Influence of Acid Strength on the Reactivity of Dimethyl Ether

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The influence of acid strength on the reactivity of dimethyl ether carbonylation over H-MOR Cai Kai, Shouying Huang, Ying Li, Zaizhe Cheng, Jing Lv, and Xinbin Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04388 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Sustainable Chemistry & Engineering

The influence of acid strength on the reactivity of dimethyl ether carbonylation over H-MOR

Kai Cai, Shouying Huang*, Ying Li, Zaizhe Cheng, Jing Lv and Xinbin Ma*

Key Laboratory for Green Chemical Technology of Ministry of Education; Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China

Corresponding author

* Shouying Huang: [email protected] * Xinbin Ma: [email protected]

1

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Abstract

Ethanol synthesis that consists of carbonylation of dimethyl ether (DME) to methyl acetate (MA) and hydrogenation of MA is considered as a potential and sustainable technology. The aim of this study was to explore the influence of strength of Brønsted acid sites in 8-membered ring (8-MR) of H-mordenite (H-MOR) on the reactivity of DME carbonylation. A series of H-MOR samples with Si/Al ratios ranging from 6 to 21 was obtained by treating with oxalic acid and ammonium hexafluorosilicate. A new method, Fourier transformed infrared spectra of temperature-programmed desorption (FTIR-TPD) of NH3 was used to determine the strength of Brønsted acid sites in different channels. By comparing the heat of NH3 desorption in 8-MR (Hdes(NH3-8MR)) of the H-MOR samples with different Si/Al ratios, we found that the acid strength decreased with the increase of Si/Al ratio. A linear relationship between Hdes(NH3-8MR) and the turnover frequency of MA formation was established, indicating the stronger acidity in 8-MR accelerates the formation rate of MA. Theoretical calculations suggested that stronger Brønsted acid sites in 8-MR side pockets (T3-O33) could reduce the activation barrier of CO insertion to form acetyl group, which is consistent with the experimental results. Key words: ethanol; dimethyl ether; carbonylation; zeolite; acidity

2


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Introduction

Nowadays, the rapid increase in global energy demand, depletion of fossil oil resource as well as rising concerns about environment and energy security have triggered a worldwide research interest in alternatives to petroleum-derived fuels and chemicals. Ethanol is considered as an ideal and viable alternative for transportation fuel and or as a potential source of hydrogen in fuel cells.1 The widespread utilization of ethanol contributes to reducing negative environmental effects, such as emissions of CO and hydrocarbons. In addition, ethanol can also serve as a versatile feedstock and green solvent in chemical industry. Therefore, the development of clean and efficient technology based on alternative resources for ethanol production is a burgeoning topic of both scientific and industrial interest, especially in countries that are committed to improve environment or to reduce their dependence on imported crude oil. Recently, a new synthetic route of ethanol from syngas has been proposed, consisting of carbonylation of dimethyl ether (DME) to methyl acetate (MA) and MA hydrogenation (as shown in Eq. 1 and 2). CH3OCH3+CO → CH3COOCH3

(1)

CH3COOCH3+2H2 → CH2CH3OH + CH3OH

(2)

The raw material DME could be easily produced from syngas (CO+H2), which can be derived from alternative sources such as biomass, coal, natural gas and organic waste etc..2 On the other hand, the only byproduct methanol could be recycled to produce DME through dehydration.3 More importantly, an energy-intensive separation process of products is avoided, owing to the absence of ethanol-H2O 3



