Research Article pubs.acs.org/acscatalysis
Bifunctional Strategy Coupling Y2O3‑Catalyzed Alkanal Decomposition with Methanol-to-Olefins Catalysis for Enhanced Lifetime Andrew Hwang and Aditya Bhan* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Bifunctional strategies exploiting the selective and catalytic decomposition of formaldehyde by Y2O3 improve the lifetime of CHA zeotypes and zeolites for methanol-toolefins catalysis 4-fold, as quantified by total turnovers, without disrupting the inherently high selectivity to light olefins. The improvement in catalyst lifetime increases with increasing proximity between H+ sites of the zeotype/zeolite and the surface of the rare earth metal oxide. This proximity effect demonstrates crucial transport of formaldehyde between and within zeotypic/zeolitic domains on catalyst lifetime. These results provide mechanistic insights revealing formaldehyde as an accelerant for the initiation and termination of chain carriers and exemplify a strategy for designing improved methanol-toolefins catalysts by optimizing (bi)functionality and reaction-transport dynamical phenomena. KEYWORDS: methanol-to-olefins, rare earth metal oxide, bifunctional catalysis, formaldehyde, deactivation, yttrium oxide mains,17,18 and incorporating metals and/or their cations.19−22 Levin and Vartuli23 demonstrated an alternative approach to improve lifetime via addition of rare earth metal oxides to fixed beds of HSAPO-34, with Y2O3 conferring the largest improvement in lifetime, and recent studies corroborate efficacy of rare earth metal species to improve lifetime of zeotypes/ zeolites for methanol-to-olefins catalysis.24−27 These reports, however, do not identify the function or the identity of active rare earth metal species nor provide mechanistic justification for their synergistic effects with zeotypes/zeolites during methanolto-olefins catalysis. Here, we reveal improved efficacy of Y2O3 to enhance lifetime by increasing the proximity between the rare earth metal oxide and the zeotype/zeolite. We identify and attribute this proximity effect to the function of Y2O3to catalytically decompose formaldehydeand evince formaldehyde as an accelerant for chain carrier initiation and termination. These results support recent results and hypotheses on the deleterious consequences of methanol dehydrogenation on chain carrier termination7,8 and provide lucidity on mechanistic precepts necessary to guide the design of improved processes and materials for methanol-to-olefins catalysis.
1. INTRODUCTION Methanol-to-hydrocarbons catalysis by microporous solid acids proceeds via an indirect, autocatalytic chain growth mechanism.1−4 Impurities5 and/or rare C1 activation events2 initiate formation of unsaturated organic moieties within inorganic confining voids resulting in hybrid organic−inorganic1 active centers which propagate C−C bond formation and scission. The organic cocatalystsolefins and aromaticsevolve spatially and temporally in both number and identity, eventually terminating as inactive polycyclic aromatic hydrocarbons.1,2 The complex network of reactions describing the propagation of active centers is summarized by a dual cycle schematic6 where olefins methylation and β-scission are coupled with aromatics methylation and dealkylation through hydrogen transfer, dehydrocyclization, and dealkylation.2,4 Termination of active chain carriers is mediated by formaldehyde formed in bimolecular transfer dehydrogenation of methanol.7,8 Methanol-to-olefins processes in practice9,10 utilize HSAPO34, a microporous silicoaluminophosphate zeotype11 of CHA topology12 (three-dimensional structure with large ellipsoidal cavities, 10 × 6.7 Å, interconnected via narrow eight-membered ring apertures, 3.8 × 3.8 Å12), which suffers short lifetimes because of the rapid accumulation of terminated cocatalysts as compelled by the narrow apertures.1,2,9,10 Investigated strategies to extend lifetime through designed modifications to CHA zeotypes and zeolites include decreasing crystallize size,13,14 optimizing site density,15,16 introducing mesoporous do© XXXX American Chemical Society
