Carbonylation of Dimethyl Ether to Methyl Acetate Over SSZ-13 Marcella Lusardi, Thomas T

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Subscriber access provided by Caltech Library Article Carbonylation of Dimethyl Ether to Methyl Acetate over SSZ-13 Marcella Lusardi, Thomas T. Chen, Matthew James Kale, Jong Hun Kang, Matthew Neurock, and Mark E Davis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04307 • Publication Date (Web): 06 Dec 2019 Downloaded from pubs.acs.org on December 6, 2019

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1 2 3 4 5 6 7 Carbonylation of Dimethyl Ether to Methyl Acetate 8 9 10 11 over SSZ-13 12 13 14 15 16 † ‡ †, ‡ † ‡ 17 Marcella Lusardi, Thomas T. Chen, Matthew Kale, Jong Hun Kang, Matt Neurock, 18 19 20 Mark E. Davis†,* 21 22 23 24 † 25 Chemical Engineering, California Institute of Technology, Pasadena, CA 91125 United 26 27 28 States 29 30 31 32 ‡ 33 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 34 35 36 55455 United States 37 38 39 40 41 ABSTRACT: The small pore zeolite with the chabazite framework topology, SSZ-13, is 42 43 44 45 found to be an active catalyst in the carbonylation of dimethyl ether (DME) to methyl 46 47 48 acetate (MA). The production of MA over SSZ-13 after 24 hours on stream at 165 ˚C and 49 50 51 52 1 bar approaches that obtained from mordenite and is significantly higher than from 53 54 55 ferrierite at comparable Si/Al of ca. 10. To understand the origin of the activity, SSZ-13 56 57 58 59 60 ACS Paragon Plus Environment 1 ACS Catalysis Page 2 of 54

1 2 3 4 materials are synthesized with variable Si/Al, characterized via several techniques 5 6 7 including multinuclear MAS NMR spectroscopy, and evaluated for their carbonylation 8 9 10 11 activity. While MA production rates increase with decreasing Si/Al, the correlation is non- 12 13 14 linear due to the effect of Si/Al on the acid site distribution within different confining 15 16 17 18 environments, and the associated impact of the latter on the rate-determining transition 19 20 21 state barrier. Enhanced MA production rates trend with acid sites located at the eight- 22 23 24 25 membered ring (8MR) that are increasingly populated as framework Al content increases. 26 27 28 Density functional theory (DFT) analyses of transition state energies as a function of 29 30 31 32 active site location support the experimental findings, where the lowest apparent barriers 33 34 35 are associated with the methoxy groups that orient within the plane of the 8MR window. 36 37 38 39 This is due to an optimal charge stabilization of the cationic transition states with the 40 41 42 negatively charged oxygens within the 8MR window. The effects of catalyst chemical 43 44 45 46 composition, separate from framework topology, are also investigated using SAPO-34 47 48 49 (the silicoaluminophosphate analog of SSZ-13). Analyses of the 1H MAS NMR signals 50 51 52 and carbonylation activity suggest that the higher acid site strength of SSZ-13 compared 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 2 Page 3 of 54 ACS Catalysis

1 2 3 4 to that of the SAPO is required for effective Brønsted acid catalysis of Koch-type 5 6 7 carbonylation pathways. 8 9 10 11 12 KEYWORDS: Chabazite, carbonylation, acetic acid, confinement, small-pore zeolite 13 14 15 16 17 18 19 1 Introduction 20 21 22 23 Methanol carbonylation is an industrially important route for the production of acetic 24 25 26 acid, an intermediate in the synthesis of various polymers, pharmaceuticals, and 27 28 29 1–4 30 textiles. The commercial process for acetic acid production is catalyzed by Rh/Ir- 31 32 33 organometallic complexes and iodide promoters. Solid acid-mediated, Koch-type 34 35 36 37 reactions provide a transition metal- and halide-free carbonylation pathway that removes 38 39 40 significant economic and environmental drawbacks associated with the commercial 41 42 43 5 44 catalytic system. With this mechanism, there is an induction period during which methyl 45 46 47 groups from methanol and/or dimethyl ether (DME) adsorb at the proton site to form 48 49 50 51 methoxy groups. DME is a preferred substrate to methanol since the latter generates 52 53 54 water as a by-product, resulting in inhibition of carbonylation steps.6 During the steady- 55 56 57 58 59 60 ACS Paragon Plus Environment 3 ACS Catalysis Page 4 of 54

1 2 3 4 state catalytic cycle, CO executes a rate-determining nucleophilic attack onto the methoxy 5 6 7 group, yielding an acetyl intermediate. Finally, methanol and/or DME reacts with the 8 9 10 11 acetyl to form methyl acetate (MA), a precursor to acetic acid, and regenerate the surface- 12 13 14 bound methyl group. 15 16 17 18 Various solid acids have been investigated for this chemistry, including 19 20 21 heteropolyacids,7,8 Brønsted acid zeolites,5,9,10 and sulfated zirconia.11,12 Certain zeolite 22 23 24 25 topologies have shown promising MA production activity due to the positive effect of 26 27 28 microporous confinement on the rate-determining step (RDS). Most notably, mordenite 29 30 31 32 (MOR—each unique topology is given a three-letter code by the International Zeolites 33 34 35 Association13) and ferrierite (FER) exhibit superior carbonylation reactivity within their 8- 36 37 38 14–16 39 member rings (8MRs). Theoretical analyses show that the cationic linear transition 40 41 42 state [O—C—CH3]* is effectively stabilized by the surrounding anionic oxygens within the 43 44 45 46 8MR cage of MOR, which decreases the rate-determining activation barrier, thus resulting 47 48 49 in enhanced MA production rates.9,17 In contrast, carbonylation rates in the larger pores 50 51 52 of MOR (12MR) and FER (10MR) are significantly lower, as are those from other zeolites 53 54 55 56 with medium- and large-pore topologies (FAU, MFI, BEA).5,18 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 54 ACS Catalysis

1 2 3 4 Zeolites (aluminosilicate molecular sieves) and zeo-type materials have vast ranges of 5 6 7 pore size, shape, and interconnectivity, giving rise to over 200 unique framework 8 9 10 19 11 structures with different confining environments. Zeolite topologies beyond the common 12 13 14 types mentioned above (MOR, FER) have been investigated for carbonylation activity, 15 16 17 20,21 18 including ETL (EU-12), PAU (ECR-18), OFF (offretite), and CHA (chabazite). CHA is 19 20 21 particularly interesting because, unlike these other frameworks, it is commercially 22 23 24 25 available as SSZ-13 (aluminosilicate CHA with Si/Al > 5). SSZ-13 is manufactured as a 26 27 22,23 28 catalyst for the selective reduction of NOX emissions in automotive exhaust. It is also 29 30 31 32 found as the aluminosilicate mineral chabazite (Si/Al < 5). CHA has a 3-dimensional, 33 34 35 small-pore, cage-containing framework composed of double 6MRs (D6MRs) that 36 37 38 39 interconnect to form 4-, 6-, and 8-member rings. These 8MRs (3.8 x 3.8 Å) are the sole 40 41 42 conduit for molecular transport and provide access to the ellipsoid CHA cage (12 Å x 7 43 44 45 24 46 Å). To the best of our knowledge, there is only one reference for the carbonylation 47 48 49 activity of CHA, reported as MA yields measured at two temperatures (US Patent 50 51 52 7,465,822 B2).21 No discussion or further analysis of the activity in CHA was provided, 53 54 55 56 and only a single CHA sample was evaluated (despite its promising MA yield that was 57 58 59 60 ACS Paragon Plus Environment 5 ACS Catalysis Page 6 of 54