azeotrope. Therefore, this synthetic route exhibits significant benefits in sustainability and economy, such as high atomic economy, mild conditions, cheap catalysts, abundant raw materials as well as technically flexibility. Solid acid catalysts, especially aluminosilicate zeolites were found active in the carbonylation of DME, in which Brønsted acid sites (BAS) serve as active centers.4-7 As an environmental-friendly porous material, zeolites have been extensively used as heterogeneous catalysts in oil refining and chemical industry due to their specific features, such as tunable acidity, well-defined micro-porosity, large specific area and high thermal stability. In acid-catalyzed reactions such as cracking, isomerization, dehydrogenation, methanol-to-olefins etc., acidic properties including quantity, strength and location play crucial roles in catalytic performance. Therefore, regulating and characterizing the acid sites in zeolites as well as exploring the structure-activity relationship are of fundamental importance in understanding and rational design of zeolite catalysts. Several approaches toward tuning acidity of aluminosilicate zeolites have been exploited, including acidic/alkaline/hydrothermal treatment, ion exchange, heteroatom incorporation and so on. Among these approaches, adjusting Si/Al ratios during synthesis or through post-treatment is the most available and efficient way to modify acidic strength. Despite intense experimental and computational efforts, however, the relationship between Si/Al ratios and acid strength as well as the influence in reactivity is still in dispute. In some cases, the discrepancies lie in zeolites with different framework structure, such as MOR,8 ZSM-59 and MCM-22.10 The acidic 4


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strength is also dependent on the location and distribution of Al atoms, which might result in proximity effect, even in zeolite with same topology. In H-MOR catalyzed DME carbonylation, numerous efforts have been devoted to exploring the relationship between the amount of acid sites and reactivity. It has been found that the formation rates of MA are proportional to the number of BAS in 8-membered ring (8-MR) channels. However, few reports focus on understanding the effect of acidic strength in H-MOR. In this work, we prepared H-MOR samples with different Si/Al ratios by post-treatment using oxalic acid with hexafluorosilicate (AHFS) in sequence. As acid treatment usually results in lattice defects such as hydroxy nest at the site where aluminum is removed, we employed the subsequent treatment with AHFS to eliminate defect sites by Si insertion. As a result, a serious of H-MOR samples with different Si/Al ratios and high crystallinity were obtained. The number of BAS in 8-MR were characterized by combining ammonia temperature-programmed desorption (NH3-TPD) and Fourier transformed infrared spectra of pyridine adsorption (Py-FTIR). In particular, Fourier transformed infrared spectra of temperature-programmed desorption (FTIR-TPD) of ammonia was employed to determine the Brønsted acid strength in different channels. Together with DFT calculations, the influence of Brønsted acid strength in 8-MR on MA formation over H-MOR was investigated.

Experimental

Catalyst preparation 5



Parent zeolite, denoted as SAR=8, was treated with oxalic acid solution with varying concentration and time, followed by removing debris and defects with AHFS . In a typical run, 10 g of the sample of SAR=8 was treated with 200 mL oxalic acid solution (0.5 M) for 6 h, 12 h, 24 h and oxalic acid (1M) for 24 h at 353 K, respectively. Subsequently, the above acid-treatment sample was suspended at 250 mL ammonium acetate buffer (1M), and then 20 mL AHFS solution (1M) was added dropwise and kept at 353 K for 6 h. Then, the obtained samples were washed with hot water and dried at 383 K for 12 h. The resulting solid was finally calcined at 773 K for 4 h with a 1 K/min heating rate to get H-MOR. The removal of defects was evidenced by FTIR spectra in the O–H stretching region (Figure S1) of SAR=15 sample before and after treatment with AHFS. The as obtained samples with different Si/Al ratios were named as SAR=X, where X stands for the actual framework Si/Al ratio determined by 29Si MAS NMR (the calculation details were given in Figure S2 ). Characterization 29Si

MAS NMR spectra were recorded on a Varian Infinityplus-300 spectrometer

to obtain the framework Si/Al ratio of samples. The experiments were performed at resonance frequencies of 78.1 MHz with the spinning rate of 7 kHz. Three bands were found in the spectra of all samples (Figure S2).The signal at -106 ppm was assigned to Si (1Al) and -112 ppm and -114 ppm were assigned to Si (0Al).11 The framework Si/Al ratios were determined based on the peak fitting results using Eq. S1 (the details were given in SI).X-ray diffraction (XRD) patterns were measured by a Rigaku D/max-2500 diffractometer using Cu Kα radiation (λ = 0.154056 nm) at 40kv and 200 6


