistries dictate the product distributions during catalysis: the olefin and aromatic cycles. The olefin cycle consists of (i) methylation of olefins via methanol to synthesize larger olefins and (ii) the cracking of C5+ olefins into lighter olefins, mostly propylene and butene,12 to create more olefin cycle carbon-chain-carriers, leading to the autocatalytic nature of the process. Similarly, the aromatic cycle is proposed to propagate through methylation of aromatic species that are either observed products or entrained within the zeolite/zeotype cavities − this would be the case for MTO over SAPO-34, as its structure contains large cavities with 8 member-ring windows unable to elute formed aromatics.7,8 The alkylated aromatic species then undergo dealkylation to form light olefins (ethylene and propylene) and recover less-substituted aromatics capable of continuing aromatic cycle propagation. The olefin and aromatic cycles are postulated to be linked by hydrogen transfer events of larger olefins (C5+) with either other olefins or methanol to form species capable of cyclizing to form aromatics species within the catalyst framework. In general, zeolite/zeotype structures7,8 or MTO process conditions6 that favor aromatic cycle propagation will lead to larger fractions of light olefins (ethylene and propylene), while favoring olefin cycle propagation will result in higher C4+ olefin selectivities. III. Direct Production of Hydrocarbons/Olefins Using Bifunctional Catalysts. The necessity for intermediate production of methanol in the MTO process can be eliminated by combining methanol synthesis and acidic functionality in bifunctional catalyst mixtures to enable direct conversion of synthesis gas into a mixture of hydrocarbons. Hydrocarbons

can be used as a feedstock for cracker to produce olefins. There are numerous reports in the literature describing implementation of bifunctional materials containing the Cu-based methanol catalyst with acidic components for this purpose. For example, a combination of the commercial low temperature methanol synthesis catalyst13 with SAPO-34 enables direct production of saturated C2−C4 hydrocarbons as reported by Chojecki et al.14 and Nieskens et al.15 While SAPO-34 was reported as a component of hybrid catalyst targeting C2−C4 hydrocarbons,16−19 BETA20 or USY21 zeolites favor formation of C2−C3 products. Gasoline range product formation was reported when ZSM-522,23 or Ferrierite24 was used. Recently Jiao et al.25 and Cheng et al.26 reported direct conversion of synthesis gas to olefins via the mixed oxidezeolite process, where SAPO-34 was combined with Cr−Zn−Al and Zn−Zr catalysts, respectively. Interestingly, using Zn-based methanol synthesis catalysts (designed for operation at high temperature) minimizes hydrogenation of olefins to paraffins and achieves total C2−C4 hydrocarbon selectivity as high as 52% (44% selectivity to C2−C4 olefins) at 17% CO conversion.25 The conversion of synthesis gas to light olefins using a bifunctional chromium oxide/zinc oxide-SAPO-34 catalyst was also reported by Nieskens et al.27 In the present paper, we aim to compare two bifunctional catalytic mixtures containing a Cu−Zn (here and after the catalyst is referred as “Cu−Zn”) catalyst designed for low temperature methanol synthesis or a Cr−Zn catalyst designed for high temperature methanol synthesis in combination with a zeolite component SAPO-34 for the direct conversion of synthesis gas to olefins. The focus of this study is the distribution of products and kinetics of product formation.