1 2 3 4 40% of that for MOR at comparable Si/Al; T = 180 ˚C). The carbonylation activity in CHA 5 6 7 is particularly interesting given its starkly different structure compared to that of MOR and 8 9 10 11 FER (2-dimensional, large-/medium-pore, channel structures). The significantly different 12 13 14 topology, along with the commercial availability of SSZ-13, motivated our investigation of 15 16 17 18 the behavior of CHA-type zeolite catalysts for the carbonylation of DME to MA. 19 20 21 In order to understand the origin of activity in CHA-type zeolites, we synthesized SSZ- 22 23 24 25 13 materials with variable Si/Al. The framework Si/Al not only impacts the density of 26 27 28 Brønsted acid sites (and, commensurately, the density of the surface-bound methyl 29 30 31 32 groups that replace them during the induction period), but also the distribution of those 33 34 35 methoxys within different confining environments.25 In this way, we systematically alter 36 37 38 39 the number density of the framework positions where the transition state can form during 40 41 42 the RDS, and determine the impact on the MA production activity. These studies facilitate 43 44 45 46 the identification of the most reactive methoxy group—the one located at the 8MR 47 48 49 window—for DME carbonylation to MA in CHA-type zeolites. This result is supported by 50 51 52 complementary DFT analyses that calculate the intrinsic activation barriers for the rate- 53 54 55 56 determining nucleophilic attack by CO as a function of methoxy location in the CHA 57 58 59 60 ACS Paragon Plus Environment 6 Page 7 of 54 ACS Catalysis

1 2 3 4 framework. We also evaluate the reactivity of a molecular sieve with the CHA topology, 5 6 7 but a different chemical composition (SAPO-34). This analysis decouples the effects of 8 9 10 11 confinement and acid site strength to elucidate further the catalyst requirements for Koch- 12 13 14 type carbonylation routes. 15 16 17 18 2 Materials and Methods 19 20 21 We obtained commercial mordenite (CBV 21A, Si/Al = 10) and ferrierite (CP914C, Si/Al 22 23 24 25 =10) from Zeolyst for comparison to our synthesized SSZ-13 samples. 26 27 28 2.1 Synthesis of CHA-type materials 29 30 31 32 SSZ-13s 33 34 35 CHA-type zeolites with Si/Al > 5 (SSZ-13s) were prepared from a gel with the molar 36 37 38 26 39 composition 1 SiO2:x Al2O3:0.2 SDAOH:0.2 NaOH:40 H2O. The alumina content, x, was 40 41 42 varied from 0.014 to 0.062. Fumed silica (Cab-O-Sil M5, Cabot), aluminum hydroxide 43 44 45 46 (Reheiss F2000), and trimethyl adamantylammonium hydroxide (25 wt.% in H2O, 47 48 49 Sachem) were used as the silica source, alumina source, and structure-directing agent 50 51 52 (SDA), respectively. In a typical synthesis, 20 wt.% sodium hydroxide solution, deionized 53 54 55 56 (DI) water, and the SDA were added to a PTFE liner. The alumina source was then added 57 58 59 60 ACS Paragon Plus Environment 7 ACS Catalysis Page 8 of 54

1 2 3 4 and stirred. After 30 min, the silica source was added, and the solution was stirred 5 6 7 overnight. The gel was crystallized at 160 ˚C and 60 rpm for approximately 10 days. 8 9 10 11 Samples were recovered by centrifugation, washed 3 times in DI H2O and 1 time in 12 13 14 acetone, and dried overnight at 100 ˚C. The as-synthesized materials were then calcined 15 16 17 18 in flowing breathing-grade air to remove the SDA by combustion at a 1 ˚C/min ramp to 19 20 21 150 ˚C, 3 hr hold at 150 ˚C, 1 ˚C/min ramp to 580 ˚C, 6 hr hold at 580 ˚C. The calcined 22 23 24 + + 25 SSZ-13s underwent an ammonium exchange to replace Na with NH4 as the charge- 26 27 28 balancing cation. In this procedure, 1 g of material was stirred in 100 mL of 1 M NH4NO3 29 30 31 32 solution at 80 ˚C for approximately 8 hr. This was repeated 3 times, and the ammonium- 33 34 35 exchanged sample was washed and dried following the same procedure above. 36 37 38 39 SAPO-34 40 41 42 The silicoaluminophosphate material with the CHA framework, SAPO-34, was 43 44 45 46 synthesized from a gel with molar composition 0.08 SiO2:0.2 Al2O3:0.2 P2O5:0.4 47 48 27 49 TEAOH:9.8 H2O. Colloidal silica (Ludox AS-30), aluminum isopropoxide, and 50 51 52 phosphoric acid (85 wt.%) were used as the sources for silicon, aluminum, and 53 54 55 56 phosphorous, respectively. Tetraethylammonium hydroxide (TEAOH, 35 wt.%) was used 57 58 59 60 ACS Paragon Plus Environment 8 Page 9 of 54 ACS Catalysis

1 2 3 4 as the SDA. In this procedure, the following materials were added followed by mixing time 5 6 7 in parentheses: SDA and 40% of the required water (0.5 hr); aluminum source (overnight); 8 9 10 11 silicon source (4 hr); 60% of the required water (1 hr); phosphoric acid (overnight). The 12 13 14 gel was then crystallized statically at 200 ˚C for 48 hr. The standard centrifugation, 15 16 17 18 washing and drying procedure was used. The dried as-synthesized material was 19 20 21 subjected to the same calcination program used to combust the SDA in the SSZ-13s to 22 23 24 + 25 remove TEAOH and obtain the H -form. 26 27 28 2.2 Characterization 29 30 31 32 Powder x-ray diffraction (PXRD) patterns were obtained on a Rigaku MiniFlex II 33 34 35 instrument using Cu Kalpha radiation (λ = 1.54 Å) and a scan rate of 0.3˚/min. All solid- 36 37 38 39 state, magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy 40 41 42 experiments were conducted on a Bruker 500 MHz spectrometer using a 4 mm ZrO2 rotor. 43 44 45 29 1 46 Si MAS NMR spectra were acquired at 99.3 MHz with H decoupling and a spinning 47 48 49 rate of 8 kHz using a 90˚ pulse length of 4 μs and a cycle delay time of 60 s. For the 50 51 52 zeolites (SSZ-13s), 128 scans were collected on the NH +-form and the spectra were 53 4 54 55 56 deconvoluted using the DMFit software to calculate the framework Si/Al values. For 57 58 59 60 ACS Paragon Plus Environment 9 ACS Catalysis Page 10 of 54

1 2 3 + 27 4 SAPO-34, 960 scans were collected on the calcined (H -form) sample. Al MAS NMR 5 6 7 spectra were acquired on the H+-form samples after dehydration at 130.2 MHz, a spin 8 9 10 11 rate of 10 kHz, a 90˚ pulse length of 0.5 μs, and a cycle delay time of 3 s for the zeolite 12 13 14 (2000 scans) and 2 s for SAPO (128 scans) samples. Particle morphology and bulk 15 16 17 18 elemental analysis were investigated using a Zeiss 1550VP field-emission scanning 19 20 21 electron microscope (SEM) equipped with an Oxford X-Max SDD energy dispersive 22 23 24 25 spectrometer (EDS). 26 27 28 The number of Brønsted acid sites was measured using quantitative 1H MAS NMR 29 30 31 + 32 spectroscopy. In a typical experiment, the freshly calcined samples (H -form) were 33 34 35 dehydrated under vacuum (10-2 torr) at 400 ˚C for 12 hr in a dehydration manifold. The 36 37 38 39 sample was transferred under Ar into a glove box for air- and moisture-free packing of the 40 41 42 NMR rotor (4 mm, ZrO2) and loaded into the Bruker 500 MHz spectrometer. Spectra were 43 44 45 46 collected at 500.1 MHz and a spinning rate of 13 kHz using a 90˚ high power pulse length 47 48 49 of 4 μs, a 15 s cycle delay time, and 16 scans. Signal intensities were referenced to 50 51 52 hexamethylbenzene and normalized by the mass packed into the rotor for quantification. 53 54 55 56 The spectra were deconvoluted using DMFit. 57 58 59 60 ACS Paragon Plus Environment 10 Page 11 of 54 ACS Catalysis