Received: March 20, 2017 Revised: April 22, 2017
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DOI: 10.1021/acscatal.7b00894 ACS Catal. 2017, 7, 4417−4422
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2. MATERIALS AND METHODS 2.1. Materials Synthesis and Characterization. Aluminum isopropoxide (Al(i−O-Pr)3, Aldrich), aluminum hydroxide (Al(OH)3, Aldrich), phosphoric acid (85 wt % H3PO4 in water, Aldrich), colloidal silica (30 wt % SiO2 in water, LUDOX AS-30), sodium hydroxide (50 wt % NaOH in water, Aldrich), tetraethylammonium hydroxide (TEAOH, 35 wt % (C2H5)4N(OH) in water, Aldrich), N,N,N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH, 25 wt % TMAdaOH in water, SACHEM), ammonium nitrate (NH4NO3, Sigma-Aldrich, ACS Reagent), and yttrium nitrate hexahydrate (Y(NO)3·6H2O, Aldrich, 99.9% (REO)) were used without further purification. SAPO-34 was prepared following a reported procedure28 with TEAOH as the organic structure directing agent using solutions of molar composition 0.15 SiO2:1.0 Al(i−O-Pr)3:1.0 H3PO4:32 H2O:1.0 TEAOH. First, Al(i−O-Pr)3 (82 g) was added to aqueous H3PO4 (46 g) and deionized water (104 g) under continuous stirring. Then, aqueous colloidal SiO2 (12 g) and deionized water (5 g) were added and stirred continuously until a homogeneous mixture formed. Finally, aqueous TEAOH (168 g) was added under continuous stirring. The resulting gel mixture was then transferred to a 0.6 L Teflon-lined stainless steel digestion bomb, sealed, and heated at 473 K under autogenous pressure for 120 h. The solid product was recovered by centrifugation, washed with deionized water, and dried overnight at 373 K. The dried SAPO-34 sample was stored with the organic structure direct agent intact to prevent modifications to the coordinative heteroatom environment29 and only detemplated (and converted to H+ form) in situ, via oxidative thermal treatment (vide infra), immediately prior to catalytic testing. SSZ-13 was prepared following a reported procedure16 with TMAdaOH as the organic structure directing agent using solutions of molar composition 0.1 Al(OH)3:1.0 SiO2:0.2 NaOH:44 H2O:0.2 TMAdaOH. First, aqueous NaOH (0.34 g) and aqueous TMAdaOH (3.6 g) were added to deionized water (10.2 g) under continuous stirring. Then, Al(OH)3 (0.16 g) was added to the reaction mixture and stirred continuously for 0.5 h. Finally, aqueous colloidal SiO2 (4.0 g) was added to the reaction mixture and stirred continuously overnight at ambient conditions. The resulting gel mixture was then transferred to a 0.045 L Teflon-lined stainless steel digestion bomb, sealed, and heated at 433 K under autogenous pressure for 264 h. The solid product was recovered by filtration, washed with deionized water, and dried at 363 K. The dried product was treated in flowing dry air at 423 K for 2 h (0.0167 K s−1) and then at 823 K for 8 h (0.0167 K s−1) to remove the organic structure directing agent. Template-free SSZ-13 was treated in aqueous 1.0 M NH4NO3 under continuous stirring for 4 h at 353 K and recovered by filtration. The exchange process was repeated thrice. The resulting NH+4 -exchanged sample was washed with deionized water, dried overnight at ambient conditions, and decomposed to H+ form via the previously described oxidative thermal treatment. Y2O3 was prepared following a reported procedure.23 First, Y(NO)3·6H2O (2.0 g) was dissolved in deionized water (20 g), and then, the pH of the mixture was adjusted to 9 by addition of concentrated ammonium hydroxide. This slurry was transferred to a Teflon-lined stainless steel digestion bomb, sealed, and heated at 373 K under autogenous pressure for 72 h. The solid product was recovered by filtration, washed with