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mA. All the samples were scanned from 3° to 50° (2θ) with a 8°/min scanning speed at room temperature. The relative crystallinity was calculated using the ratio of the sum of peak areas of the peaks (200), (330), (150), (202) and (350) by assuming 100% crystallinity of the parent mordenite (SAR = 8). N2 adsorption-desorption analysis was performed at 77 K using a Micromeritics ASAP-2020 analyzer, samples were degassed at 473 K for 24 h before the analysis. Total surface area and pore volume were calculated by the BET method and Horvath-Kawazoe equation, respectively. And t-plot method was applied to estimate microporous area, external surface area and micropore volume. The total content of Al were determined on an inductively coupled plasma optical emission spectroscope (ICP-OES) (Vista-MPX, Varian). All the samples were dissolved using HF aqueous solution, subsequently complexed with excessive amount of H3BO3 and diluted before measurements. 27Al

magic angle spinning nuclear magnetic resonance (27Al MAS NMR) was

performed on a Varian Infinityplus-300 spectrometer. The spectra were recorded at a resonance frequency of 104.2 MHz with a spinning rate of 8 kHz. Scanning electron microscope (SEM) to characterize the morphological of the samples was performed on a field emission scanning electron microscope (FESEM, S-4800, HITACHI, Japan). Transmission electron microscopy (TEM) was performed on a JEM-2100F instrument. The samples were suspended in ethanol for 30 min under ultrasonication, the mixture was dropped on a Cu parallel-bar TEM grid followed by drying in the air. 7



Temperature-programmed desorption of ammonia (NH3-TPD) was carried out on a Micromeritics Autochem II 2920 instrument with 0.1 g H-MOR sample. The samples were pretreated at 473 K for 1 h under Ar atmosphere to remove impurities. After the pretreatment, the samples were cooled down to 423 K and exposed to a flow of NH3/He for 1 h, and then NH3/He was switched to Ar for 1 h to remove physically adsorbed NH3. The samples were heated from 373 K to 873 K at a rate of 10 K/min under He flow. The amount of desorbed NH3 was measured by a thermal conductivity detector (TCD) in the meantime. In order to eliminate the interference of H2O on TCD signal, ethylene glycol (EG) cooled by liquid nitrogen was chosen as cold trap to condense H2O before the desorption gas enter into TCD. In order to selectively determine the BAS in the 12-MR, Fourier transformed infrared spectra of pyridine adsorption (Py-FTIR) were performed on a Thermo Scientific Nicolet 6700 connected to a vacuum system. Samples (20 mg) were pressed into self-support wafers and placed into the in-situ cell with CaF2 windows. Prior to adsorption of pyridine, the samples were evacuated to 10-4 Pa at 723 K for 1 h. After in-situ cell was cooled down to 423 K, a spectrum was recorded as background. Subsequently, the sample was saturated in pyridine vapor for 30 min, and then evacuated for 30 min to remove gaseous and physically adsorbed pyridine. All spectra were collected in the range of 40001100 cm-1 by averaging 32 scans at a resolution of 4 cm-1. The concentrations of Brønsted and Lewis acid sites were calculated based on the bands at ~1540 cm-1 and ~1450 cm-1 respectively, according to the literature.12 To further compare the strength of Brønsted acid site at 8-MR of samples with 8


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different

Si/Al

ratios,

Fourier

transformed

infrared

spectra

of

temperature-programmed desorption (FTIR-TPD) of ammonia was also performed on the Thermo Scientific Nicolet 6700. After evacuation of the samples at 723 K for 1 h, IR spectra were recorded from 373 K to 873 K with different heating rates (5 K/min, 10 K/min, 15 K/min, 20 K/min), shown as N(T) (Figure S3a). And then the samples were pretreated with 10% NH3/He for 30 min after the samples were cooled down to 423 K, followed by evacuation for 30 min to remove physically adsorbed NH3. Spectra were collected at the same time for the purpose of calculating the amount of BAS based on the band at 1430 cm-1. Subsequently, IR spectra were recorded with the same procedure before ammonia adsorption, shown as A (T). As the vibration frequency of zeolite framework would vary with the temperature, we obtained the relationship between band position and temperature by performing linear fitness of spectra N(T) (shown in Figure S3). The result (Eq. S2) was applied to correct the spectra before analysis. The difference spectra, A(T) - N(T), were deconvoluted into two peaks (~3590 cm-1 for the BAS in 8-MR and ~3610 cm-1 for the BAS 12-MR) in the OH region.5 A differential change in the absorption intensity at 3590 cm-1 with respect to the corresponding temperature, -d{A(T) - N(T)}/dT, was calculated to determine the ammonia desorption temperature TP with different heating rates. The heat of ammonia desorption was determined as follows 13: ln

