Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation

Journal Pre-proof Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation Baoyu Wang (Conceptualization) (Methodology) (Investigation) (Data curation)Writing- original draft), Ximing Yan (Validation) (Writing - review and editing) (Funding acquisition), Xingyuan Zhang (Supervision) (Project administration), Haiyang Zhang (Formal analysis), Faping Li (Resources)

PII:

S0926-3373(20)30060-6

DOI:

https://doi.org/10.1016/j.apcatb.2020.118645

Reference:

APCATB 118645

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 July 2019

Revised Date:

21 December 2019

Accepted Date:

14 January 2020

Please cite this article as: Wang B, Yan X, Zhang X, Zhang H, Li F, Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118645

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation Baoyu Wanga,c,1, Ximing Yanb,c,1,*, Xingyuan Zhangb,*, Haiyang Zhangc, Faping Lid a

College of Marine Sciences, Beibu Gulf University, Qinzhou, 535011, PR China

b CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, PR China c

Guangxi Colleges and Universities Key Laboratory of Beibu Gulf Oil and Natural Gas Resource Effective Utilization, College of

Chemistry and Chemical Engineering, Beibu Gulf University, Qinzhou 535011, China Hong Yuan Jiangmen Chemical With Science Co., Ltd, Jiangmen 529700, China

Jo

ur

na

lP

re

-p

ro of

d

1 *

These authors contributed to the work equally and should be regarded as co-first authors Corresponding authors. E-mail addresses: [email protected] (X. Yan), [email protected] (X. Zhang).

-p

ro of

Graphical abstract

Highlights

re

• Citric acid has a significant dealumination and realumination effect.

lP

• The amount of external protonic sites is critical to obtaining high catalytic activity. • Relatively weak Brønsted acid sites may be the primary active sites. • The turnover frequency can be used to reflect the catalytic performance.

Jo

ur

na

• The ring opening decomposition of trioxane may not necessarily be the initial step.

Abstract: Beta zeolite exhibits satisfactory catalytic activity in the synthesis of polyoxymethylene dimethyl ethers. However, their microporous channels have adverse effects on mass transfer of macromolecules. In this work, it was found that the textural and acidic properties of catalysts could be improved by citric acid treatment. Extensive characterization confirmed that citric acid induced significant pore-expanding on catalysts, and its dealumination and realumination effect could lead to reduced amount of total acid sites and increased amount of Brønsted acid sites (BAS). Meanwhile, a good positive correlation was found between amount of external weak BAS and initial reaction rate. It was speculated that BAS with relatively weak acid strength might be the primary active sites in

ro of

this reaction. Compared with the blank sample, the initial reaction rate of optimal catalyst was increased by 100%, and the conversion of trioxane was increased by 86.5 %, indicating a significantly improved catalytic activity.

Keywords: Polyoxymethylene dimethyl ethers; Beta zeolite; Citric acid; Textural properties; Acidic properties

-p

1. Introduction

It is well known that diesel engines are widely used in transportation, construction machinery,

re

ship power industries owing to the strong power, high combustion heat efficiency, long service life, and excellent safety. However, it is criticized a lot due to the emission of high-polluting exhaust

lP

gas [1]. Even for high-performance diesel engines, the emitted excess NOx and soot are still big threats to human health [2]. Therefore, how to improve the environmental performance of diesel

na

engines while maintaining its power, stability, and economic efficiency is an urgent issue worthy of attention [3]. Studies have confirmed that the addition of oxygen compounds in diesel can reduce the formation of harmful substances such as soot during combustion [4]-[6].

ur

Polyoxymethylene dimethyl ethers (PODEn) are a general term for low molecular weight acetal polymers. Studies have shown that PODEn (n = 3 ~ 5) have not only similar

Jo

physicochemical properties to diesel [7], but also higher oxygen content and higher cetane number than diesel [8],[9], which can reduce the kinematic viscosity of diesel [10]. Also, they can reduce the freezing point of diesel with lower n-alkane content [11], so they are considered a new type of environmentally friendly diesel additives [12]. Based on systematical study on the combustion and emission performance of diesel blended with PODEn, it was found that mixing a proper amount of PODEn in diesel could not only increase diesel combustion performance by reducing the negative occurrence of ignition delay [13]-[15], but also change the emission balance of NOx and soot by

increasing exhaust gas recirculation rate while maintaining soot content [6],[7],[15],[16]. Moreover, pollutant emissions such as THC, CO in the exhaust gas can also be effectively reduced by adding PODEn in diesel [10],[17]. In short, the addition of PODEn to diesel is one of the most convenient and effective measures to improve diesel quality and environmental performance, which has broad application prospects [18]. TRIox

MeOH

DMM

PODE n

PFn an oxymethylene group provider a methyl-end group provider

ro of

FA

Scheme 1. Synthesis routes for the synthesis of PODEn.

-p

Baranowski et al. reviewed [19] that the raw materials for synthesis of PODEn and found there were mainly two types of compounds: one is compound that provides oxymethylene group

re

(─CH2O─), including formaldehyde (FA), trioxane (TRIox) and paraformaldehyde (PFn). The other is compound which provides methyl-end group (─CH3), including methanol (MeOH),

lP

dimethoxymethane (DMM), etc. The synthesis routes are shown in Scheme 1. Water is present in all synthetic systems except for the synthetic route of DMM and TRIox [19],[20]. PODEn

na

synthesis in water environment usually has shortcomings including high reaction temperature, long reaction time, large amount of by-products, and difficulty in separating products from water [21]-[23]. Moreover, the presence of water induced hydrolysis of PODEn to form methanol and

ur

formaldehyde. Studies have shown that when the water content exceeds 5 ωt%, hydrolysis reaction has an un-ignorable effect on the synthesis, which will significantly reduce selectivity and

Jo

yield [21]. Therefore, PODEn synthesis in the anhydrous environment has attracted more attention [20],[24],[25].