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MATERIALS AND METHODS The following chemicals were purchased from Sigma-Aldrich and used as received: chromium(III) nitrate nonahydrate (99%), zinc nitrate hexahydrate (99%), and ammonium carbonate (99%). Catalysts Synthesis. Methanol Synthesis Catalysts. Low temperature Cu-based methanol catalyst “HiFuel” (here and after referred as “Cu−Zn”) was purchased from Alfa Aesar and used after resizing to 60−80 mesh. The expected bulk particle envelope packing density of the sieved Cu−Zn catalyst is 1.09 g cm−1. A high temperature Cr-based methanol catalyst (hereafter referred to as “Cr−Zn”) was synthesized by coprecipitation according to known literature procedures.28 In brief: manual coprecipitation was carried out in a 2 L, 3-neck flask at 80 °C (maintained in the range ±5 °C), under gentle stirring (300 rpm). The process was started by dripping to a buffer (DI water, 500 mL) simultaneously: (i) an aqueous mixture of nitrate salts (ratio Cr/Zn = 4/10; c0 = 1.4 mol/L) and (ii) an aqueous solution of the precipitating agent (ammonium carbonate, 1.5 mol/L) targeting the pH set point = 7 (±0.2). The total addition time was 90 min. The total amounts of added solutions (i) and (ii) were approximately 350 and 550 mL, respectively. The precipitate then aged at the process temperature for 3 h in the mother liquor and was recovered by centrifugation. Finally, the precipitate cake was dried, crushed, and calcined at 500 °C (4 h dwell time) to obtain the mixed Cr−Zn oxide. The final material was resized to 60−80 mesh. The expected bulk particle envelope packing density of the sieved Cr−Zn catalyst is 0.95 g cm−1. B

DOI: 10.1021/acs.iecr.7b02401 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Zeotype Synthesis (SAPO-34). Synthesis of the zeotype component (SAPO-34) was performed in accordance with literature procedures.29 A SAPO-34 sample was calcined to remove the template prior to catalytic measurements. Calcination was performed in air using the following procedure: ambient temperature → 600 °C at 5 °C/min, dwell at 600 °C for 4 h. Calcined SAPO-34 was pelletized, then crushed, and sieved to 60−80 mesh size (expected particle envelope packing density of the sieved catalyst = 0.51 g cm−1). Characterization. We determined the chemical composition of the catalysts via wavelength Dispersive XRF (WDXRF). Measurements were performed at room temperature with an Axios PANalytical spectrometer using an X-ray tube with a rhodium anode. UniQuant was used to process the obtained data. The detected elements (Na and higher) are reported as metal weight percentage of the inorganic oxide composition. The crystalline composition was determined by X-ray diffraction (XRD). XRD D8 ADVANCE (BRUKER) was used to measure the ground residues at a 35 kV and 45 mA Xray generator condition and with CoKα1/2 radiation. The diffractogram was recorded in coupled 2θ mode using a LYNXEYE detector (solid angle: 2.89°) at the 2θ range of 10° to 98°, 0.05 step size, and 3 s step time. On the primary side, a variable slit was applied to realize a constant radiated sample length of 15 mm. Furthermore, a Fe filter and a 2.5° soller were mounted. On the secondary side, a soller of 2.5° was used in combination with a 6 mm fixed slit. The sample was mounted on a silicon sample holder with a recess. The XRD patterns, after 2θ calibration against a reference standard, background fitting, and Kα2 stripping, were analyzed on the ‘JADE 9.6 Plus XRD Pattern Processing, Identification & Quantification’ program (Materials Data, Inc. 1995−2013) which uses the International Centre for Diffraction Data (ICDD) database of XRD patterns to identify the crystalline phase(s) observed. Textural properties were determined by performing N2physisorption at −196 °C on a TriStar II 3020 with a connected SMART Prep 065, both from Micromeritics. Prior to the measurement, all samples were evacuated at 200 °C for 2 h under a nitrogen atmosphere. Data analysis was performed with Microactive software V3.00. The BET area was determined by direct fitting of points in the P/P0 = 0.05− 0.15 domain, constraining the upper boundary of the relative pressure window using the two-point BET method.30 Total pore volume was measured at P/P0 = 0.95, where no interparticle condensation occurred. The external surface area and microporous volume were determined by the t-method, obtaining a linear fit in the De Boer thickness of at least 10 data points. Temperature-programmed reduction (TPR) was used to evaluate the reducibility of Cr/Zn mixed oxides. These experiments were performed on AutoChem 2920 from Micromeritics. In a typical experiment, 100 mg of sample was placed in a quartz tube and exposed to a mixture of 10 vol % H2/He. The sample was then heated using a rate of 10 °C/min from RT to 800 °C with a flow-rate of 20 cm3 min−1 (STP). The outlet composition of the gas mixture was measured as a function of time using a thermal conductivity detector (TCD) and a mass spectrometer (MS). For Scanning Electron Microscopy (SEM) analysis, the powder samples were used as received avoiding a conductive coating processing in order to preserve fine morphological features. The SEM images of materials placed on a conductive