1 2 3 4 2.3 DME Carbonylation experiments 5 6 7 The DME carbonylation activity of the CHA-type zeolite catalysts was evaluated in a 8 9 10 11 fixed bed reactor (8 mm ID). All reactions were conducted at 165 ˚C and 1 bar. The 12 13 14 catalyst particles were sized between 150 and 600 μm and 150 mg was packed onto a 15 16 17 18 bed of quartz wool. The catalyst bed was pretreated in-situ in flowing breathing-grade air 19 20 21 at 2 ˚C/min to 550 ˚C, and subsequently held at 550 ˚C for 6 hr to convert the sample to 22 23 24 + 25 the H -form and/or remove surface adsorbates. After cooling to the reaction temperature, 26 27 28 reagents were introduced at 75 sccm total flowrate (2% DME, 3% He with Ar (5%), 95% 29 30 31 32 CO). The reactor effluent line was heated to prevent condensation of species and 33 34 35 samples were measured by an online gas chromatograph (GC, Agilent 7890A) paired 36 37 38 39 with a flame ionization detector (FID). 40 41 42 2.4 Computational methods 43 44 45 46 The adsorption, reaction, and activation energies for the different proposed elementary 47 48 49 steps in DME carbonylation were calculated at different sites within CHA and MOR using 50 51 52 periodic plane-wave density functional theory (DFT) calculations as implemented in the 53 54 55 56 Vienna Ab initio Simulation Program (VASP). Gradient corrections were calculated using 57 58 59 60 ACS Paragon Plus Environment 11 ACS Catalysis Page 12 of 54

1 2 3 28 4 the Perdew-Burke-Erzenhof (PBE) exchange-correlation functional and the core 5 6 7 electrons were described using projector augmented wave (PAW) based pseudo- 8 9 10 29 11 potentials. The valence electrons were represented by a plane-wave basis set with an 12 13 14 energy cutoff of 400 eV. A (1  1  1) k-point mesh at the single Γ-point was used to 15 16 17 18 sample the Brillouin zone due to the large size of the CHA unit cell and breaking of 19 20 21 symmetry associated with Al framework substitution. The Grimme-type D3 corrections 22 23 24 25 with Becke-Johnson (BJ) damping were used to account for dispersive interactions of 26 27 28 adsorbates with the zeolite framework.30,31 29 30 31 32 The self-consistent field (SCF) and geometry optimization calculations were converged 33 34 35 to 10-6 eV and 0.05 eV/Å, respectively. The reaction path between the reactant and 36 37 38 39 product states was calculated using the climbing image nudged elastic band (CI-NEB) 40 41 42 method32,33 to a force tolerance of 0.3 eV/Å. The transition states of the different paths 43 44 45 34 46 were subsequently refined using the dimer method to a force convergence of 0.05 eV/Å. 47 48 49 Stricter convergence criteria for the SCF with energies converged to 10-8 - 10-6 eV and 50 51 52 geometry optimization with forces converged to 0.01 - 0.05 eV/Å were tested and found 53 54 55 56 to change the reaction energy and the intrinsic activation energy for CO addition to CH3 57 58 59 60 ACS Paragon Plus Environment 12 Page 13 of 54 ACS Catalysis

1 2 3 -1 4 in CHA by less than 2 kJ mol , suggesting that the original convergence criteria is 5 6 7 sufficient for this study. Small perturbations of the transition state toward the reactant and 8 9 10 11 product followed by optimization resulted in the formation of the reactant and the product, 12 13 14 respectively, thus confirming the transition state and the reaction coordinate. Harmonic 15 16 17 18 frequency calculations and partial Hessian vibrational analysis were performed to 19 20 21 determine the zero point vibrational energies, vibrational enthalpies, and vibrational free 22 23 24 25 energies that were subsequently used to calculate the enthalpies, entropies, and free 26 27 28 energies for the reactant, product, and transition state structures. Further details 29 30 31 32 regarding the computational methods are included in the SI. 33 34 35 For this analysis, the SSZ-13 (CHA framework type) structure was constructed from the 36 37 38 @ 39 Materials Studio structural library from Dassault Systemes with unit cell Si12O24 and 40 41 42 lattice parameters a = b = c = 9.421 Å and α = β = γ = 94.200˚.35 The CHA topology is a 43 44 45 46 high symmetry framework containing a single unique T site and four crystallographically- 47 48 49 distinct O sites. The previously established O site nomenclature36 relates to each O site’s 50 51 52 association to different rings of the framework: O1 site connects two 4MRs and one 8MR; 53 54 55 56 O2 site connects one 4MR, one 6MR, and one 8MR; O3 site connects two 4MRs and one 57 58 59 60 ACS Paragon Plus Environment 13 ACS Catalysis Page 14 of 54

1 2 3 4 6MR; and O4 site connects one 4MR and two 8MRs. The CHA zeolite was modeled with 5 6 7 a 2  2  2 supercell of 288 atoms with a single Al atom substituted at the sole T site of 8 9 10 11 CHA at the 8MR windows connecting the larger cages (Si/Al = 95). The substituted Al site 12 13 14 was balanced by a hydrogen atom to create a Brønsted acid site. 15 16 17 18 19 20 21 3 Results and Discussion 22 23 24 25 26 3.1 Physicochemical characterizations 27 28 29 30 The PXRD patterns for the as-synthesized zeolites are illustrated in Figure 1a. All 31 32 33 29 34 samples show features attributed to a single, CHA phase. Si MAS NMR spectra (Figure 35 36 37 1b) for the SSZ-13s show primary resonances at -111, -105, and -101 ppm, 38 39 40 41 corresponding to Q4 ((Si-O)4-Si; Si(0Al)), Q3 ((Si-O)3-Si-(O-Al); Si(1Al)) and Q3 ((Si-O)3- 42 43 44 Si-OH; Si(0Al)) sites, respectively.22 As the gel Si/Al decreases (i.e., increasing Al 45 46 47 48 content), the resonance at -101 ppm shifts towards -100 ppm due to increased 49 50 51 contributions from Q2 sites ((Si-O)2-Si-(O-Al)2; Si(2Al)) that manifest at -99 ppm. 52 53 54 Deconvolution of the spectra enabled the calculation of the framework Si/Al values (Table 55 56 57 58 59 60 ACS Paragon Plus Environment 14 Page 15 of 54 ACS Catalysis