Because of adjustable textural and acidic properties, easy regeneration after deactivation, and

high environmental friendliness, zeolite catalysts are widely used in acid catalysis reactions [26]-[28]. PODEn synthesis represents a typical acid catalysis process, for which BEA, MFI, and FAU microporous zeolites have been demonstrated to have better activities [29]-[32]. However, such microporous zeolites are restricted by pore channels, which adversely affect the mass transfer

and diffusion of macromolecular reactants and products, especially under low-temperature [27]. Fu et al. simulated the molecular sizes of PODEn (Table 1), and found that it was difficult for PODEn to enter the microporous structure of the zeolite and participate in the reaction when n > 3 [32]. Lautenschütz et al. also confirmed that acid strength of zeolites had little effect on the reactivity at low temperatures, but the mesoporous surface area played a decisive role in the synthesis of PODEn [29]. At the same time, Xue et al. modified the mesoporous SBA-15 with aluminum isopropoxide, which demonstrated good activities in PODEn synthesis [33]. Therefore, how to optimize the pore channel structure, improve the mass transfer efficiency of the

ro of

macromolecule inside the catalyst, and improve the catalyst stability while maintaining proper acidity of microporous zeolite are the key problems to be solved in PODEn synthesis over microporous zeolite [27]. Table 1

-p

The optimized models of PODE3, PODE5, and PODE8 by Gaussian 09 [32].

Size of optimized models (a × b nm)

n=3

0.70 × 0.52 nm

re

PODEn

n=5

1.28 × 0.32 nm 1.49 × 0.28 nm

lP

n=8

Acid-base post-treatment is regarded as one of the most convenient methods for adjusting the

na

textural and acidic properties of microporous zeolites [34]-[36], which has been widely used in industrial production [37][40]. Baranowski et al. used a combination of NaOH and HCl to adjust

ur

the textural properties of ZSM-5 zeolite, which significantly enhanced the synthesis of PODEn [31]. However, the "etching and desilication" effect of the lye is limited by the silica to alumina

Jo

ratio, the catalyst crystallinity is greatly lowered, and framework even collapses at high temperature, so stability is deteriorated [41]. On the contrary, acid treatment method can also achieve the purpose of regulating textural and acidic properties, which has little effect on the zeolite framework, so that the catalyst can maintain good stability [34],[42]. In this paper, the effect of textural and acidic properties on reaction activity is investigated. Moreover, the relationship between textural properties, acidic properties, and reaction activity is analyzed to speculate the potential active sites.

2. Experimental section 2.1. Catalyst preparation The solid powder of beta zeolite (Si/Al = 20) in the protonic form (H+) was purchased from Catalyst Plant of Nankai University, Tianjin, China. The pseudo-boehmite was purchased from Shandong Aluminum Inc., China. Sesbania powder was provided by Sinopec Research Institute of Petroleum Processing. DMM, TRIox, MeOH, nitric acid, citric acid, and tetrahydrofuran (THF) were purchased from Aladdin Industrial Inc., Shanghai, China. First, the beta zeolite was mechanically mixed with pseudo-boehmite (as the binder) at a

ro of

mass ratio of 2:3, followed by addition of an appropriate amount of sesbania powder (as the extrusion assistant) and nitric acid solution (as the peptizer agent, 5 ωt%), resulting in the dough-like mixture. Then, the mixture was extruded into cylindrical shape, with a diameter of 1.5 mm, then dried at 110 °C for 10 h and calcined at 550 °C for 4 h, and finally milled into 20 ~ 40

-p

mesh particles with the geometric mean diameter of 0.63 mm, which is denoted as BEA (adopted as the blank sample in this experiment). Subsequently, acid treatment process was achieved on

re

BEA via excessive impregnation method with the citric acid solution at 80 °C for 5 h, and the resulting samples were calcined at 550 °C for 4 h in air. According to different concentrations of

lP

the citric acid solution, the citric acid treatment catalysts were remarked as 0.3CT/BEA, 0.6CT/BEA, 0.9CT/BEA, 1.2CT/BEA, respectively.

na

2.2. Characterization

The X-ray diffraction (XRD) (Bruker D8 Advance X-ray diffractometer, DE) was employed to measure the crystalline structure of the catalyst. The catalyst was irradiated

ur

using Cu Kα (λ = 0.15406 nm) radiation with a Nickel filter at a tube voltage of 40 kV and a tube current of 30 mA. The catalyst scanning was carried out over a 2θ range from 5 ° to