( ) β

TP

2

=―

∆Hdes RTP

+ ln

∆HdesA

(

RC

)

(3)

where β is the ramp rate, TP is the temperature at peak max, Hdes is the heat of NH3-desorption from BAS, A is the quantity of adsorbed NH3 at saturation, R is gas 9



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constant and C is a constant related to the desorption rate. By plotting 2lnTp - ln β versus 1/TP the heat of ammonia desorption can be determined by using Eq. (4), where s is the slope of the plot. Hdes= s × R

(4)

Catalytic reaction Catalytic activity tests were conducted in a fixed bed reactor with inner diameter

of 8 mm. Typically, 500 mg catalyst (40-60 mesh) was loaded in the middle of the stainless reactor. Pretreatment of catalyst was performed under N2 flow (75 mL/min） at 473 K for 9 h to remove adsorbed water. The reactant mixture (DME/CO = 1/49, mol/mol) was then introduced to the reactor with a flow rate of 100 mL/min and the reaction pressure was maintained at 1.5 MPa. The effluent was analyzed by an online gas chromatograph (Agilent 7890B), which was equipped with a thermal conductivity detector and a flame ionization detector. The conversion of DME (XDME), selectivity to MA (SMA), Space time yield of MA (STYMA) and turnover frequency of MA (TOFMA) were calculated by equations in Supporting Information (Eq. S3-S6). The initial XDME and YMA (reaction time at 1.5 h) were chosen as descriptor of activity for all the samples in this work, taking rapid deactivation into consideration.14 Detailed activity information was listed in Table S1. Computational methods Density functional theory (DFT) calculation was also applied to investigate the influence of the strength of Brønsted acid site in 8-MR on the reactivity of DME carbonylation. A cluster model containing 21T centers was used to present the local 10


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environment of 8-MR channels, which is derived from the periodic MOR structure and terminated with H atoms along with the direction of corresponding Si–O bond to maintain charge neutrality. According to literature,7, 15 only the T3-O33 site was taken into consideration. The BAS were created by replacing one of Si atoms at T3 site by an Al atom, and then a proton attached to O33 site was added to compensate the negative charge. The acid strength was modulated by tuning the length of terminal Si– H bond (rSi-H) of zeolite models, as used in literature.16-17 And the rSi-H was respectively set to 1.2 Å, 1.6 Å, 2.0 Å, and 2.4 Å in this work, as displayed in Figure S16. Deprotonation energy (EDPE) was used as a descriptor of acid strength that represent the energy required to remove the acidic proton from the zeolite framework (ZEO–OH) to form a zeolite framework with a negative charge, ZEO–O- . EDPE = EZEO ― O ― ― EZEO ― OH

(5)

where EZEO ― OH and EZEO ― O ― are the energies of neutral H-form zeolite, zeolite anion after removing proton, respectively. A smaller EDPE value means that heterolytic cleavage of H atoms from framework is more easily to occur, indicative of stronger acid strength. Since previous experimental studies have confirmed that the formation of the acetyl species through the reaction of methyl groups with CO is the rate-limiting step. Herein, activation barrier of this reaction step on MOR models with different acid strength were calculated to represent the ease of DME carbonylation to MA. The activation barrier (Eact) was calculated by the following formula: (6)

Eact = ETS ― ER 11



where ETS and ER are the energies of transition states and the sum of the energy of intermediates respectively. DFT calculations were performed using DMol3 package in Material Studio. All geometry optimizations and energy calculations were performed using Generalized gradient approximation (GGA) with Perdew-Wang (PW91) density function with the double numeric polarized (DNP) basis set. The boundary SiH3 groups of the cluster model were fixed, while other atoms of the cluster model and adsorbed molecules were allowed to relax during geometry optimizations. Complete linear synchronous transit and quadratic synchronous transit (Complete LST/QST) approach was used for searching the transition states of the reaction, and the optimized transition state structure was examined by frequency calculations.