1 2 3 37 4 1). An example of the deconvolution is shown in Figure S1. Bulk Si/Al values measured 5 6 7 by SEM-EDS were within 15% of the framework Si/Al values. 8 9 10 11 (a) 12 (b) 13 14 SSZ-13 (35) SSZ-13 (35) 15 16 SSZ-13 (30) SSZ-13 (30) 17 18 SSZ-13 (22) SSZ-13 (22) 19 20 SSZ-13 (19) SSZ-13 (19) 21 SSZ-13 (15) 22 SSZ-13 (15) 23 SSZ-13 (13) 24 SSZ-13 (13) 25 SSZ-13 (9) 26 SSZ-13 (9) 27 10 15 20 25 30 35 40 -80 -90 -100 -110 -120 -130 28 Chemical shift (ppm) 29 2 (degrees) 30 Figure 1. Characterizations of SSZ-13s. (a) PXRD patterns for as- 31 32 29 + 33 synthesized SSZ-13s. (b) Si MAS NMR spectra for SSZ-13s (NH4 -form). 34 35 36 37 38 Table 1. SSZ-13 characterization data. 39 40 41 a 42 Sample x Al2O3 Framework Bulk Brønsted acid site 43 b c -1 d 44 (mol/mol Si/Al Si/Al density (mmol g ) 45 SiO ) 46 2 47 48 SSZ-13 0.062 9.2 9.8 1.55 49 50 (9) 51 52 SSZ-13 0.042 12.9 11.6 1.08 53 54 (13) 55 56 57 58 59 60 ACS Paragon Plus Environment 15 ACS Catalysis Page 16 of 54

1 2 3 4 SSZ-13 0.035 14.9 13.9 0.95 5 6 (15) 7 8 SSZ-13 0.026 18.8 17.0 0.73 9 10 (19) 11 12 SSZ-13 0.025 21.5 18.7 0.52 13 14 (22) 15 16 17 SSZ-13 0.016 29.9 29.3 n. q. 18 19 (30) 20 21 SSZ-13 0.014 35.3 33.3 n. q. 22 23 (35) 24 25 a 26 content in synthesis gel 27 28 bmeasured by 29Si MAS NMR spectroscopy 29 30 c 31 measured by SEM-EDS 32 33 dmeasured by 1H MAS NMR spectroscopy and referenced to hexamethylbenzene 34 35 36 n.q.: not quantified since 4 ppm signal intensities for these samples are too low for 37 38 accurate quantification 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 16 Page 17 of 54 ACS Catalysis

1 2 3 4 5 6 7 8 9 SSZ-13 (30) 10 11 12 13 14 SSZ-13 (22) 15 16 17 18 SSZ-13 (9) 19 20 21 100 80 60 40 20 0 -20 22 Chemical shift (ppm) 23 24 25 26 27 Figure 2. 27Al MAS NMR spectra for SSZ-13s (dehydrated H+-form). 28 29 30 31 27Al NMR spectra on selected SSZ-13 samples (Figure 2) show a strong resonance 32 33 34 35 centered at 58 ppm corresponding to tetrahedrally-coordinated Al in the zeolite framework 36 37 38 and a weaker peak near 0 ppm, indicative of extraframework Al. SEM images of the 39 40 41 42 zeolites show particles with a cube-like morphology (Figure S2) typical of chabazite and 43 44 45 diameters between 2 and 5 μm (Table S1). 46 47 48 49 1 50 3.2 Bronsted acid site characterization: H MAS NMR spectroscopy 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 17 ACS Catalysis Page 18 of 54

1 2 3 1 4 The H MAS NMR spectra measured on the dehydrated SSZ-13s are reported in Figure 5 6 7 3a. An example deconvolution through DMFit is presented in Figure S3. The spectra show 8 9 10 38,39 11 a resonance near 1.8 ppm that corresponds to protons in silanols (Si-OH). 12 13 14 Resonances for protons in aluminum-rich (e.g., extraframework Al) environments arise 15 16 17 39 18 between 2 and 3.5 ppm. Near 4 ppm, the signal manifests from bridging hydroxyl groups 19 20 21 that form from Al incorporation into the framework.39,40 The integration of this resonance 22 23 24 25 gives the total Brønsted acid site density (Table 1), which linearly scales with framework 26 27 28 Al content (Figure 3b). Since the signals at 4 ppm for samples SSZ-13 (30) and SSZ-13 29 30 31 32 (35) are very weak (Figure 3a), accurately fitting these spectra is difficult. As a result, 33 34 35 Brønsted acid site densities for these samples are not reported. 36 37 38 39 40 41 42 43 (a) 44 SSZ-13 (35) 45 46 SSZ-13 (30) 47 SSZ-13 (22) 48 49 50 SSZ-13 (19) 51 SSZ-13 (15) 52 53 SSZ-13 (13) 54 55 SSZ-13 (9) 56 57 10 8 6 4 2 0 58 Chemical shift (ppm) 59 60 ACS Paragon Plus Environment 18 Page 19 of 54 ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 (b) 17 2.0 18 ) 1

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30 n o 31 r B 32 33 0.0 34 5 10 15 20 25 35 Si/Al (29Si NMR) 36 37 38 39 40 41 42 43 44 Figure 3. Brønsted acid site characterizations. (a) 1H MAS NMR spectra for dehydrated 45 46 47 + 48 SSZ-13s (H -form). (b) Total Brønsted acid site density measured as a function of 49 50 51 framework Si/Al for the SSZ-13s. 52 53 54 55 56 3.3 DME carbonylation activity in CHA-type zeolites 57 58 59 60 ACS Paragon Plus Environment 19 ACS Catalysis Page 20 of 54

1 2 3 4 3.3.1 MOR, FER, CHA comparison 5 6 7 8 The DME carbonylation activity for MOR, FER, and CHA frameworks at similar Si/Al 9 10 11 12 (~10) is reported in Figure 4a. MOR is the most active at early time on stream, but the 13 14 15 MA production rate decays by more than a factor of 2 over time. The activity loss in MOR 16 17 18 19 is attributed to deactivation—while the 8MRs are extremely selective (>99%) to MA, 20 21 22 hydrocarbon formation mechanisms within the 12MRs of MOR are more kinetically 23 24 25 26 favored compared to carbonylation routes, thus allowing the formation of carbonaceous 27 28 29 deposits that restrict transport and cause the catalyst to deactivate.9,10,14 This result is 30 31 32 33 consistent with the weight loss in the TGA profiles for the spent catalysts between 300 34 35 36 and 650 ˚C (Figure S4) that shows that MOR produces more coke-like deposits (2 to 3x, 37 38 39 40 Table S2) than the other frameworks. The relatively smaller 10MR channels in FER inhibit 41 42 43 excessive formation of these deposits, resulting in a stable MA production rate over time 44 45 46 10,41 47 on stream for this catalyst (Figure 4a). However, the maximum rate of MA production 48 49 50 over FER is an order of magnitude less than that over MOR, despite the fact that the 51 52 53 carbonylation activity in both topologies is attributed to their 8MRs. The enhanced rate in 54 55 56 57 58 59 60 ACS Paragon Plus Environment 20 Page 21 of 54 ACS Catalysis

1 2 3 4 MOR is attributed to the specific dimensions of its 8MRs, specific sites within the 8MR, 5 6 7 and the preferred orientation of the linear transition state within the small pores and side 8 9 10 17 11 pockets. Interestingly, the carbonylation reactivity over CHA ranks intermediate 12 13 14 between that of MOR and FER, thereby surpassing FER as the second-most active 15 16 17 5,9,15,18,42 18 zeolite framework for carbonylation reported in the literature. After 24 hr on 19 20 21 stream, the total MA produced over CHA is within 12% of that produced over MOR (Figure 22 23 24 25 4b). 26 27 28 29 30 31 0.25 (a) 32 MOR (Si/Al = 10)

) FER (Si/Al = 10) 1

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1 2 3 4 5 6 7 8 9 10 11 12 500 13 (b) 14

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T 100 25 26 27 28 0 29 MOR FER CHA 30 31 32 33 34 35 36 37 38 39 Figure 4. DME carbonylation results. (a) Methyl acetate production rates measured over 40 41 42 43 time for MOR (open triangles), FER (open circles), and CHA (filled squares) frameworks. 44 45 46 (b) Total methyl acetate produced after 24 hr on stream over each framework. 47 48 49 50 51 The carbonylation activity in the CHA-type framework is interesting, particularly because 52 53 54 its framework is so different from those of the other zeolite carbonylation catalysts (MOR, 55 56 57 58 59 60 ACS Paragon Plus Environment 22 Page 23 of 54 ACS Catalysis