excess deionized water, and dried overnight at 358 K. The dried product was treated in flowing dry air at 873 K for 3 h. Powder X-ray diffractograms were collected using a Siemens D-500 diffractometer with a 2.2 kW sealed cobalt source (Co Kα, λ = 1.79 Å) and recalculated and reported for Cu Kα radiation (λ = 1.54 Å). Scanning electron micrographs were collected with a JEOL 6700 field emission gun microscope at an acceleration voltage of 5 kV. The elemental compositions of SAPO-34 and SSZ-13 samples were measured by inductively coupled plasma optical emission spectroscopy (Analytical Geochemistry Lab, University of Minnesota). Micropore volumes and BET surface areas were calculated from measured N2 physisorption isotherms at 77 K collected using a Micromeritics ASAP 2020 analyzer with prior sample degassing (≤6 μmHg at 363 K and heating under vacuum at 723 K for 4 h (0.0167 K s−1)). Brønsted acid site densities of HSAPO-34 and HSSZ-13 samples were measured using NH3 temperatureprogrammed desorption: HSAPO-34/HSSZ-13 (∼0.050 g) was treated in flowing 500 ppm of NH3 (1.67 cm3 s−1, 1.01% NH3 in He, Praxair, certified standard) at 323 K for ≥2 h, purged in flowing He (Minneapolis Oxygen, 99.997%; 1.67 cm3 s−1) for ≥8 h, and ramped to 823 K (0.167 K s−1) in flowing He and Ar (Matheson, UHP/Zero; used as an internal standard) while continuously monitoring the effluent via online mass spectrometry (MKS Cirrus, m/z = 16, 17, 18, 40) to quantify desorbed NH3; the H+ site density was calculated by assuming unit stoichiometry between H+ and NH3 desorbed from 423 to 823 K. The base site density of Y2O3 was measured using CO2 temperature-programmed desorption: Y2O3 (∼0.100 g) was treated in flowing 1 kPa CO2 (Airgas, research grade; 1.67 cm3 s−1) at 303 K for ≥2 h, purged in flowing He for ≥8 h (1.67 cm3 s−1), and ramped to 873 K (0.167 K s−1) in flowing He and Ar while continuously monitoring the effluent via online mass spectrometry (m/z = 16, 28, 40, 44) to quantify desorbed CO2; the base site density was calculated by assuming unit stoichiometry between a base site and desorbed CO2. Powder X-ray diffractograms, scanning electron micrographs, and temperature-programmed desorption profiles of SAPO-34, SSZ-13, and Y2O3 samples are reported in Figures S1, S2, and S3, respectively, of the Supporting Information (SI). Elemental analyses, textural properties, and site densities of SAPO-34, SSZ-13, and Y2O3 samples are reported in Table S1 of the SI. 2.2. Catalytic Testing. Reactions were performed on catalyst beds composed of 180−250 μm aggregates of HSAPO34 (0.025−0.040 g), HSSZ-13 (0.040 g), Y2O3 (0.005−0.010 g), or mixtures thereof diluted with quartz sand (180−250 μm; washed in 2 M HNO3, rinsed with deionized water, and treated in flowing dry air at 1273 K for 12 h; ≤0.1 gcat g−1 sand). Stratified catalyst bed configurations utilized inert quartz wool to separate stratum of HSAPO-34 or HSSZ-13 mixed with quartz sand from stratum of Y2O3 mixed with quartz sand. Interpellet physical mixtures were prepared by mixing 180−250 μm aggregates of HSAPO-34 or HSSZ-13 with 180−250 μm aggregates of Y2O3 and quartz sand. Intrapellet physical mixtures were prepared by mixing HSAPO-34 or HSSZ-13 powders with Y2O3 powders using an agate mortar and pestle and then pelletizing, crushing, and sieving to obtain 180−250 μm aggregates. Catalyst beds were held between inert quartz wool plugs in a “U”-shaped tubular quartz reactor (0.4 cm i.d.) placed inside a resistively heated furnace (National Element FA120). The reaction temperature was regulated by an electronic controller 4418
DOI: 10.1021/acscatal.7b00894 ACS Catal. 2017, 7, 4417−4422
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Figure 1. Total turnovers (left, bars) for methanol-to-olefins reactions on catalyst beds configured with (i) HSAPO-34/HSSZ-13 only, (ii) Y2O3 packed downstream of HSAPO-34/HSSZ-13, (iii) Y2O3 packed upstream of HSAPO-34/HSSZ-13, (iv) an interpellet physical mixture of HSAPO34/HSSZ-13 with Y2O3, and (v) an intrapellet physical mixture of HSAPO-34/HSSZ-13 with Y2O3. The relative increase in total turnovers (right, ◊) is the total turnovers for catalyst bed configurations (ii)−(v) normalized to the total turnovers for configuration (i). Reaction conditions: 673 K, 12 kPa MeOH, 24 wt % Y2O3 (for (ii)−(v)), and 1.2 × 102 molMeOH (molH+·ks)−1.