Jo

80 ° at a scanning rate of 4 °C·min-1. The crystalline phases were identified by matching them with reference patterns in the Joint Committee on Powder Diffraction Standards (JCPDS) database. The Brunauer-Emmett-Teller (BET) specific surface area and porosity texture of catalyst were analyzed by N2 physisorption at -196 °C (Micromeritics ASAP2400 static nitrogen adsorption analyser, US). Before measurement, the catalyst was degassed under high vacuum at 300 °C for 8 h. The specific surface area was calculated using the BET method in the relative pressure range of 0 < p/p0 < 0.1. The total pore

volume and the pore size distributions were obtained in the desorption branch and using the Barrett-Joyner-Halenda (BJH) method at a relative pressure (p/p0) of 0.99. Si, Al Elemental analyses were carried out with an Axios MAX XRF analyser (PANalytical B.V., NL). The transmission electron microscopy (TEM) images were obtained using Tecnai G2 F20 field emission transmission electron microscope (FETEM) (FEI company, US). Before measurements, the samples were suspended in ethanol and sonicated for 10 min, then placed over a carbon coated holey Cu microgrid. The ammonia temperature programmed adsorption-desorption

(NH3-TPD)

experiments

were

carried

out

on

the

US

ro of

Autosorb-1C-TCD-MS chemisorption analytical instrument with argon used as the carrier gas. Firstly, the catalyst was loaded in a quartz reactor and preheated in high purity argon

(30 mL·min-1) at 550 °C for 1 h to remove the adsorbed moisture. After that, it was

saturated with NH3 at 100 °C for 1 h before removing the physically adsorbed NH3 by

-p

argon flow. Finally, the desorption operation was carried out from 100 °C to 600 °C at a

heating rate of 10 °C·min-1. The amount of NH3 desorbed was monitored by a mass

re

spectrometer. Pyridine and 2,6-di-tert-butylpyridine (hereafter denoted dTBPy) were selected as probe molecules to analyze the acid types at different positions of the catalysts

lP

by Fourier transform infrared spectroscopy (US MAGNAIR-IR560 IR). Prior to FTIR studies，the powder samples were pressed into a self-supported wafer and pretreated at

na

450 °C for 1.0 h under high vacuum, then the excess of probe molecular vapor was adsorbed for 1.0 h and physisorbed molecules were subsequently removed by evacuation, finally subjected to high vacuum desorption at respectively 200 °C and 350 °C for 1.0 h before

ur

recording the spectra. The FTIR spectra were recorded between 4000 cm-1 and 1000 cm-1, where the resolution was 4 cm-1 for Py-FTIR and resolution was 2 cm-1 for dTBPy-IR.

Jo

High-resolution 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR) spectra in the solid state were recorded on a Bruker AVANCE III 600 spectrometer at a resonance frequency of 156.4 MHz using a 4 mm HX double-resonance MAS probe. The spectra were obtained at a rotation speed of 15 kHz and a 1 s recycle delay. The

27

Al chemical shift was

referenced to 1 M aqueous Al(NO3)3. Solid-state 29Si DD/MAS NMR spectra were recorded on an Agilent 600 DD2 spectrometer (magnetic field strength 14.1 T) at a resonance frequency of 109.13 MHz for

29

Si using the dipole decoupling magic-angle spinning (DD/MAS). The powder

samples were placed in a triple-channel probe using 4.0 mm ZrO2 rotors at room temperature. The spectra were obtained at a rotation speed of 8 kHz, and a 10 s recycle delay. The scanning number was 5000. The Si signal of tetramethylsilane at 0 ppm was used as the reference of 29Si chemical shift. 2.3. Catalytic activity tests PODEn was synthesized by DMM and TRIox in a 0.25 L autoclave reactor at 50 °C under autogenic pressure, with stirring speed was set to 400 rpm to eliminate the external mass transfer resistance. In the synthesis process, the DMM/TRIox molar ratio was 2:1, and catalyst loading

ro of

was 0.2 ~ 2 ωt%. The reaction equation is shown in Eqs. (1).

（1）

-p

The product distribution was quantified off-line using an SH-Rxi-5Sil MS capillary

column (30 m × 0.25 mm × 0.25 µm, SHIMADZU GCMS-QP2020, JP) connected to a

re

flame ionization detector, and identified by GC/MS analysis. Initially, the column temperature was held at 40 °C for 5 min, then ramped to 250 °C at a rate of 30 °C·min -1

lP

and held for 10 min. Nitrogen was used as the carrier gas, and THF was used as standard internal substance. In addition, trace formaldehyde in the product was quantitatively analyzed by sodium sulfite titration method using thymolphthalein as the indicator.

na

The activity evaluation indexes of catalysts include the conversion of trioxane ( X TRIox ), the selectivity of PODEj-k ( SPODE j k ) and the yield of PODEj-k ( YPODE j k ), which

Jo

ur

were calculated by Eqs. (2) ~ (4) as follows: X TRIox 

mTRIox ,feed  mTRIox ,product mTRIox ,feed

100%

（2）

k

   i

SPODE j k 

i= j

mTRIox，feed  mTRIox，product

100%

YPODE j k  X TRIox  SPODE j k 100%

（3） （4）

Where mTRIox , feed and mTRIox , product are the mass of trioxane in the feedstock and product (g), respectively, and [αi] is mass of trioxane consumed to produce PODEi (2≤i≤8, i ∈N). 3. Results and discussion 3.1. Physicochemical characteristics The XRD patterns show (Fig. 1) the main diffraction peaks of samples was presented at 2θ of 7.60 °, 13.4 °, 14.4 °, 21.2 ° and 22.4 °, at which in the XRD peak range of beta zeolite (JCPDS PDF-48-0074). Meanwhile, the characteristic diffraction peaks attributed to γ-Al2O3 are located at

ro of

2θ of 37.63 °, 39.52 °, 45.84 ° and 66.82 ° (JCPDS PDF-29-0063). Hence, based on the XRD results, the crystal phases of BEA and γ-Al2O3 could be seen in the spectra, but no additional

diffraction peak appeared. The results indicated that the citric acid treatment had no destructive

-p

effect on the crystal structure of beta zeolite, and the intensity of characteristic peaks increased.