Results and discussion

Textural and structural properties The Si/Al ratio and Al content of samples were summarized in Table 1. We successfully modulated the Si/Al ratios by changing the oxalic acid concentrations and treatment duration, which range from 8 to 26. A decrease of Al content in SAR=15 suggests that the treatment with 0.5 M oxalic acid for 6 h resulted in a massive Al leaching. And the severer treatment conditions (such as 0.5 M, 12 h), the more Al species removed, but the tendency of Al extraction became moderate. Two signals at about 54 ppm and 0 ppm were observed in the 27Al MAS NMR spectra (Figure 1a), attributable to tetrahedrally coordinated Al species (AlF) in 12


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zeolitic framework and octahedrally coordinated extra-framework Al species (AlEF), respectively.18 As the dealumination conditions became much harsher, the intensity of both AlF and AlEF signal weakens, indicating that the aluminium content in samples decreased gradually, which is consistent with the ICP-OES results. The proportion of AlEF was obtained by evaluating the intensity ratio of AEF/Atotal from peak deconvolution of NMR spectra. Notably, the impacts of extra-framework Al species on catalytic performance among different samples can be excluded, as evidenced by similar fraction of extra-framework Al species that ranges from 19% to 22%. The powder XRD patterns of samples with different Si/Al ratios were displayed in Figure 1b. All samples exhibited typical characteristic diffraction peaks of MOR zeolite,19 suggesting that all of them preserved well-defined MOR crystal structure after post-treatment. Relative crystallinities estimated from the five most intense peaks (2θ = 9.8˚, 19.6˚, 22.3˚, 25.7˚ and 26.3˚) showed a slight decrease as the treatment conditions became harsher, but are still higher than 90%, indicating that dealumination process has little influence on framework integrity of MOR. Unlike dealumination of zeolite with inorganic acid (such as HCl, HNO3) or steam that usually result in obvious loss of crystallinity,20 the strategy combined oxalic acid with AHFS in this case could obtain MOR of higher Si/Al ratio without significant zeolitic framework deterioration. The co-existence of micropores and mesopores was observed from N2 adsorption-desorption isotherms (Figure S5) of all the samples.21 As listed in Table 1, both the micropore area and volume slightly increased, suggesting that the mild 13



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dealumination process has little impact on micropores after acid treatment. The mesopore volume increased from 0.07 cm3/g to 0.124 cm3/g with the increase of treatment time and oxalic acid concentration, owing to creation of mesopore during the acid treatment process. Table 1. The composition, relative crystallinity and textural properties of samples with different Si/Al ratios Sample

a

Si/Al

a

Altotalb (mmol/g)

Relative Crystallinity (%)

Surface area (m2/g)

AlEFc (%)

Pore volume (cm3/g)

BET

Micropore

External

Total

Micropore

Meopre

SAR=8

7.8

1.43

100

19.2

415.5

369.2

46.3

0.242

0.172

0.070

SAR=15

14.7

0.94

96.2

22.8

433.1

374.5

58.6

0.254

0.173

0.081

SAR=18

18.2

0.72

93.4

22.1

442.7

377.3

65.4

0.290

0.176

0.114

SAR=24

23.7

0.64

90.1

19.1

442.4

373.1

69.3

0.297

0.174

0.123

SAR=26

26.3

0.53

93.3

22.4

448.9

375.7

73.2

0.301

0.176

0.124

Calculated from 29Si MAS NMR based on peak deconvolution ;

b Obtained

from ICP-OES results;

c Calculated

from 27Al MAS NMR based on peak deconvolution;

The SEM images of all the samples exhibit similar irregular morphology with average particle sizes in the range of 500-800 nm (Figure S6). The severer treatment conditions, the rougher surface could be observed. This is in accord with the results of N2 adsorption that the external surface area increases gradually with post-treatment. The TEM images (Figure S7) of the parent sample (SAR=8) and the samples with different post-treatment conditions (SAR=18, SAR=26) display well-crystallized lattice structure, demonstrating that the MOR structure is maintained after post-treatment. Note that some amorphous debris and defect sites were found on the treated samples, which is consistent with the observations from SEM images. 14