lifetime of HSAPO-34 for methanol-to-olefins catalysis, consistent with reported effects of impurities on methanol-tohydrocarbons lifetime.34,35 Total turnovers of HSAPO-34 and HSSZ-13 increaseby factors of 2.5 and 1.9, respectively, relative to configurations without Y2O3when 180−250 μm aggregates of zeotype/ zeolite are physically mixed with 180−250 μm aggregates of Y2O3. Total turnovers of HSAPO-34 and HSSZ-13 increase furtherby factors of 4.2 and 3.8, respectively, relative to configurations without Y2O3when the intimacies of the mixtures are enhanced by physically mixing powders of HSAPO-34/HSSZ-13 with powders of Y2O3 prior to forming 180−250 μm aggregates. The observed increase in total turnovers with increasing mixture intimacy implies that a stable, gas-phase species formed within the zeotype/zeolite that adversely effects catalyst lifetime egresses through the eightmembered ring apertures of CHA cavities to the Y2O3 surface where it is adsorbed, converted, or otherwise transformed to free or bound moieties inconsequential to catalyst lifetime. Formaldehyde matches the criteria prescribed by the observed proximity effect: (i) formaldehyde is a product of acid-catalyzed bimolecular dehydrogenation of methanol either by methanol, via dehydrative disproportionation to methane,8,36−38 or olefins, via transfer hydrogenation to paraffins;7,8 (ii) formaldehyde terminates active olefinic and aromatic chain carriers via alkylation-mediated routes accelerating accumulation of unreactive carbonaceous deposits;7,8 (iii) the kinetic diameter of formaldehyde (4.5 Å39) is similar to that of propane (4.3 Å40) consistent with the (sufficiently) facile transport of formaldehyde through the eight-membered apertures of CHA cavities at temperatures relevant to methanol-to-olefins catalysis; and (iv) the base sites exposed on the Y2O3 surface catalyze selectively the decomposition of formaldehyde to CO (vide infra). We hypothesize that Y2O3 improves lifetime by scavenging formaldehyde via decomposition to innocuous CO and H2 thereby preventing formaldehyde from alkylating organic cocatalysts hosted inside the zeotype/zeolite and inducing
(Watlow 96) and measured using a K-type thermocouple wrapped and tied around the outer wall of the reactor near the axial center of the catalyst bed. Catalyst beds were treated in flowing air (1.67 cm3 s−1, Matheson, UHP/Zero) at 823 K (0.0167 K s−1) for 6 h prior to testing. Liquid methanol (Fluka Analytical, ≥99.9%), aqueous formaldehyde solution (16% (w/ v) in water, Pierce Chemical), and deionized water were delivered using a syringe pump and vaporized into heated gas lines (≥393 K) with flowing He (Minneapolis Oxygen, 99.997%). He flow rates were metered using thermal mass flow controllers (Brooks SLA58580). Compositions of reactor influent and effluent mixtures were measured by gas chromatography using flame ionization detection (HP-PLOT Q, 30 m × 0.530 mm × 40 μm) and thermal conductivity detection (Porapak Q, 13 ft × 1/8 in, 100−80 mesh) with He as the reference gas. Formaldehyde and formic acid were not observed because of their low responses in both flame ionization and thermal conductivity detectors, 30,31 and molecular hydrogen was either not observed or not quantified because of its low response using the described chromatographic protocols.