On the one hand, it may be because citric acid causes elution of extra-framework aluminum and

re

amorphous substances in the pores during the acid pickling process. On the other hand, it is possible that some of the removed aluminum species are replenished to the silicon hydroxyl



lP

structure cavity.

 beta γ-Al2O3







 

na



1.2CT/BEA



ur

Intensity (a.u.)

0.9CT/BEA

Jo

0.6CT/BEA

0.3CT/BEA

BEA 10

20

Fig. 1.

30

40

2θ (°)

50

60

XRD patterns of different samples.

70

2.0

B

1.5

dV/dlog(D)

Quanitity adsorbed (cm3·g-1)

A

BEA 0.3CT/BEA 0.6CT/BEA 0.9CT/BEA 1.2CT/BEA

1.0

0.5

0.0 0

10

20

30

40

0.0

0.2

0.4

0.6

Relative pressure (p/p0)

ro of

Pore size (nm)

0.8

1.0

Fig. 2. Nitrogen adsorption-desorption isotherms (A) and pore size distribution (B) of different samples.

-p

The textural properties such as BET surface area, pore volume, and pore size of catalysts are

shown in Table 2. N2 adsorption-desorption isotherms and pore size distribution of different

re

samples are shown in Fig. 2. The desorption isotherm curves of all samples increase slowly in the low-specific pressure zone, showing hysteresis loops in the medium-specific pressure zone, which

lP

suggests all samples have microporous and mesoporous structures simultaneously. This is due to that catalysts were obtained by mixing beta zeolite with pseudo-boehmite followed by extrusion

na

molding, and the pseudo-boehmite was converted into γ-Al2O3 after calcination, so the microporous and mesoporous structures exist simultaneously in the catalysts. In addition, it can be

ur

seen that with the increase of citric acid treatment concentration, the samples have gradually increased adsorption amount in the high specific pressure zone, the hysteresis loop is regularly

Jo

narrowed layer by layer, the adsorption curve tends to be parallel with the desorption curve, and the hysteresis loop shape transits from the initial H2 type to H1 type (IUPAC classification). The above results indicate that citric acid treatment can increase pore volume and pore size and significantly widen the pore distribution. The increase of DBJH from 6.13 nm in the blank sample to 10.75 nm (Table 2) confirms significant pore-expanding effect of citric acid treatment. The citric acid treatment exerts an impact on pore structure from three main aspects: (1) dealumination of γ-Al2O3; (2) elution of some extra-framework aluminum and amorphous substances in beta

zeolite; (3) dealumination and realumination of beta zeolite [42]. Table 2 Textural properties of different samples. Specific area (m2·g-1)

Pore volume (cm3·g-1)

Pore size (nm)

Samples a

b

SBET

c

Smicro

d

Sexter

VTotal

e

VBJH

f

Daverage

g

DBJH

381.5

157.7

223.8

0.427

0.343

4.47

6.13

0.3CT/BEA

401.4

173.0

228.4

0.567

0.481

5.65

8.07

0.6CT/BEA

412.6

188.9

223.7

0.640

0.543

6.20

9.71

0.9CT/BEA

415.1

207.1

207.4

0.667

0.562

6.43

10.84

1.2CT/BEA

418.7

215.4

203.3

0.680

0.577

6.50

10.75

a

The BET specific surface area of catalyst,

catalyst,

d

b

The total pore volume of catalyst,

ro of

BEA

The microporous specific surface area of catalyst, c The external specific surface area of e

The mesoporous pore volume of catalyst,

The average pore size of catalyst,

g

The

-p

mesoporous pore size of catalyst.

f

As shown in the FETEM images of pure beta zeolite (Fig. 3a) and pure γ-Al2O3 (Fig. 3b), the

re

pure γ-Al2O3 shows a typical layered or wrinkled laminated structure, while pure beta zeolite is made up of larger particles formed from aggregated particles with diameters of 20 ~ 50 nm. It was

lP

worth noting that there were obvious light contrast areas in Fig. 3d, which indicated that there

Jo

ur

na

were a large number of intracrystal mesopores on the sample treated with citric acid.

ro of -p re lP

FETEM images of beta zeolite (a), γ-Al2O3 (b), BEA (c) and 0.6CT/BEA (d).

na

Fig. 3.

The changes in the local structure of the Si atoms and the Al atoms of samples modified by citric acid were characterized by

29

Si MAS NMR and

27

Al MAS NMR spectroscopy. A total of

ur

five peaks were detected in 29Si MAS NMR spectra (Fig. S1A), which were assigned to Q4 (-117 ppm and -114 ppm), Q3 (-107 ppm and -111 ppm) and Q2 (-98 ppm) sites, respectively [43].