1 2 3 4 FER). Given its slower deactivation rate compared to MOR, and its commercial 5 6 7 availability, CHA-type zeolites (SSZ-13s) merit further investigation. 8 9 10 11 3.3.2 CHA: activity vs Si/Al 12 13 14 The kinetics of MA production in MOR (and to a smaller degree, FER) have been 15 16 17 18 extensively investigated, and the locations of the Brønsted acid sites that give rise to the 19 20 21 very high selectivity within the framework have been specifically identified.5,9,14,15,17,42–44 22 23 24 25 To elucidate the origin of activity in the CHA topology, we investigated the MA production 26 27 28 rates in SSZ-13s at varied Si/Al (9 ≤ Si/Al ≤ 35). The activities of these materials are 29 30 31 32 summarized in Figure 5. 33 34 35 36 0.15 37 SSZ-13 38 (9) ) 1 - (13) 39 r h (15) 40 1 - T

41 O (19) T 0.10 g (22)

42 l 43 o (30) m

m (35)

44 (

45 n o i t

46 c

u 0.05 47 d o r

48 p

49 A 50 M 51 52 0.00 53 0 200 400 600 800 1000 1200 1400 54 Time (min) 55 56 57 58 59 60 ACS Paragon Plus Environment 23 ACS Catalysis Page 24 of 54

1 2 3 4 Figure 5. MA production rates as a function of time for the SSZ-13s. Samples with 5 6 7 overlapping activity correspond to SSZ-13 (19)/SSZ-13 (22); and SSZ-13(30)/SSZ- 8 9 10 11 13(35). 12 13 14 15 On a per gram basis, the MA production rates increase with decreasing Si/Al. This 16 17 18 general trend is expected given that the surface coverage of methyl groups has shown to 19 20 21 5 22 reach saturation (DMEadsorbed / Alframework = 0.5) for various topologies. Thus, decreasing 23 24 25 Si/Al (i.e., increasing Brønsted acid site density) would result in larger methoxy surface 26 27 28 29 coverages, enhancing the probability of nucleophilic attack by CO. We prepared a sample 30 31 32 with low Si/Al (2.6) using a recipe previously reported in our group45 to investigate the 33 34 35 36 activity at low Si/Al. However, we were not able to maintain structural integrity of the 37 38 39 sample at reaction conditions. This is a common challenge with Al-rich zeolites in their 40 41 42 43 acid forms. 44 45 46 If all Brønsted acid sites are equally reactive for carbonylation, the MA production would 47 48 49 50 increase linearly with decreasing framework Si/Al (increasing Brønsted acid site density— 51 52 53 see Figure 3b). However, we find this not to be the case. Figure 6 shows the non-linear 54 55 56 57 58 59 60 ACS Paragon Plus Environment 24 Page 25 of 54 ACS Catalysis

1 2 3 4 correlation between the maximum rates of MA production and framework Si/Al. These 5 6 7 rates plotted as a function of total Brønsted acid site density are reported below. 8 9 10 11 12 ) 1 -

13 r 0.14 h

14 1 - T

15 O 0.12 T g

16 l 17 o 0.10 m

18 m (

19 n 0.08 o i t

20 c

u 0.06

21 d o 22 r p 0.04 23 A M 24 e t 0.02 25 a r

26 x a 0.00

27 M 28 5 10 15 20 25 30 35 29 29 30 Si/Al ( Si NMR) 31 32 33 34 Figure 6. Maximum rates of MA production measured over the SSZ-13s as a function of 35 36 37 Si/Al. 38 39 40 41 + 42 Framework Si/Al, among other factors (e.g., the presence or absence of Na during the 43 44 45 synthesis)22,46 strongly influences the distribution (i.e., siting, density) of cations required 46 47 48 3+ 4+ 25,47– 49 to compensate the negative charge in the lattice generated by Al /Si substitution. 50 51 52 49 Monovalent cation distribution, in turn, dictates the distribution of methoxy groups in 53 54 55 56 the framework. We postulate that the non-linearity in these rates as a function of Si/Al 57 58 59 60 ACS Paragon Plus Environment 25 ACS Catalysis Page 26 of 54

1 2 3 4 originates from the sensitive relationship between MA rates and methoxy location, as 5 6 7 evidenced by prior experimental and computational work in MOR.6,9,17,43 This relationship 8 9 10 11 arises from the stabilization (or lack thereof) via confinement of the transition state formed 12 13 14 by CO attack on the methoxy group, which decreases (or increases) the activation barrier 15 16 17 18 of the RDS. There are three primary positions for monovalent, extraframework cations to 19 20 21 site in SSZ-13 (Figure 7) associated with the crystallographically-distinct O atoms.25,47,49 22 23 24 25 Cations in site SI nest in the D6MR, in site SII sit above the plane of the 6MR, and in site 26 27 28 SIII’ orient at the 8MR. While small cations (e.g., H+) can fit in the D6MR, it is not 29 30 31 32 considered an active center because it is inaccessible by the gas-phase reactants 33 34 35 involved in the carbonylation reaction. This leaves sites SII and SIII’ as candidates for 36 37 38 39 methoxy positions in the SSZ-13 catalysts. 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 26 Page 27 of 54 ACS Catalysis

1 2 3 4 Figure 7. Monovalent cation locations in CHA: site SI (sphere, in D6MR); site SII (cube, 5 6 7 above 6MR into cage); site SIII’ (triangular prism, at 8MR). 8 9 10 11 25 23 12 Prior work using Na multiple quantum MAS NMR spectroscopy and DFT 13 14 15 calculations has shown that site SII is preferentially occupied at higher Si/Al in SSZ-13 16 17 18 materials. As framework Al content increases, site SIII’ becomes increasingly populated 19 20 21 22 as more than one Al per cage cannot be avoided. As Al content increases further, 23 24 25 occupation in Site SI becomes apparent. These trends in the site distribution when 26 27 28 29 considered in conjunction with measured MA production rates as a function of Si/Al are 30 31 32 suggestive that active sites located at SII are less kinetically favorable for DME 33 34 35 36 carbonylation than those at SIII’. This, in turn, would lead to the fact that CO attack on a 37 38 39 methoxy group at the 8MR (window or cage) gives a lower transition state barrier 40 41 42 43 compared to those at the 6MR (cage). 44 45 46 3.3.3 DFT analysis of rate-determining transition state barriers in SSZ-13 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 27 ACS Catalysis Page 28 of 54

1 2 3 4 The activation enthalpies, activation free energies, and the overall reaction energies for 5 6 7 the different elementary steps in the carbonylation of DME to methyl acetate at the four 8 9 10 11 crystallographically-distinct O sites (Figure 8) in SSZ-13 were calculated using DFT. 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Figure 8. 2  2  2 supercell of zeolite CHA with Al substituted at the sole T site to form a 43 44 45 46 Brønsted acid site. Close-up viewpoint of the Brønsted acid site depicts the four 47 48 49 crystallographically-distinct O sites. Atom colors: Al (purple), Si (yellow), O (red), and H 50 51 52 (white). 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 28 Page 29 of 54 ACS Catalysis

1 2 3 4 The carbonylation of DME to MA is thought to proceed by the elementary steps listed in 5 6 7 Scheme 1. 8 9