carbon adhesive were obtained with NOVA nanoSEM 600 (FEI, Eindhoven, The Netherlands) operated in low vacuum mode with an accelerating voltage of 5 kV and spot 4.5. The images were recorded using a GAD backscattered electron detector. Catalytic Test. Bifunctional hybrid catalyst mixtures were prepared by physically mixing a methanol synthesis component (Cr−Zn or Cu−Zn) with SAPO-34 in equal volumetric proportion unless specified otherwise. The mixing was carried out in a plastic vial using 60−80 mesh size of a mixed oxide and SAPO-34. Homogeneous distribution was achieved by shaking the as-prepared mixture for 1 min. Catalytic tests were performed in a tubular stainless steel fixed-bed microreactor (i.d. 3 mm). The bottom of the stainless steel reactor was equipped with a metal frit to support the catalyst bed. Total catalyst bed loading was 100−400 μL. Total gas flow through a catalyst bed was 6 mL/min. Synthesis gas contained hydrogen, carbon monoxide, helium as internal standard, and nitrogen as balance gas. Detailed compositions of the tested synthesis gas mixtures are given in Table S1. Reactions were carried out at 20 bar and between 380 and 410 °C. Catalysts were heated (5 °C min−1) in nitrogen flow at ambient pressure until the desired temperature was reached. Reactors were then slowly pressurized to 20 bar, followed by an introduction of a synthesis gas mixture. Time zero indicates introduction of synthesis gas. After changing synthesis gas composition, the reaction was allowed to reach steady state (∼6 h time-on-stream). See Table S1 for more details. Reaction products were analyzed by means of gas chromatography. Online analysis of components (N2, H2, He, CO, CO2, C1−C5 alkanes, and olefins) was performed periodically using a Maxum GC system. Mass balance in all experiments was 95−100% based on carbon. Calculation of Conversion and Selectivities. Carbon monoxide conversion [%]: c prod XCO = × 100 ctotal (1) where n

c prod =

∑ n × Ci /He − CO/He i=1

(2)

n

ctotal =

∑ n × Ci /He

(3)

i=1

Ci and He are GC concentrations (mol %) of the i-component with n carbon atoms in the structure and helium concentration (internal standard), respectively, in the reactor effluent. Selectivity (carbon basis) [%]: Si =

n × Ci /He × 100 c prod

(4)

Carbon yield [%]: Yi = XCO/100 × Si

(5)

Reported values are within ±5% (relative error).

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RESULTS AND DISCUSSION Characterization. Quantitative bulk elemental and structural analysis for both mixed oxides are summarized in Table S2.

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Figure 1. SEM micrographs in back scattered electron contrast of mixed Cr−Zn and Cu−Zn oxides (horizontal field of view 2.98 and 1.49 μm). Gray tone differences provide elemental composition variation. The efficiency of production of backscattered electrons is proportional to the sample material’s mean atomic number, which results in image contrast as a function of composition (i.e., lighter vs heavier).

respectively). Note that, at the magnification employed, the catalysts appeared homogeneous with hardly any spatial variations of elemental composition. The textural properties were evaluated by nitrogen adsorption at −196 °C. The isotherms are shown in Figure S3. The Cr−Zn mixed oxide catalyst showed a type II isotherm (IUPAC classification), characteristic of macroporous material. The shape of hysteresis loop is close to an H3 type and indicates the presence of slit-shaped pores. Such pores may result from the spatial arrangement of nanoparticles. The Cu− Zn catalyst showed a type IV isotherm typical for mesoporous materials (2−50 nm). The hysteresis was characterized by a plateau at high P/P0 and by parallel adsorption/desorption branches due to adsorption in unconnected mesopores with a relatively narrow pore size distribution. The BET surface areas of both catalysts are summarized in Table S3 and range from 20 m2 g−1 (Cr−Zn sample) to 100 m2 g−1 (Cu−Zn sample). The reducibility of Cr−Zn and Cu−Zn catalysts was evaluated by H2-TPR. The TPR patterns are shown in Figure S4. The TPR pattern of the Cr−Zn catalyst (Figure S4a) is