Jo

Among them, the two peaks for Q4 sites came from two different stacking orders of polymorph A (Si(0Al)A) and polymorph B (Si(0Al)B), respectively. The two peaks of Q3 were attributed to silanol groups defect sites (Si-OH) and Si(1Al) tetrahedrons, respectively. Q2 site was assigned to Si(2Al) tetrahedrons. As the catalyst was prepared by mixing beta zeolite with pseudo-boehmite, the

27

Al MAS

NMR spectra (Fig. S1B) contained the overlap of peaks, which was relatively complex. According to the

27

Al MAS NMR spectrum of pure beta zeolite (Fig. S2) and literature [44][45], it can be

known that the peak at 68 ppm was attributed to the superposition of tetrahedral aluminum of alumina (AlⅣ) and ordered extra-framework tetrahedral aluminum of beta zeolite (AlT1). A hump within the range from 20 to 50 ppm was due to the superposition of the amorphous Penta-coordinated aluminum of alumina (AlⅤ) and the distorted extra-framework tetrahedral aluminum of beta zeolite (AlT4). The two peaks at 50 ~ 60 ppm were attributed to the framework tetrahedral aluminum occupying T1 and T2 sites (AlT3) and the framework tetrahedral aluminum occupying T3 ~ T9 sites (AlT2), respectively. Meanwhile, a broad peak between -20 ppm and 20 ppm was ascribed to the superposition of octahedral aluminum of alumina (AlⅥ, 11 ppm),

ppm) of beta zeolite. Table 3

ro of

well-ordered octahedral aluminum (AlO1, 0 ppm) and distorted octahedral aluminum (AlO2, -14

Percentage of the peak areas determined by deconvolution of the 29Si MAS NMR spectra. Si(0Al)B (%)

Si(1Al)

BEA

38.88

23.78

21.08

0.3CT/BEA

34.18

32.25

22.44

0.6CT/BEA

28.48

42.42

0.9CT/BEA

35.07

31.34

1.2CT/BEA

43.36

Si-OH

nSi/nAl

0.77

17.7

11.13

0

17.8

20.66

8.45

0

19.4

19.76

13.83

0

20.2

18.67

8.98

0

21.4

re

15.48

lP 29.00

a

Si(2Al)

-p

Si(0Al)A (%)

The ratio of silicon atoms to aluminum atoms on the framework, which is obtained by the calculation formula in literature [46].

na

a

Samples

Table 4

ur

Percentage of the peak areas determined by deconvolution of the 27Al MAS NMR Spectra. Relative peak areas (%) a

Samples

Ⅳ

Jo

Al

Al

Ⅴ

Al

Ⅵ

AlT2

AlT3

AlO1

AlO2

SiO2 (ωt%)

a

Al2O3 (ωt%)

BEA

23.89

6.47

31.80

4.66

3.82

15.71

13.65

38.93

59.70

0.3CT/BEA

23.78

6.18

30.18

5.02

4.44

16.15

14.55

46.24

53.21

0.6CT/BEA

23.53

6.82

29.76

4.98

5.33

15.73

13.84

47.93

51.37

0.9CT/BEA

22.90

6.78

29.28

4.23

5.98

16.12

14.71

50.81

48.55

1.2CT/BEA

22.68

6.87

29.97

4.15

6.52

15.96

13.85

52.48

46.31

a

The mass fraction of silica and alumina was obtained by XRF analysis.

The spectra of 29Si MAS NMR and 27Al MAS NMR spectroscopy were fitted with Gaussian

functions for quantitative deconvolution of overlapping peaks (Fig. S3), and the relative areas of these different peaks are shown in Table 3 and 4, respectively. After citric acid treatment, the Si(2Al) site signal on the catalyst was significantly weakened, but the Si(1Al) site signal increased first and then decreased, confirming that citric acid treatment had both framework dealumination and realumination effect on beta zeolite. Table 4 shows that citric acid treatment led to an increase in AlT3 signal of beta zeolite. On the one hand, it was probably due to that citric acid treatment reduced total aluminum content of the catalyst, while AlT3 site framework aluminum remained stable [44] and resulting in a relative increase of AlT3 content. On the other hand, it may be due to

ro of

that partial realumination occurred at T1 and T2 sites, resulting in an increased signal at AlT3 site. In addition, AlT2 site signal also increased first and then decreased, but decreasing rate was lower than that of total aluminum. According to Si/Al results in Table 3, it can be confirmed that the

framework realumination also occurred at this position.

-p

dealumination of beta zeolite framework probably occurred at T3 ~ T9 sites [42][44], and a part of

NH3-TPD was carried out to determine the relative strength and relative amount of acid sites,

re

in which the relative strength can be determined by desorption temperature position, and the area under desorption peak represents the relative amount of acid sites in the samples. Due to the

lP

uneven distribution of acid sites on the surface of alumina, the high-temperature desorption peak (Tm > 400 °C) of blank sample BEA was significantly widened (Fig. 4A). However, after citric

na

acid treatment, the peak position of each sample moved towards the low temperature, and the high-temperature peak shifted significantly to the position of the middle-temperature peak (200 °C < Tm < 400 °C). This indicates that citric acid treatment could adjust the acid distribution and

Jo

ur

increase the ratio of medium and strong acids.

Fig. 4.

NH3-TPD profiles (A) and Py-IR spectra (B) of different samples.