3. RESULTS AND DISCUSSION We evaluated the lifetime of HSAPO-34 and HSSZ-13 for methanol-to-olefins catalysis in five different fixed bed configurations, with and without Y2O3, depicted in Figure 1: zeotype/zeolite only, Y2O3 packed downstream of the zeotype/ zeolite, Y2O3 packed upstream of the zeotype/zeolite, interpellet physical mixture of zeotype/zeolite with Y2O3, and intrapellet physical mixture of zeotype/zeolite with Y2O3. We apply total turnoversthe cumulative amount of methanol converted to hydrocarbon products per H+ before complete deactivationas a direct measure of catalyst lifetime32,33 (consult Section S2 of the SI for exposition). Total turnovers of both HSAPO-34 and HSSZ-13 are unchanged when Y2O3 is packed downstream of the zeotype/ zeolite. The number of total turnovers of HSAPO-34 increases modestlyby a factor of 1.3when Y2O3 is packed upstream of HSAPO-34 but unchanged for HSSZ-13 when analogously configured. These results suggest that Y2O3 renders benign trace impurities in the methanol feed that otherwise decrease 4419
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ACS Catalysis their termination.8 The efficacy of Y2O3 to improve lifetime increases with increasing proximity between the H+ sites of the zeotype/zeolite and the surface of Y2O3 because the propensity for formaldehyde formed inside the zeotype/zeolite to diffuse to and decompose at the Y2O3 surface, relative to the propensity for formaldehyde to induce active center termination, increases when the average distance between zeotypic/ zeolitic domains and the Y2O3 surface decreases. This synergy between the rare earth metal oxide and the zeotype/zeolite is analogous to a synergistic interaction between metal clusters and acid domains in naphtha reforming and hydroisomerization where (cyclo)alkenes of desired size and substitution are scavenged via (de)hydrogenation at metal domains to prevent undesired secondary reactionse.g., cracking, oligermerization, and ring-openingwithin acid domains.41 Y2O3 is an irreducible rare earth metal sesquioxide with stable cubic and monoclinic phases.42 Surface carboxylate formation upon exposure to pyridine,43 mono- and bidentate (bi)carbonate formation upon exposure to CO2,44 and CO2 temperature-programmed desorption measurements42,45 evince exposure of base sites on the surface of Y2O3 which catalyze, for example, 2-propanol dehydrogenation43 and selective dehydration of 1,4-butanediol to terminal, unsaturated alcohols.46 Formaldehyde decomposes to CO with high yield and selectivity, 71% and 100%, respectively, on Y2O3 (Section S3 of SI). The high yield and selectivity appear stable (Section S3 of SI); turnover numbers for formaldehyde decomposition, normalized by the number of base sites enumerated via CO2 temperature-programmed desorption, exceed unity demonstrating catalytic turnover of base sites for formaldehyde decomposition cycles. Methanol forms CO in comparatively small yields, 0.1% (Section S3 of SI), on Y2O3 either via dehydrogenation to formaldehyde and subsequent decomposition or from the decomposition of adventitious carbonylcontaining impurities; the latter explanation is consistent with the effects of impurities on catalyst lifetime34 and the observed increase in total turnovers when Y2O3 is packed upstream of HSAPO-34. Thus, the two predominant routes for CO formation during methanol-to-olefins reactions on catalyst beds with added Y2O3 are the Y2O3 catalyzed decomposition of formaldehyde derived from transfer dehydrogenation of methanol within zeotypic/zeolitic domains and the Y2O3 facilitated decomposition of impurities in the methanol feed. Figure 2 shows profiles of CO selectivity during methanol-toolefins reactions for catalyst beds composed of HSAPO-34 and Y2O3 in the configurations depicted in Figure 1. CO is formed in negligible quantities without added Y2O3. CO selectivities are also negligibly small when Y2O3 is packed downstream of HSAPO-34, demonstrating that formaldehyde and impurities are fully consumed at the zeotype stratum before reaching the Y2O3 stratum. CO selectivities, however, are non-negligible when Y2O3 is packed upstream of HSAPO-34. The site time yield of CO (per base site) on a catalyst bed composed of only Y2O3 matches that for the configuration with Y2O3 packed upstream of HSAPO-34 (Figure S5 of SI), implying that the sole provenance of CO when Y2O3 is packed upstream of HSAPO-34 is the decomposition of impurities and/or dehydrogenation and decomposition of methanol at the Y2O3 surface. Physically mixing Y2O3 with HSAPO-34 provides an additional pathway for CO formation via decomposition of formaldehyde formed within zeotypic domains, yet the CO selectivities for the interpellet physical mixtures are similar to
Figure 2. CO selectivity versus turnover number during methanol-toolefins reactions on catalyst beds configured with HSAPO-34 only (▼), Y2O3 packed downstream of HSAPO-34 (purple ▶), Y2O3 packed upstream of HSAPO-34 (green ■), an interpellet physical mixture of HSAPO-34 with Y2O3 (red ●), and an intrapellet physical mixture of HSAPO-34 with Y2O3 (blue ⧫). Dotted lines are guides for the eye. Reaction conditions: 673 K, 12 kPa MeOH, 24 wt % Y2O3, and 1.1 × 103 molMeOH (molH+·ks)−1.