The bimetallic Cr−Zn mixed oxide featured well-crystallized ZnO and ZnCr2O4 domains; interestingly, no Cr2O3 phase was detected (Figure S1). We could assume that, having an excess of zinc, all the chromium reacted into a spinel structure. Another plausible interpretation was that Cr2O3 domains were well dispersed, as deduced by comparing the actual XRF data to the computed elemental makeup ensuing from the quantitative interpretation of the diffractograms. For the low-temperature methanol synthesis catalyst (Cu−Zn), the phase assignment was possible only for the main components i.e., copper oxide and zinc oxide (Figure S2). Due likely to a low concentration of the other elements (Al, Mg) and a high dispersion, no phase containing Al or Mg was detected. For the high-temperature methanol Cr−Zn catalyst, the SEM analysis (see Figure 1) revealed agglomerates of globular particles with diameters up to 100 nm. The Cu−Zn catalyst showed a significantly finer structure with a narrower distribution of particle size around 20 nm. This comparison agreed with an evaluation of the computed size of primary crystallites for the XRD-visible oxide phases formulating each catalyst (15−30 vs 4−5 nm, D

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Figure 2. Reaction pathways in the conversion of synthesis gas to olefins over a hybrid catalyst that consists of a methanol catalyst and zeolite SAPO34.

Figure 3. Catalytic performance of bifunctional mixtures containing Cr−Zn (red circles) or Cu−Zn (blue triangles) methanol catalyst time-onstream. Conditions: synthesis gas composition: H2/CO = 2, P = 20 bar, 6 mL/min flow, T = 400 °C. Total catalyst loading was 400 μL (∼320 mg) and 200 μL (∼160 mg) for Cr−Zn/SAPO-34 and Cu−Zn/SAPO-34 mixtures, respectively. a) Carbon monoxide conversion, b) selectivity to C2− C5 paraffins, and c) selectivity to C2−C5 olefins.

characterized by a main peak centered at 285 °C and two overlapping peaks centered at 350 and 450 °C, respectively. Previous studies have demonstrated that the consumption of hydrogen is associated with surface reduction of Cr(VI) species to Cr(III).31,32 Based on integration of the TPR curve, we estimate that 5−8% of the chromium atoms are in the form of Cr(VI) (see Table S3). The TPR pattern of Cu−Zn catalyst is shown in Figure S4b. The reduction pattern is characterized by the main peak centered at 200 °C and ascribed to the reduction of CuO to metallic Cu.33−35 The integration of the TPR curve estimates that 90% of copper is reduced to a metallic state under the reaction conditions (Table S3). Catalytic Results. Catalytic conversion of synthesis gas to a mixture of olefins is illustrated in Figure 2. In the first step, carbon monoxide is hydrogenated to methanol over metallic sites (M sites) of the methanol synthesis catalyst (Cr−Zn or Cu−Zn), followed by dimethyl ether (DME) formation that can take place over acid sites on either component. The methanol synthesis equilibrium is unfavorable under the experimental conditions chosen for hybrid chemistry (high T and relatively low P). Oxygenates formed under these conditions react further over SAPO-34 (zeotype). This reaction shifts methanol formation equilibrium by removing oxygenates and results in the formation of light olefins. The latter likely occurs via conventional methanol-to-olefins routes.36,37 Due to the presence of hydrogen in the reaction mixture, the formed olefins can also undergo posthydrogenation to paraffins over metallic sites of methanol catalyst or, to some extent, over acidic sites of SAPO-3438 (the concentration of strong acid sites determined by NH3-TPD in SAPO-34 is 0.76 mmol g−1 as reported in ref 15). We postulate methane formation originates from the direct hydrogenation of methanol over metallic sites of the methanol component of the hybrid catalyst. Under