To further distinguish the type of acid sites on the catalyst under different acid strengths, the samples were analyzed by pyridine-infrared spectroscopy (Fig. 4B), and the calculation results of the analytical data are listed in Table 5. First, as the citric acid treatment concentration increased, the height of characteristic absorption peaks corresponding to Lewis acid sites (LAS) decreased significantly at 1444 cm-1, 1453 cm-1, 1600 cm-1, and 1620 cm-1, while height of characteristic absorption peak corresponding to BAS increased to some extent at 1544 cm-1 [39][47]. The blank sample has a relatively high amount of LAS and a low amount of BAS. However, with the increase of citric acid treatment concentration, the BAS to LAS ratio on both weak acid sites and

ro of

strong acid sites was significantly improved (Table S1). These results confirm that the citric acid treatment can significantly reduce amount of LAS while increasing amount of BAS. Secondly, a characteristic absorption peak corresponding to the BAS appeared near the wavenumber 1637 cm-1 [39], and the intensity of the characteristic peak increased first and then decreased with the

-p

increase of acid treatment concentration. This may be due to that the dealumination of citric acid occurred at the original weak acid sites (especially the weak LAS), which made the acid sites

re

covered or form Al(OH)2+ structure, producing many silanol structure cavities. Afterward, under the action of hydroxyl group in the citric acid molecule, realumination occurred in the silanol

lP

structure cavity, so that BAS was regenerated as Brønsted acid ratio increased [48]. However, dealumination was intensified under extra-high citric acid concentration, which would adversely

Table 5

na

affect the BAS and the LAS of the catalysts.

Amount of BAS in samples measured in quantitative experiments of Pyridine and dTBPy Sorption. a

PyH+ (μmol·g-1)

ur

Samples

c

a

d

S

e

T

c

dTBPyH+ (μmol·g-1)

W

d

S

f

e

Accessibility factor

T

dTBPyH+ / PyH+

24.8

58.1

82.9

9.0

11.3

20.3

0.245

0.3CT/BEA

26.1

62.2

88.3

12.1

12.2

24.3

0.275

0.6CT/BEA

35.9

73.0

108.9

15.3

15.2

30.5

0.280

0.9CT/BEA

31.5

84.3

115.8

13.4

14.3

27.7

0.240

1.2CT/BEA

23.8

88.0

111.8

10.0

13.0

23.0

0.201

Jo

BEA

W

b

PyH+ means amount of BAS (1544 cm-1) by pyridine sorption [49], b dTBPyH+ means amount of external BAS (1610 cm-1) by dTBPy

sorption [50], c W means relatively weak acid sites (The desorption temperature of probe molecules is between 200 ~ 350 °C), d S means strong acid sites, (The desorption temperature of probe molecules greater than 350 °C), e T means total acid sites, f Accessibility factor means the total amount of external BAS divided by the total amount of BAS.

ro of -p re

Fig.5.

Spectra of adsorbed dTBPy on different samples

lP

The external acidity was also detected through the dTBPy-FTIR spectra. The 1610 cm−1 band of the dTBPyH+ could be used as adequate for characterizing external BAS (Fig. 5) [50]. Results showed that compared with the sample BEA, with the increase of citric acid concentration, the

na

amount of external BAS exhibited a trend of increasing first and then decreasing, where the amount of external BAS of sample 0.6 CT/BEA was the largest. This may be due to that citric acid

ur

had both "dealumination - realumination" effect and "pore-expanding" effect on the catalyst. However, an excessive high concentration of citric acid can lead to dealumination, making the

Jo

negative effect on external acidity overpower the positive pore-expanding effect. Based on the results of Py-IR, it was found that the accessibility factor of the catalyst also exhibited a trend of increasing first and then decreasing (Table 5), confirming that sample 0.6CT/BEA might have a relatively higher utilization rate of BAS than other samples. It was worth noting that, even if the concentration of citric acid reached 1.2 M, the amount of external BAS was still higher than that of the sample BEA, indicating that the external framework aluminum content of the sample 1.2CT/BEA was still higher than that of the sample BEA. However,

29

Si MAS NMR spectra

results had confirmed that the framework aluminum content of sample 1.2CT/BEA was actually lower than that of sample BEA (Table 3). Therefore, we speculated that the citric acid treatment can transform the partially inaccessible aluminum in beta zeolite into accessible aluminum (external surface aluminum), which also indirectly confirms that citric acid does have a pore-expanding effect on beta zeolite.

Fig. 6.

-p

ro of

3.2. Catalytic activity and initial reaction pathway

Catalytic performances of different samples (A) (50 °C; 30 min; nDMM/nTRIox: 2; 0.2 ωt% catalyst) and effect of reaction time on trioxane concentration (B) (50 °C, nDMM/nTRIox: 2; 0.2 ωt% catalyst).

re

All samples were evaluated in synthesis of PODEn using DMM and TRIox as reactants, and their catalytic performances are shown in Fig. 6A. The results show that citric acid concentration

lP

has a significant effect on the conversion of trioxane and yield of PODE3-5. For blank sample BEA, the conversion of trioxane and yield of PODE3-5 were only 26.96 % and 9.63 %,

na

respectively, and the product PODE5 was not detected. However, the conversion and yield increased rapidly with the increase of citric acid concentration, and a small amount of PODE6-8

ur

was detected. XTRIox and YPODE35 of 0.6CT/BEA reached 50.29 % and 24.69 %, which were increased by 86.5% and 156%, respectively. Fig. 6B shows that the concentration of trioxane

Jo

gradually decreased with the extension of reaction time for all samples. Through comparison of these curves, it can be found that the initial reaction rate of sample 0.6CT/BEA was double faster than that of the blank sample. However, a higher concentration of citric acid treatment will result in a simultaneous decrease in conversion, yield, and initial reaction rate. For heterogeneous catalytic reactions, the catalytic performances of samples are usually affected under the joint influence of textural and acidic properties [51][52].