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Quantitation of acid sites The strong basicity and accessibility allows NH3 adsorption as a powerful tool to probe acidity of microporous materials. NH3-TPD profiles of H-MOR samples were shown in Figure 1c. In generally, NH3-TPD profiles of aluminosilicate zeolites consist of two desorption peaks, which are designated as l-peak and h-peak respectively according to low and high desorption temperatures. In some cases, a shoulder peak along with the h-peak can be observed, which was named m-peak. It is accepted that the h-peak is due to NH3 molecules strongly adsorbed on BAS sites, which has been demonstrated by TPD-MS, IR, chemical titration as well as NMR22-24. As for the l-peak, a majority of literatures attribute it to NH3 adsorbed on weak acid sites (mainly existed as Lewis acid sites),22, 25 whereas some other researchers declare that l-peak is due to ammonia physically adsorbed on zeolites as evidenced by complete disappearance of the l-peak after long-term evacuation. It is noted that a shoulder peak at medium temperature (600 K) exists in each sample. In previous studies,24,26 Na-form MOR that displays only l- and m-peak, is totally inactive in carbonylation of DME to MA. Therefore, it is reasonable to attribute the m-peak to NH3 interacting with Lewis acid sites. It is obvious that the center of h-peak shifts towards lower temperature (from 797 K to 754 K) with increasing the Si/Al ratio, indicative of a decrease in acid strength of BAS. On the basis of peak decomposition, we calculated the total amount of BAS through integrating the area of h-peak. As listed in Table 2, the concentration of BAS decreased gradually with the increase of Si/Al ratio, which is in accordance with the 15



tendency of framework Al content characterized by combination of ICP-OES and 27Al MAS NMR. Furthermore, infrared spectra of NH3 adsorption (NH3-IR) (Figure S4) was also applied to determine the concentration of BAS based on the band intensities at 1430 cm-1, which was ascribed to NH3 adsorbed on BAS, according to literature.27 It is noticeable that the concentration of BAS calculated from NH3-TPD and NH3-IR results were pretty close, indicating the reliability of quantitative assessment for total BAS in the samples.

Figure 1.

27Al

MAS NMR profiles (a) XRD patterns (b) NH3-TPD profiles (c)

pyridine-IR spectra (d) of the MOR samples with different Si/Al ratios Pyridine as probe molecule could selectively adsorbed on the acid sites in 12-MR channels because its size is smaller than 12-MR channels but larger than 8-MR 16


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channels of MOR.28-29

Figure 1d presents the FTIR spectra in the range of

14001700 cm-1. The bands at 1450 cm-1 and 1540 cm-1 are ascribed to pyridine adsorbed on Lewis and Brønsted acid sites, respectively. With the increase of Si/Al ratio, the intensities of both two peaks decreased, suggesting that the amount of Lewis and Brønsted acid sites in 12-MR reduced gradually as the dealumination conditions became severer, which has the same trend as total concentration of BAS calculated from NH3-TPD. Table 2. Quantity and distribution of acid sites and TOFMA in MOR samples Bframeworka

Bframeworkb

AlFc

(mmol/g)

(mmol/g)

(mmol/g)

B8-MRd

TOFMA

F8-MRe

(h-1)

(%)

F8-MRf

Sample (mmol/g)

(%)

Hdes(NH3-8MR)g kJ/mol

SAR=8

0.990

1.134

1.150

0.462

8.38

46.7

48.1

130.2

SAR=15

0.683

0.789

0.726

0.413

6.83

60.5

60.8

123.4

SAR=18

0.604

0.658

0.576

0.379

6.11

62.7

62.2

118.1

SAR=24

0.472

0.539

0.520

0.302

5.11

64.0

63.4

112.1

SAR=26

0.452

0.476

0.436

0.293

4.91

64.8

65.3

108.7

a

Calculated from NH3-TPD based on h-peak;

b Calculated c

from NH3-IR based on the band at 1430 cm-1 (εNH4+ = 0.11 cm2 /mol);

Calculated by using AlF =AlTotal ×FF-Al (%), where AlTotal is obtained by ICP-OES, FF-Al (%) is

calculated from 27Al NMR based on peak deconvolution; d,e

Calculated by using B8-MR = Bframework -B12-MR, where Bframework is obtained by NH3-TPD, B12-MR

is obtained according to the band at 1540 cm-1 in Py-IR (Integrated molar extinction coefficients of pyridine = 1.67 cm/mol); f,g

Calculated from FTIR-TPD of ammonia based on peak deconvolution.