experimental conditions, water that is formed as a byproduct of CO hydrogenation and olefin formation reacts rapidly with unconverted carbon monoxide via water−gas shift (WGS) to form carbon dioxide and additional hydrogen. Figure 3 shows the performance of both hybrid mixtures in the conversion of synthesis gas to olefins at 20 bar and 400 °C (H2/CO = 2). Catalysts loadings were selected to adjust space velocities and compare catalytic performance at similar conversion levels. Both hybrid mixtures showed high activity for synthesis gas conversion. The decrease in catalytic activity with time observed for the hybrid catalyst containing a Cu−Zn component is likely to be caused by sintering of small copper particles in the Cu−Zn component17,39 under synthesis gas (not due to deactivation of SAPO), in good agreement with previously reported data for the Cu−Zn/SAPO-34 system.15 For both of the hybrid mixtures, the reaction products are mainly C2−C4 hydrocarbons, small quantities of methane, and carbon dioxide originating from a water−gas shift reaction that occurs rapidly under experimental conditions. Figure 3b and Figure 3c clearly demonstrate the difference in the product formation between two hybrid catalysts containing either the Cr−Zn or the Cu−Zn component and emphasize the importance of material selection for target product formation. Practically no olefins were observed in the case of the Cu-based catalyst due to a very high hydrogenating activity of the Cu−Zn catalyst. While in the case of the hybrid catalyst containing Cr− Zn, mainly paraffins were formed, but C2−C4 olefin formation was also observed, pointing to a lower hydrogenation ability of Cr−Zn compared to Cu−Zn. Figure 4 shows the selectivity of hydrocarbon products formed for the two hybrid mixtures. While some olefin formation was observed for Cr−Zn/SAPO-34 (C2−C4 olefin selectivity 11%), Cu−Zn/SAPO-34 showed only paraffin E

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Figure 4. Product composition in direct conversion of synthesis gas to olefins over bifunctional catalysts containing Cr−Zn or Cu-based methanol component. Conditions: H2/CO = 2, P = 20 bar, 6 mL/min flow, T = 400 °C. Total catalyst loading was 400 μL (∼320 mg) and 200 μL (∼160 mg) for Cr−Zn/SAPO-34 and Cu−Zn/SAPO-34 mixtures, respectively. Measurements were gathered with 50 vol % loading of SAPO-34. Data were collected between 7 and 9 h time-onstream. Full bar: paraffins, upward diagonal texture: olefins.

Figure 5. Measured ratio of carbon yield of C3 and C2 products over (striped) Cu−Zn/SAPO-34 and (solid) Cr−Zn/SAPO-34 systems at 380, 400, and 410 °C with 50 vol % SAPO-34 loading and at 400 °C with 75 vol % SAPO-34 loading. The data labels on each bar indicate the measured CO conversion for the reaction. Total catalyst loadings for Cu−Zn/SAPO-34 systems were 100 μL (∼80 mg) at 380 °C and 200 μL (∼160 mg for 50% and ∼135 mg for 75%) in all other cases. The total catalyst loading for the Cr−Zn/SAPO-34 systems was 300 μL (∼240 mg) in all cases. The feed mixture was 6 mL/min of 30% CO, 60% H2, and balance inerts.