Fig. 7.

Correlation between mesoporous pore size and conversion of trioxane (A) and correlation between

ro of

amount of external BAS and initial reaction rate (B).

It was worth noting that when citric acid concentration was low, the conversion of TRIox was positively correlated with the mesoporous pore size (DBJH) of samples (Fig. 7A). However, when

citric acid concentration increased to 0.9 M, the mesoporous pore size remained unchanged after

-p

reaching the maximum, but the conversion of trioxane continued to decrease. This is probably due

to that microporous size of beta zeolite in the blank sample is relatively small, which makes low

re

accessibility of active sites, resulting in lower conversion and yield. The accessibility of active sites can affect the overall reaction rate, and the presence of a larger pore size is of critical

lP

significance to improve the accessibility of active sites. Therefore, the pore-expanding effect is very favorable for accelerating mass transfer efficiency of macromolecules. However, mass transfer effect was weakened with the increase of citric acid concentration, then the acidic

na

property played a decisive role in the catalytic activity. In this case, the catalytic activity decreased with the decrease in the amount of external BAS (Fig. 7B).

ur

Previous experiments have confirmed that the pure γ-Al2O3 has no catalytic activity for PODEn synthesis (Table S2), which is consistent with the conclusions of Wu et al. [30], indicating

Jo

that the active sites of samples are mainly derived from beta zeolite. It was found that the initial reaction rate increased with the increase of external protonic sites. Especially, there was a positive correlation between the amount of external weak BAS and the initial reaction rate for all samples other than the blank sample (Fig. 7B), which was probably due to that the blank sample was greatly affected by mass transfer restrictions in alumina. Therefore, it is speculated that weak BAS on the beta zeolite may be the primary active sites in the reaction. In addition, lots of studies reveal that in the catalytic reaction with the BAS as the main active sites, LAS usually has partial

synergistic catalysis [53], and Baranowski et al. [54] have confirmed LAS are active in paraformaldehyde decomposition, PODEn growth, and acetalization. However, whether LAS has the same synergistic effect in this study, which should be further confirmed by subsequent studies. 100

A

80

0.6CT/BEA 0.9CT/BEA 1.2CT/BEA

B

1.20

TOF (s-1)

40

1.15

1.10

20

1.05 0.8

0

1.0

1.2

1.4

Mass fraction of catalyst (t%) 0.0

0.4

0.8

1.2

Mass fraction of catalyst (t%)

1.6

Effect of catalyst addition amount on trioxane conversion (A) and TOF (B).

-p

Fig. 8.

ro of

XTRIox (%)

1.25

60

The turnover frequency (TOF) can be used to reflect the level of catalyst activity.

re

Considering the combined effect of textural and acidic properties, the value of TOF was calculated by Eqs. (5) and (6), as shown in Fig. 6. The results showed that the simultaneous reduction of

lP

external specific surface area and framework aluminum content resulted in decreased TOF value when the citric acid concentration exceeded 0.6 M. Then, the effects of catalyst addition amount on conversion of trioxane and TOF were investigated by changing the addition amount of

na

0.6CT/BEA, 0.9CT/BEA, and 1.2CT/BEA (Fig. 8). It was confirmed that the three catalysts could reach equilibrium conversion under increased addition (Fig. 8A), and a good linear relationship

Jo

8B).

ur

existed between TOF and catalyst addition amount when the reaction reached equilibrium (Fig.

nTRIox，feed  X TRIox mcat  CAl  t

（5）

   1 M   Sexter  Al2 O3 2 CAl  2     M Al2O3   S n BET  1 2  M Al2O3  M SiO2  Si n   Al   

（6）

TOF=

Where nTRIox , feed is mole mass of trioxane in the feedstock (mol), mcat is mass of catalysts (g), CAl is the molar content of the outer surface framework aluminum of the beta zeolite per unit mass (mol·g -1), t is reaction time (s), M Al O is the molar mass of Al2O3 2

(g·mol-1), M SiO

2

3

is the molar mass of SiO2 (g·mol-1), nSi n is the number ratio of silicon atoms to aluminum atoms in the zeolite Al

framework.

The catalyzed synthesis of PODEn using DMM and TRIox as materials in anhydrous environment belongs to a typical acid catalysis reaction. The reaction pathway mainly consists of two parts, i.e. the initial reaction part and the chain growth part [19]. Different opinions have been proposed for the initial reaction part. Some people believe that TRIox is first decomposed into three formaldehyde monomers [CH2O] by acid catalysis, and then inserted into DMM and/or a lower PODEn in form for chain growth, resulting in a higher PODEn [30],[55]. However, Goncalves et al. [56] based on the DFT theory proposed that in the initial stage of the reaction,

ro of

dimethoxymethane needs to remove methanol under the action of acid center to form [Al–O–CH2O–Me] intermediate, and then TRIox can be inserted into this intermediate as a whole

or monomers [CH2O] after decomposition. There is a competitive relationship between the two insertion approaches, in which the former is relatively more advantageous in the initial stage of

-p

reaction or when the amount of acid sites is low. Using kinetic research method, Brice et al. [57] inferred that TRIox would not decompose at low temperatures and low amount of acid sites.