Characterization of Acid strength Previous studies have proved that only the BAS located inside 8-MR channels could selectively catalyze MA production, while the BAS in 12-MR is responsible for 17



coke formation.7 Therefore, determining the variation tendency of acid strength of BAS in 8-MR is challenging and prerequisite to understand how it affects this reaction. As only the overall acid strength could be obtained from NH3-TPD, in this work, a new characterization method of FTIR-TPD of NH3 was undertaken in order to interpret the acid strength of Brønsted acid site at 8-MR for different H-MOR samples. Taken the SAR=8 as an example , the spectra of ammonia desorption recorded during elevating temperature from 373 K to 873 K with 5 K/min heating rate was shown in Figure 2a. The strong adsorption peak at 1430 cm-1 associated with bending vibration of NH4+ cation and 3400-2500 cm-1 related to N‒H stretches were observed.30 The NH4+ was formed through interacting with BAS, as confirmed by the appearance of the negative bands at OH region (37503500 cm-1). In H-MOR, there are two types of bridge hydroxyl groups functioning as BAS, which can be detected by FTIR with bands at ∼3610 cm-1 for acidic H+ in 12-MR channels and at ∼3590 cm-1 for H+ in 8-MR side pockets.5 During the heating process, the overlapped band of the two bands recovered with time, and the shift in frequency is due to different desorption rate of probe molecule (as shown in local magnification of Figure 2a). Concurrently, the positive bands related to NH3 adsorption diminished gradually, confirming the removal of NH3. All the curves corresponding to different desorption temperatures were fitted with a mixed Gauss-Lorenz profile. The curve fitting results of sample SAR=8 at 423 K was shown as Figure 2b. We calculated the fraction of BAS located in 8-MR using the ratio of peak areas derived from curve fitting results and compared it with the values obtained on the basis of NH3-TPD and Py-IR results 18


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(listed in Table 2). It is found that the percentage of BAS located in 8-MR derived from the two methods are very close, proving the feasibility and reliability of our curve fitting.

Figure 2. FTIR-TPD of NH3 for the sample of SAR=8: (a) IR Spectra during elevating temperature from 373 K to 873 K with heating rate of 5 K/min. (b) Deconvolution of the OH band (black) at 423 K, corresponding to OH in 8-MR (red) and 12-MR channels (blue). (c)The desorption peak temperature (TP) of NH3 absorbed on 8-MR with different heating rates. (d) Linear fit of 2lnTp - lnβ versus 1/TP. The desorption peak temperature (TP) of NH3 adsorbed on 8-MR with different heating rates was obtained from the differential of the peak area at 3590 cm-1 with respect to the temperature (Figure 2c). The desorption peak gradually shifts to higher 19



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temperature with increasing the heating ramp rates. The linear fit of 2 lnTp - lnβ versus 1/TP using Eq.(3) was shown as Figure 2d. The heat of NH3-desorption (Hdes(NH3-8MR)) from BAS in 8-MR of SAR=8 was determined as 130.2 kJmol-1K-1 based on Eq.(4), which is close to the data in literature.31 The other samples were also calculated with this method (Figure S8S15). With the increase of Si/Al ratio, the Hdes(NH3-8MR) decreased gradually from SAR=8 to SAR=26, indicating that the strength of BAS at 8-MR side pockets decreased with the increase of Si/Al ratio. Catalyst evaluation showed that the conversion of DME decreased gradually with the increase of Si/Al ratio, while all the samples exhibit excellent selectivity to MA. For the sake of clarity, the turnover frequencies (TOFs) of MA is plotted as a function of Hdes(NH3-8MR) in Figure 3a. It is seen that there is a linear correlation between them giving an R2 = 0.97, which indicates that the intrinsic acid strength plays an essential role in the DME carbonlylaion reaction. Therefore, the Hdes(NH3-8MR) can be used as a descriptor to predict the reactivity of DME carbonylation on zeolites, thus inspiring the design of zeolite catalysts. As we mentioned above, experimental and quantum-chemical investigations have revealed the site specificity of H-MOR for DME carbonylation and the T3-O33 positions in the 8-MR channels were selectively to produce MA,7,