formation. Noteworthy, a lower methane and higher C4 selectivity was observed for Cu−Zn/SAPO-34 compared to Cr−Zn/SAPO-34 (see conversion levels in Figure 3a). Detailed data on product composition for the two hybrid catalysts at 400 °C are given in Table S4. It is worth noting that oxygenates (methanol and dimethyl ether) were not detected among reaction products, indicating their full conversion to hydrocarbons over SAPO-34. Notably, we noticed a difference in the C3/C2 hydrocarbon formation between the two hybrid catalysts as illustrated in Figure 4 and in Table S4. This important experimental observation points to a potential difference in reaction pathways of hydrocarbon formation originating from the difference in methanol component as SAPO-34 was equivalent for both hybrid catalysts. Such a difference in product distribution has important implications for industrial feasibility of this process and is of high fundamental interest. Therefore, we decided to study this effect in more detail. Analysis of C3/C2 Product Distribution and Kinetic Model for Cr−Zn/SAPO-34. Comparison between Cu−Zn and Cr−Zn-Based Systems. Herein, the “C3/C2 ratio” signifies the total carbon yield of C3 products (propylene and propane) normalized to the total C2 product carbon yield (ethylene and ethane). For both Cr−Zn/SAPO-34 and Cu−Zn/SAPO-34 systems, this ratio varies with time-on-stream, but all observed trends and differences are maintained (average values are reported). The C3/C2 ratio is consistently higher for Cr−Zn/ SAPO-34 systems compared to Cu−Zn/SAPO-34 systems at the same conversion level for all measured conditions of 380− 410 °C and with 50 or 75 vol % SAPO-34 loading (Figure 5). This observation is in agreement with the simplified reaction scheme shown in Figure 6. In this reaction network, the mixedmetal oxide serves only to synthesize methanol and hydrogenate olefins and methanol to form paraffins and methane, respectively. Furthermore, we propose carbon chain growth occurs predominantly in SAPO-34 via the well-studied methanol-to-hydrocarbons process over zeotype materials − Bjørgen et al.12 describe the mechanism of methanol-tohydrocarbons over H-ZSM-5 in detail based on isotopic

transient measurements. In the case of the Cu−Zn/SAPO-34 system, we postulate that any formed olefins that leave the SAPO-34 cages quickly convert into paraffins, a much less reactive and, thus, terminal product. Resultantly, the olefin pressure throughout the reactor is much lower in the Cu−Zn/ SAPO-34 system, leading to a lower propagation rate of the SAPO-34 olefin cycle compared to that of the Cr−Zn/SAPO34 system. The propagation of this cycle would be expected to result in a comparatively higher propylene make from ethylene methylation or from cracking mechanisms. Ilias and Bhan40 observed that the 13C content of ethylene was distinct from other light olefins upon cofeeding 13C propylene with 12C dimethyl ether over H-ZSM-5, leading the authors to argue that ethylene is largely not produced from olefin cycle cracking reactions. We extend this postulate to conclude that olefin cycle propagation in the mixed methanol-synthesis catalyst/SAPO-34 system will promote propylene formation compared to ethylene production. The C3/C2 ratio in the Cu−Zn/SAPO-34 system is relatively unchanged upon increasing the SAPO-34 fraction from 50 to 75 vol % (1.23 and 1.19, respectively). In contrast, this ratio increases from 1.5 to 2.1 for the same change in the Cr−Zn system. The virtual absence of olefin in the bulk vapor phase in the Cu−Zn system, due to fast olefin hydrogenation, rationalizes the observed invariance with SAPO-34 loading. Under negligible bulk olefin partial pressure, each SAPO-34 particle acts relatively independently, with only local methanol partial pressure differentiating the reaction rates (similar at lower conversions). Therefore, we postulate that relative C3 and C2 production rates will be virtually equivalent at low CO conversions across all SAPO-34 particles, resulting in the observed SAPO-34 loading invariance. The effect of SAPO-34 loading in the Cr−Zn/SAPO-34 system is analyzed below. The difference in C3/C2 ratios between the Cr−Zn/SAPO34 and Cu−Zn/SAPO-34 catalyst systems demonstrates the role of the relative hydrogenation behavior in affecting SAPO34 product distributions. We postulate that selection of synthesis gas-to-methanol catalysts to balance methanol synthesis and olefin hydrogenation rates adds an additional F

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Figure 6. Schematic of the proposed reaction network for syngas conversion over the hybrid system of Cr−Zn and SAPO-34 catalysts.

degree of control in dictating olefin product distribution beyond just reaction conditions investigated for traditional methanol-to-hydrocarbons processes. This effect presents future avenues of research in the development of models and predictions to describe a target methanol-synthesis catalyst behavior to optimize propylene yield out of the combined dualcatalytic system. Effects of Process Conditions on the C3/C2 Ratio for the Cr−Zn System. On its own, the methanol-to-hydrocarbons process over zeotype materials contains many possible series and parallel reaction pathways that evolve as the reaction proceeds with increasing conversion.12 Therefore, we establish the influence of feed composition and SAPO-34 loading fraction on resulting C3/C2 ratios at similar conversions (