10 + + 11 (1) CH3OCH3 (gas) + H -Zeolite ⇌ CH3OCH3--H -Zeolite (DME 12 adsorption) 13 14 + + 15 (2) CH3OCH3--H -Zeolite ⇌ CH3OH + H3C -Zeolite (DME 16 hydrogenolysis) 17 18 + + 19 (3) CO(gas) + H3C -Zeolite ⇌ CO--H3C -Zeolite (CO 20 adsorption) 21 22 + + 23 (4) CO-- H3C -Zeolite ⇌ CH3CO -Zeolite (CO 24 addition) 25 26 + + 27 (5) CH3OCH3 (gas) + CH3CO -Zeolite ⇌ CH3OCH3--CH3CO -Zeolite (DME 28 adsorption) 29 30 + + 31 (6) CH3OCH3--CH3CO -Zeolite ⇌ CH3COOCH3--H3C -Zeolite (Acetyl 32 esterification) 33 34 + + 35 (7) CH3COOCH3--H3C -Zeolite ⇌ CH3COOCH3 (gas) + H3C -Zeolite (MA 36 desorption) 37 38 39 Scheme 1. Elementary steps involved in the carbonylation of DME to MA over zeolites 40 41 42 43 Previous experimental results have shown that DME readily adsorbs in different 44 45 46 5,6,18 47 framework topologies but undergoes a slow induction period where adsorbed DME 48 49 50 intermediates react with the Brønsted acid sites to form persistent surface methoxy 51 52 53 54 intermediates along with gas phase methanol. As such, we assume herein that the 55 56 57 58 59 60 ACS Paragon Plus Environment 29 ACS Catalysis Page 30 of 54

1 2 3 4 Brønsted acid sites are fully converted to methoxy intermediates at steady-state 5 6 7 conditions. DFT-calculated activation enthalpies and free energies for elementary steps 8 9 10 11 (3)-(7) at the O2 site within SSZ-13 are shown in Figures S5 and S6, respectively. This 12 13 14 site, as discussed below, was found to be the most active. The resultant enthalpy and 15 16 17 18 free energy pathways in SSZ-13 presented in Figures S5 and S6 are consistent with 19 20 21 previous literature for DME carbonylation steps in MOR43 where the largest barrier and 22 23 24 25 rate limiting step involves the addition of CO to the bound methoxy species to form an 26 27 -1 -1 28 acetyl intermediate (step 4: ΔHA = 102.6 kJ mol ; ΔGA = 129.5 kJ mol ). The bound acetyl 29 30 31 32 can subsequently react with a second DME molecule to form MA that desorbs and a 33 34 -1 -1 35 surface methoxy intermediate (step 6: ΔHA = 47.8 kJ mol ; ΔGA = 91.9 kJ mol ), thus 36 37 38 39 regenerating the active methoxy site. Based on these results, the energetic analyses at 40 41 42 the remaining O sites within SSZ-13 focus solely on steps (3) and (4) (CO adsorption and 43 44 45 46 addition) that are thought to control the rates of carbonylation. 47 48 49 Figure 9 shows the transition states structures for the rate-determining CO addition to 50 51 52 a bound CH at each of the four distinct O sites. The transition states can lie in the plane 53 3 54 55 56 of the 8MR window (sites O1 and O2, Figure 9a and b), or orient into the cage from the 57 58 59 60 ACS Paragon Plus Environment 30 Page 31 of 54 ACS Catalysis

1 2 3 4 6MR (site O3, Figure 9c) or the 8MR (site O4, Figure 9d). Regardless of the originating 5 6 7 O site, the transition states for CO addition to the bound methoxy species appear to have 8 9 10 11 very similar geometries, with H3C--Ozeolite distances between 2.01 and 2.05 Å and O≡C-- 12 13 14 CH3 distances between 1.95 and 1.98 Å. Furthermore, the O≡C--CH3 bond angles varied 15 16 17 18 by only a few degrees (between 168˚ and 174˚) for the four O sites. The reaction energies, 19 20 21 intrinsic activation energies, and apparent activation energies for the CO addition 22 23 24 25 transition state at each of the O sites are shown in Table 2. Although the geometries of 26 27 28 the different transition states are nearly identical, the calculated intrinsic enthalpic and 29 30 31 -1 -1 32 free energy barriers at the O1 (ΔHA = 101.5 kJ mol ; ΔGA = 126.7 kJ mol ) and O2 (ΔHA 33 34 -1 -1 35 = 102.6 kJ mol ; ΔGA = 129.5 kJ mol ) sites are similar and markedly lower in energy 36 37 38 -1 -1 39 than those over the O3 (ΔHA = 114.0 kJ mol ; ΔGA = 137.3 kJ mol ) and O4 (ΔHA = 112.0 40 41 -1 -1 42 kJ mol ; ΔGA = 136.5 kJ mol ) sites. 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 31 ACS Catalysis Page 32 of 54

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 + 30 Figure 9. Transition states for CO addition to CH3 originating from the four O sites: (a) 31 32 33 O1 site, (b) O2 site, (c) O3 site, and (d) O4 site. The dotted green lines indicate close 34 35 36 + 37 contact interactions (< 3.50 Å) between the CH3 or (C)≡O of the transition state and the 38 39 40 oxygens of the SSZ-13 framework. Atom colors: Al (purple), Si (yellow), O (red), C (grey), 41 42 43 44 and H (white). 45 46 47 48 Table 2. DFT-calculated overall reaction enthalpies and intrinsic and apparent activation 49 50 51 52 energies for CO addition to methoxy in CHA at different O sites (P = 1 bar, T = 165 °C) 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 32 Page 33 of 54 ACS Catalysis

1 2 3 4 Cationi Reaction Enthalpy Intrinsic Barrier Apparent Barrier 5 O -1 -1 -1 a 6 c (kJ mol ) (kJ mol ) (kJ mol ) 7 Sit 8 positio 9 e ΔHrxn ΔHA -TΔSA ΔGA ΔHapp ΔGapp 10 n 11 12 O1 SIII’ -38.6 101.5 25.2 126.7 71.3 127.1 13 14 O2 SIII’ -46.6 102.6 26.9 129.5 69.1 125.7 15 16 O3 SII -23.5 114.0 23.3 137.3 83.6 135.6 17 18 O4 -29.5 112.0 24.5 136.5 86.2 141.3 19 20 aApparent enthalpy barriers are calculated with the assumption that the surface is saturated by 21 CH . 22 3 23 24 25 26 27 28 The transition state for CO addition is linear in nature as a result of the CO backside 29 30 31 attack on the CH3 and shown by the angles of 168˚ to 174˚. The reaction proceeds via a 32 33 34 concerted increase in the H C-O bond and decrease in O≡C--CH distance. The 35 3 zeolite 3 36 37 38 positively charged CH3 carbenium that forms is stabilized via its interactions with the 39 40 41 negatively charged O of the zeolite to which it was initially bound and by its interaction 42 43 44 45 with the C of the attacking CO. The H3C-Ozeolite bond length is essentially the same for 46 47 48 the 4 different O sites for carbonylation that were examined. As such, the stabilization 49 50 51 52 that results from the single Ozeolite alone would be very similar for all 4 sites. The other 53 54 55 framework oxygens in the local zeolite cage or ring, however, can interact and stabilize 56 57 58 59 60 ACS Paragon Plus Environment 33 ACS Catalysis Page 34 of 54