15

herein, only

T3-O33 site was taken into consideration in this work. The models presenting the H-MOR catalysts with different acid strength could be obtained through tuning Si‒H bond length (Figure S16), as evidenced by decrease of the EDPE values from 303.2 to 254.8 kcal/mol with the terminal Si–H bond lengths elongating from 1.2 to 2.4 Å. 20


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Compared with the EDPE of different hydroxyl groups (e.g. 326 kcal/mol of silanol,32 326 kcal/mol of BF3/γ-Al2O3,33 295 kcal/mol of HY,34 258 kcal/mol of H3PW12O4035), it is conclusive that the acid strength of cluster models we choose covers the solid acid from weak-, strong-, to super-acid, and the values are also very close to the previous DFT calculations.36 As the CO insertion of methoxyl species (derived from DME association) to form acetyl groups have been recognized as the rate-determining step, we calculated the activation barriers (Eact) of this step on the models with varying acid strength. The optimized geometries of reactant, product, and transition state determined by the method mentioned above were shown as Figure S17. It can be seen that the activation barriers decrease from 29.5 kcal/mol to 24.1 kcal/mol with the EDPE ranges from 303.2 kcal/mol to 254.8 kcal/mol, which means the stronger BAS favors the reactivity of DME carbonylation. This result was consistent with the experimental results that increase acid strength could improve the space time yield of MA (STYMA). Based on the fitting results of the relationship between the deprotonation EDPE of acid sites at T3-O33 and activation barriers for DME carbonylation (shown in Figure 3b), the linear correlation confirms that the MA formation rate is proportionally dependent on the acid strength of BAS in 8-MR.

21



Figure 3. The correlation of (a) TOFMA with ΔHdes(NH3-8MR) (b) activation barriers of rate-limiting step in DME carbonylation and EDPE of T3-O33.

Conclusions

The reactivity of DME carbonylation over BAS at 8-MR of H-MOR with varying acid strength has been systematically investigated by experimental and theoretical method in this work. H-MOR samples with different Si/Al ratios ranging from 8 to 26 were obtained by sequential post-treatment with oxalic acid and AHFS. Calculation about Hdes(NH3-8MR) based on FTIR-TPD showed that the strength of BAS located at 8-MR channels decreased gradually with dealumination, in accord with the results of NH3-TPD. Concurrently, the turnover frequency of MA formation with respect to the BAS at 8-MR channels decreased with the increase of Si/Al ratio, demonstrating that stronger acidity could enhance the reactivity for DME carbonylation. DFT calculations showed that stronger acid at 8-MR channels could reduce the activation barriers of CO insertion to form acetyl groups, thus to promote the activity of DME carbonylation. These results reveal the relationship between acid strength of H-MOR and reactivity of DME carbonylation, which may provide 22


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inspiration for design of highly active zeolite for Brønsted acid catalyzed reactions.

Supporting Information

FTIR spectra of OH stretching region, 29Si MAS NMR profiles, NH3-IR spectra, N2 adsorption−desorption isotherms, SEM and TEM images, FTIR-TPD spectra, equations to calculate TOFMA, summaries of catalytic activities, optimized geometries of the cluster models and structures of reaction pathway (PDF)

Acknowledgment

This work was supported by the National Natural Science Foundation of China (21325626, 21406120) and the Program of Introducing Talents of Discipline to Universities (B06006).

23



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Synopsis: H-MOR with stronger acid sites is more efficient for dimethyl ether carbonylation, which is a key step for a sustainable route to produce ethanol.

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Influence of Acid Strength on the Reactivity of Dimethyl Ether

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