that for the configuration with Y2O3 packed upstream when compared at an identical number of turnovers. The gain in CO formed via decomposition of formaldehyde, formed within zeotypic domains, is apparently offset by the loss in CO derived from impurities (or sequential methanol dehydrogenation and decomposition). The number of total turnovers for methanolto-olefins catalysis, however, is twice as large for the interpellet physical mixture compared to the configuration with Y2O3 packed upstream of HSAPO-34 (Figure 1), demonstrating that formaldehyde formed within zeotypic domains is more consequential to active center termination than adventitious impurities. CO selectivities at all turnover numbers are largest for the intrapellet physical mixture because the enhanced proximity between H+ sites and the Y2O3 surface renders more likely formaldehyde decomposition at the Y2O3 surface, thus engendering the largest number of total turnovers. These trends in CO selectivity corroborate our hypotheses on the function of Y2O3 to catalyze formaldehyde decomposition and the benefits afforded by proximity to improve catalyst lifetime. Next, we probe the role of formaldehyde in the transient evolution of chain carriers by examining trends in conversion and hydrocarbon selectivity with mixture intimacy. Figure 3a shows conversion versus time-on-stream (scaled by molar space velocity) during methanol-to-olefins reactions on catalyst beds of HSAPO-34 and both interpellet and intrapellet physical mixtures of HSAPO-34 and Y2O3. Conversion reaches a lower maximum at later times-on-stream with increasing proximity between the H+ sites of the zeotype and the surface of Y2O3, suggesting the influence of formaldehyde on the initiation of chain carriers. The time-on-stream derivative of conversion, shown in Figure 3b, reflects the acceleration/ deceleration of product formation during autocatalysis, providing a metric to assess and compare rates of chain carrier initiation and termination. The acceleration of product formation decreases (less positive ordinate values in Figure 3b) with increasing proximity implicating relevant formaldehyde in the initiation of chain carriers, and the deceleration of product formation also decreases (less negative ordinate 4420
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Figure 3. (a) Conversion and (b) the time-on-stream derivative of conversion versus time-on-stream during methanol-to-olefins reactions on catalyst beds configured with HSAPO-34 only (▼), an interpellet physical mixture of HSAPO-34 with Y2O3 (red ●), and an intrapellet physical mixture of HSAPO-34 with Y2O3 (blue ⧫). The inset in (b) shows magnification of the region near the minima of ordinate values. Dotted lines are guides for the eye. Times-on-stream are scaled by molar space velocity. Reaction conditions: 673 K, 12 kPa MeOH, 24 wt % Y2O3, and 1.1 × 103 molMeOH (molH+·ks)−1.