1 2 3 + 4 the carbenium ion that forms in the transition state. The CH3 of the transition states at 5 6 7 the O1, O2, O3, and O4 sites are distinctly different with 3, 4, 4, and 2 close contact 8 9 10 + 11 interactions (< 3.5 Å, dotted green lines in Figure 9) between CH3 and other zeolite 12 13 14 oxygens, respectively. The positively charged CH3-C≡O transition state can be further 15 16 17 18 stabilized by the interactions of its carbonyl C with the negatively charged oxygens in the 19 20 21 zeolite framework. CO addition to methyl groups at the O1 and O2 sites, which orient 22 23 24 25 within the 8MR window, results in 3 close contact interactions between the C of CO in the 26 27 28 O≡C--CH3 transition state and the O atoms in the zeolite framework. This is in contrast to 29 30 31 32 CO addition to the methyl groups bound to the O3 or O4 sites, which orient into the cage 33 34 35 from the 6MR or 8MR, respectively, where the CO in these transition states show little 36 37 38 39 interaction with the zeolite framework. Thus, the total number of close contacts, i.e., 40 41 42 stabilizing interactions between the transition state and zeolite framework, appear to be 43 44 45 46 indicative of the propensity for CO addition at each of the O sites. The O1 and O2 sites 47 48 49 have the greatest number of stabilizing interactions with 6 and 7 total close contacts, 50 51 52 respectively. The lack of stabilization of the CO in the O3 and O4 transition states leads 53 54 55 56 to the higher intrinsic activation enthalpies relative to the O1 and O2 sites, with differences 57 58 59 60 ACS Paragon Plus Environment 34 Page 35 of 54 ACS Catalysis

1 2 3 -1 4 of ~12 kJ mol . Thus, the lowest intrinsic barriers are obtained for the linear transition 5 6 7 states formed at O1 and O2, which correspond to the SIII’ site in the plane of the 8MR. 8 9 10 11 The increased stabilization of the positively charged O≡C--CH3 transition state at the 12 13 14 framework oxygens at the O1 and O2 sites in the CHA framework results in greater 15 16 17 18 entropic penalties at the O1 and O2 sites compared to the O3 and O4 sites. The difference 19 20 21 in entropic penalties between the O1 and O2 sites and the O3 and O4 sites, however, are 22 23 24 -1 25 only 2 to 3 kJ mol . In contrast, the O1 and O2 sites show enthalpic stabilization of ~12 26 27 28 kJ mol-1, where the enthalpic contributions tend to greatly outweigh entropic penalties for 29 30 31 32 this carbonylation reaction. As such, the intrinsic free energy barriers are the smallest for 33 34 35 the O1 and O2 sites. In addition to the intrinsic barriers, the apparent enthalpy and free 36 37 38 39 energy barriers, which account for the adsorption of CO into SSZ-13, were also 40 41 42 calculated. They show the same trends with lower apparent barriers for the O1 and O2 43 44 45 46 sites. The computational results and analyses discussed here suggest that the highest 47 48 49 MA production rates would be obtained for methyl groups at the O1 and O2 sites 50 51 52 corresponding to the SIII’ position within the plane of the 8MR window due to the 53 54 55 56 stabilizing effect of zeolite confinement on the linear transition state. The results are also 57 58 59 60 ACS Paragon Plus Environment 35 ACS Catalysis Page 36 of 54

1 2 3 4 consistent with the experimentally observed trends of nonlinearly increasing MA 5 6 7 production rates with decreasing Si/Al and subsequently increasing site SIII’ occupation. 8 9 10 11 12 13 14 3.4 DME carbonylation activity of SAPO-34 15 16 17 18 The experimental and computational results for the DME carbonylation in SSZ-13s are 19 20 21 consistent with prior findings in MOR and FER in that confinement is a critical parameter 22 23 24 25 for catalytic activity. To determine the sensitivity of this reaction to the local strength of 26 27 28 the acid site, we investigated the activity of SAPO-34. SAPO-34 is the 29 30 31 4+ 5+ 32 silicoaluminophosphate analogue of SSZ-13. In SAPO-34, Si substitutes for P in the 33 34 35 aluminophosphate (ALPO) framework with the CHA topology, generating a negative 36 37 38 3+ 4+ 39 charge in the lattice that requires a charge-compensating cation (similar to Al /Si 40 41 42 substitution in zeolites). In the H+-form, these catalysts exhibit Brønsted acidity that is 43 44 45 26,50 46 reported to be weaker than that for zeolites (e.g., SSZ-13). Further, both confining 47 48 49 environments (at the 6MR in the cage and at the 8MR) are represented by protons at this 50 51 52 Si content (9 at.%) in SAPO-34.51 Thus, the impact of acid site strength can be evaluated 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 36 Page 37 of 54 ACS Catalysis

1 2 3 29 27 4 independently from confinement effects. PXRD, MAS NMR ( Si, Al) and SEM-EDS 5 6 7 analyses of the SAPO-34 material are presented in Figures S7-S10. 8 9 10 11 The DME carbonylation activity of the SAPO-34 sample compared to activities of the 12 13 14 SSZ-13s are shown in Figure 10 as a function of the total Brønsted acid site density. The 15 16 17 18 time-dependent rates are presented in Figure S11. The activity of this sample is very low, 19 20 21 despite its large density of acid sites. The Brønsted acid signal in the 1H MAS NMR 22 23 24 25 spectrum for SAPO-34 (Figure 11) shows two contributions. The shoulder that spans from 26 27 28 the dominant signal at 3.7 ppm towards 4 ppm is attributed to zeolite-like domains that 29 30 31 4+ 3+ 5+ 32 can form in SAPO when two Si atoms replace Al and P , creating SiO2 domains or 33 34 26,52 3+ 35 “islands”. Al bound to the edges of SiO2 domains are zeolite (aluminosilicate)-like, 36 37 38 51,53 39 which gives rise to zeolite acidity at these SiO2 island edges. The dominant resonance 40 41 42 at 3.7 ppm corresponds to Brønsted acid sites in silicoaluminophosphate domains (i.e., a 43 44 45 4+ 5+ 46 single Si /P substitution), which is shifted upfield relative to that for the SSZ-13s (4 47 48 49 ppm).37,54 Upfield chemical shifts are consistent with shielding effects due to higher 50 51 52 electron densities that give rise to weaker acid sites.26,50 Decreased acid site strength can 53 54 55 56 restrict the DME carbonylation pathway in two primary ways: 57 58 59 60 ACS Paragon Plus Environment 37 ACS Catalysis Page 38 of 54

1 2 3 4 (i) The first step in this reaction involves an induction period, during which the 5 6 7 Brønsted acid sites interact with the oxygen in DME. This results in a concerted 8 9 10 11 proton transfer to the oxygen of DME, the elimination of methanol, and the 12 13 14 formation of a bound methyl to the oxygen of the zeolitic framework.43 If the 15 16 17 18 strength of that interaction is insufficient, the formation of methoxy groups is 19 20 21 inhibited, resulting in low surface coverage of critical intermediates. 22 23 24 25 (ii) Even if the surface is completely saturated, the surface methoxy groups that 26 27 28 replace the protons likely bear higher electron densities, as well. Consequently, 29 30 31 32 they would be poorer candidates for nucleophilic attack by CO. 33 34 35 Evidence of the high surface coverage of methoxy groups over SAPO-34 even at mild 36 37 38 39 temperatures has been shown extensively during studies of this material as a catalyst in 40 41 42 the methanol-to-olefins reaction.55–59 This would suggest that weaker electrophilicity of 43 44 45 46 the methoxys (ii) is the primary cause of the low activity of this material. This is supported 47 48 49 by the higher barriers calculated for the rate-determining CO addition step at the four 50 51 52 crystallographically-distinct O sites in SAPO-34 for a methoxy-saturated surface (Table 53 54 55 56 S3). Similar close contact interactions between the transition state and the framework 57 58 59 60 ACS Paragon Plus Environment 38 Page 39 of 54 ACS Catalysis