values in Figure 3b) with increasing proximity consistent with mechanistic proposals on formaldehyde-mediated termination of chain carriers.7,8 These trends permit designation of formaldehyde as an accelerant for both the initiation and termination of chain carriers. Hydrocarbon selectivities reflect the relative composition of chain carriers during methanol-to-hydrocarbons catalysis; most salient, larger ethylene selectivities reflect a composition of chain carriers favoring aromatics, and larger C5+ selectivities reflect a larger proportion of olefinic chain carriers.33,47−50 Table 1 lists instantaneous hydrocarbon selectivities for methanol-to-olefins reactions on HSAPO-34 and both interpellet and intrapellet physical mixtures of HSAPO-34
and Y2O3 collated at identical turnover numbers. Instantaneous hydrocarbon selectivities are insensitive to proximity regardless of turnover number suggesting formaldehyde as uninfluential in composing chain carriers, once-initiated, culminating in cumulative hydrocarbon selectivitiesfractional contributions from each hydrocarbon product to the total turnovers8also insensitive to proximity. Thus, the additional functionality to catalytically decompose formaldehyde provided by Y2O3 improves lifetime without disrupting the high cumulative hydrocarbon selectivity to ethylene and propylene (76−77 C %). The high overall selectivities to ethylene and propylene are also undisrupted for the majority of the zeotype turnovers upon physical addition of Y2O3 to HSAPO-34 because CO selectivities are significant only for a small fraction of the total turnovers (note the logarithmic scale in Figure 2; consult Section S5 of SI for exposition).
Table 1. Instantaneous Hydrocarbon Selectivities at Turnover Numbers of 1, 10, 100, and 500 molC mol−1 H+ and Cumulative Hydrocarbon Selectivities for Methanol-toOlefins Reactions on HSAPO-34 only (S), an Interpellet Physical Mixture of HSAPO-34 and Y2O3 (inter), and an Intrapellet Physical Mixture of HSAPO-34 and Y2O3 (intra)a
4. CONCLUSIONS Total turnovers of CHA zeotypes and zeolites for methanol-toolefins catalysis increase 2- and 4-fold upon addition of Y2O3 in interpellet and intrapellet physical mixtures, respectively, while maintaining high selectivity to ethylene and propylene. Y2O3 provides an additional catalytic function to selectively decompose formaldehyde, an accelerant for chain carrier termination formed via transfer dehydrogenation of methanol within zeotypic/zeolitic domains. The increased efficacy of this synergistic interaction to improve total turnovers with increasing proximity between the H+ sites of the zeotype/ zeolite and the base sites exposed on the Y2O3 surface demonstrates critical the transport of formaldehyde within and between zeotypic/zeolitic domains on chain carrier termination. These results demonstrate effectiveness of bifunctional strategies to enhance performance of methanol-to-olefins catalysts and reveal opportunity to design materials diverse in composition and morphology that enable coupling between acid sites that mediate light olefins formation and base sites that mediate alkanal decomposition within distances for molecular diffusion.
instantaneous hydrocarbon selectivity/C% turnover number 1
molC mol−1 H+
10 molC mol−1 H+
100 molC mol−1 H+
500 molC mol−1 H+
cumulative
S inter intra S inter intra S inter intra S inter intra S inter intra
CH4
C2
C3
C4
C5+
2.5 3.3 4.1 1.7 1.6 2.5 0.63 0.72 1.1 0.64 0.48 0.54 0.85 0.84 0.91
18 19 19 23 24 21 28 28 27 31 31 30 30 30 29
44 46 46 45 45 46 47 46 45 47 47 47 47 47 47
15 13 12 14 13 13 16 14 13 17 17 17 16 16 16
21 20 18 16 17 18 8.3 10 14 4.0 3.7 5.4 6.0 6.0 7.3
Reaction conditions: 673 K, 12 kPa MeOH, 24 wt % Y2O3, and 1.1 × 103 molMeOH (molH+·ks)−1.
a
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00894. Results of materials characterization, definition of turnover number, results of reactions of formaldehyde and methanol on Y2O3, profiles of selectivity transients, and supplementary figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Aditya Bhan: 0000-0002-6069-7626 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support for this research from The Dow Chemical Company and the National Science Foundation (CBET 1055846) and thank Dr. Dean M. Millar and Dr. Yu Liu (The Dow Chemical Company) for preparation of HSAPO-34, HSSZ-13, and Y2O3 samples.
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DOI: 10.1021/acscatal.7b00894 ACS Catal. 2017, 7, 4417−4422