1 2 3 4 and minor differences in the HC3-Ozeolite bond distances (Figure S12) indicate that the 5 6 7 confining environments and the geometries of the transition states are similar regardless 8 9 10 11 of framework chemical composition. Yet, the intrinsic enthalpic barriers, particularly for 12 13 -1 -1 14 the most active O1 (SSZ-13: ΔHA,O1 = 101.5 kJ mol ; SAPO-34: ΔHA,O1 = 111.8 kJ mol ) 15 16 17 -1 -1 18 and O2 (SSZ-13: ΔHA,O2 = 102.6 kJ mol ; SAPO-34: ΔHA,O2 = 111.2 kJ mol ) sites, are 19 20 21 ~ 10 kJ mol-1 larger in SAPO-34 than in SSZ-13. The larger barriers for the rate- 22 23 24 25 determining step might explain the lower MA production activity of SAPO-34 compared 26 27 28 to SSZ-13. These results suggest that the acid strength afforded in zeolites is required 29 30 31 32 for effective catalysis in the DME carbonylation reaction through the Koch-type 33 34 35 mechanism. 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 39 ACS Catalysis Page 40 of 54

1 2 3 4 5 ) 1 - 6 r 0.14

h SSZ-13

1 7 - T SAPO-34

O 0.12 8 T g

9 o

10 m 0.10 m ( 11 n

o 0.08 i

12 t c

13 u d 0.06 o 14 r p

15 A 0.04 M 16 e t a

17 r 0.02 x

18 a 19 M 0.00 20 0.0 0.5 1.0 1.5 2.0 21 Bronsted acid site density (mmol g-1) 22 23 24 25 Figure 10. Maximum rate of MA production as a function of Brønsted acid site density 26 27 28 29 (measured by 1H MAS NMR) for CHA materials: SSZ-13s (solid squares) and SAPO-34 30 31 32 (open star). 33 34 35 36 37 38 SAPO-34 39 40 41 42 SSZ-13s 43 44 45 46 decreasing 47 Si/Al 48 49 50 51 52 8 6 4 2 0 -2 53 Chemical shift (ppm) 54 55 56 57 58 59 60 ACS Paragon Plus Environment 40 Page 41 of 54 ACS Catalysis

1 2 3 1 4 Figure 11. H MAS NMR spectra for dehydrated SSZ-13s (fine black curves) and SAPO- 5 6 7 34 (thick green curve). 8 9 10 11 12 4 Conclusion 13 14 15 CHA-type zeolites (SSZ-13s) are active catalysts for the carbonylation of dimethyl ether 16 17 18 to methyl acetate, an industrially important reaction for acetic acid production. At 19 20 21 22 comparable Si/Al, the total MA produced after 24 hr over CHA is within 12% of that 23 24 25 produced over MOR and is over 6x more than FER. To develop an understanding of the 26 27 28 29 reactivity of the CHA framework, SSZ-13 materials with a wide range of Si/Al were 30 31 32 synthesized and characterized through various physicochemical techniques and 33 34 35 36 reactivity experiments. Results from this investigation suggest that methoxy groups that 37 38 39 give the highest MA production rates in CHA are located at the 8MR, while those that give 40 41 42 43 lower rates reside in the CHA cage above the 6MR. DFT results support the experimental 44 45 46 findings, calculating the lowest activation barrier for the RDS of nucleophilic attack by CO 47 48 49 50 at the methoxy group oriented within the plane of the 8MR window. These results further 51 52 53 emphasize the critical role that confinement plays on this reaction mechanism, particularly 54 55 56 57 58 59 60 ACS Paragon Plus Environment 41 ACS Catalysis Page 42 of 54

1 2 3 4 in the context of the positively-charged transition state stabilized by close proximity to the 5 6 7 negatively charge zeolitic framework. Lastly, through a comparison of SAPO-34 and SSZ- 8 9 10 11 13 reactivities, we demonstrated that a “zeolitic” acid site strength is required for this 12 13 14 reaction to effectively proceed, even in similar confining environments. 15 16 17 18 19 20 21 ASSOCIATED CONTENT 22 23 24 25 26 AUTHOR INFORMATION 27 28 29 30 Corresponding Author 31 32 33 *Mark E. Davis, [email protected]. 34 35 36 37 38 Author Contributions 39 40 41 42 The manuscript was written through contributions of all authors. All authors have given 43 44 45 approval to the final version of the manuscript. 46 47 48 49 50 Funding Sources 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 42 Page 43 of 54 ACS Catalysis

1 2 3 4 The authors thank Chevron for supporting the experimental work and the University of 5 6 7 Minnesota for the computational efforts. 8 9 10 11 12 Supporting Information. Experimental and model fit MAS NMR spectra, SEM images, 13 14 15 energetics vs reaction coordinate diagrams, SAPO-34 characterizations. This information 16 17 18 is available free of charge on the ACS Publications website. 19 20 21 22 ACKNOWLEDGMENT 23 24 25 The authors gratefully acknowledge Dr. Sonjong Hwang and Dr. Stacey Zones for their 26 27 28 29 helpful insight on the synthesis and characterization of the materials used in this 30 31 32 investigation. The authors also acknowledge the computing resources provided by the 33 34 35 36 Minnesota Supercomputing Institute (MSI) at the University of Minnesota that contributed 37 38 39 to the calculations reported in this study. 40 41 42 43 44 REFERENCES 45 46 47 (1) Sunley, G. J.; Watson, D. J. High Productivity Methanol Carbonylation Catalysis 48 49 50 51 Using Iridium The Cativa TM Process for the Manufacture of Acetic Acid. 2000, 58, 52 53 54 293–307. 55 56 57 58 59 60 ACS Paragon Plus Environment 43 ACS Catalysis Page 44 of 54

1 2 3 4 (2) Tomas, R. A. F.; Bordado, J. C. M.; Gomes, J. F. P. P-Xylene Oxidation to 5 6 7 Terephthalic Acid : A Literature Review Oriented toward Process Optimization and 8 9 10 11 Development. Chem. Rev. 2013, 113, 7421–7469. 12 13 14 (3) Bejblova, M.; Prochazkova, D.; Cejka, J. Acylation Reactions over Zeolites and 15 16 17 18 Mesoporous Catalysts. 2009, 23, 486–499. 19 20 21 (4) Koh, J.; Kim, H. J.; Lee, B. S.; Kim, S. D. Effect of Acid Donors on the Dyeing of 22 23 24 25 Nylon Fiber with Acid Milling Dyes. 2018, 19, 2533–2540. 26 27 28 (5) Cheung, P.; Bhan, A.; Sunley, G. J.; Law, D. J.; Iglesia, E. Site Requirements and 29 30 31 32 Elementary Steps in Dimethyl Ether Carbonylation Catalyzed by Acidic Zeolites. J. 33 34 35 Catal. 2007, 245, 110–123. 36 37 38 39 (6) Bhan, A.; Allian, A. D.; Sunley, G. J.; Law, D. J.; Iglesia, E. Specificity of Sites within 40 41 42 Eight-Membered Ring Zeolite Channels for Carbonylation of Methyls to Acetyls. J. 43 44 45 46 Am. Chem. Soc. 2007, 129, 4919–4924. 47 48 49 (7) Kazantsev, M. S.; Luzgin, M. V; Stepanov, A. G. Carbonylation of Dimethyl Ether 50 51 52 with CO on Solid 12-Tungstophosphoric Acid : In Situ Magic Angle Spinning NMR 53 54 55 56 Monitoring of the Reaction Kinetics. J. Phys. Chem. C 2013, 117, 11168–11175. 57 58 59 60 ACS Paragon Plus Environment 44 Page 45 of 54 ACS Catalysis

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