re

To explore the initial reaction pathway of PODEn, the study on whether ring-opening decomposition of trioxane would occur in the absence of DMM was carried out (Table S2). The

lP

presence of formaldehyde was not detected in the product, which suggests that under such experimental conditions, the ring-opening decomposition of trioxane may not necessarily be the initial step for PODEn synthesis. Through experiment, the presence of methanol and PODE4 was

na

also observed in the product even in very short reaction time (Table S3). This experimental phenomenon is not consistent with the pathway of "the first decomposition and then insertion" as

ur

was proposed by most people, in line with the "trioxane direct insertion" pathway proposed by Goncalves [56]. Therefore, the initial reaction pathway of PODEn synthesis with as materials of

Jo

DMM and TRIox using 0.6CT/BEA as catalyst under the experimental conditions herein can be described as follows: (1) dimethoxymethane releases methanol upon coordination with a BAS, forming [Al–O–CH2O–Me] intermediate; (2) TRIox is directly inserted into [Al–O–CH2O–Me] intermediate and rapidly releases [CH2O] to form [Al–O ( CH2O ) n–Me] intermediates; (3) these [Al–O ( CH2O ) n–Me] intermediates are combined with methanol to form PODE2-4, respectively. Scheme 2 shows the possible initial reaction pathway for the synthesis of PODEn using DMM and TRIox as materials.

ro of

Scheme 2.

Suggested initial reaction pathway for the synthesis of PODEn from DMM and TRIox.

4. Conclusion

-p

In this work, the textural and acidic properties of catalysts were regulated by citric acid treatment, and then they were used to catalyze the synthesis of PODEn using DMM and TRIox as

re

materials. Citric acid treatment has a dual-function of expanding pore size and regulating acid distribution, so it can effectively improve the mass transfer and diffusion ability of

lP

macromolecules and the external acidity, resulting in an accelerated initial reaction rate. However, the effect of textural properties on the catalytic activity will gradually decrease with the increase

na

of citric acid concentration. To this end, the acidic property, especially the external protonic sites, becomes the most important factor affecting the overall catalytic activity. A good positive correlation was found between amount of external weak BAS and initial reaction rate, indicating

ur

that weak BAS may be the primary active sites in this reaction. Using 0.6CT/BEA as catalyst and set the molar ratio of dimethoxymethane to trioxane to 2:1, reaction temperature to 50 °C and

Jo

reaction time to 30 min, the conversion of trioxane and the yield of PODE3-5 reached 50.29 % and 24.69 %, respectively. Compared with the blank sample, the initial reaction rate of optimal catalyst increased by 100%, the conversion of trioxane increased by 86.5 %, indicating excellent catalytic activity. In addition, the study also found that under the experimental conditions herein, the ring opening decomposition of trioxane may not necessarily be the initial step for PODEn synthesis reaction.

Credit Author Statement Baoyu Wang:

Conceptualization, Methodology, Investigation, Data curation, Writing- Original draft preparation.

Ximing Yan:

Validation, Writing- Reviewing and Editing, Funding acquisition.

Xingyuan Zhang: Haiyang Zhang:

Formal analysis.

Resources.

ro of

Faping Li:

Supervision, Project administration.

-p

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

re

Acknowledgments

The authors are grateful to the Natural Science Foundation of Guangxi Zhuang Autonomous

lP

Region (2017GXNSFBA198145) and the Foundation of Education Department of Guangxi

Jo

ur

na

Zhuang Autonomous Region (2019KY0457) for financial support.

Reference [1]

A.C. Lewis, D.C. Carslaw, F.J. Kelly, Vehicle emissions: Diesel pollution long under-reported, Nature 526 (2015) 195.

[2]

S.C. Anenberg, J. Miller, R. Minjares, L. Du, D.K. Henze, F. Lacey, C.S. Malley, L. Emberson, V. Franco, Z. Klimont, C. Heyes, Impacts and mitigation of excess diesel-related NOx emissions in 11 major vehicle markets, Nature 545 (2017) 467-471.

[3]

J. Lee, J.R. Theis, E.A. Kyriakidou, Vehicle emissions trapping materials: successes, challenges, and the path forward, Appl. Catal. B-Environ. 243 (2018) 397-414. P.S. de Caro, Z. Mouloungui, G. Vaitilingom, J.C. Berge, Interest of combining an additive with diesel–ethanol blends for use in diesel engines, Fuel 80 (2001) 565-574.

[5]

M.M. Roy, J. Calder, W. Wang, A. Mangad, F.C.M. Diniz, Cold start idle emissions from a modern Tier-4 turbo-charged

diesel

engine

fueled

with

diesel-biodiesel,

diesel-biodiesel-ethanol,

and

-p

diesel-biodiesel-diethyl ether blends, Appl. energy 180 (2016) 52-65. [6]

ro of

[4]

S.E. Iannuzzi, C. Barro, K. Boulouchos, J. Burger, Combustion behavior and soot formation/oxidation of

[7]

re

oxygenated fuels in a cylindrical constant volume chamber, Fuel 167 (2016) 49-59.

H. Liu, Z. Wang, J. Wang, X. He., Y. Zheng, Q. Tang, J. Wang, Performance, combustion and emission

Energy 88 (2015) 793-800.

L. Lautenschütz, D. Oestreich, P. Seidenspinner, U. Arnold, E. Dinjus, J. Sauer, Physico-chemical properties

na

[8]

lP

characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/diesel blends,

and fuel characteristics of oxymethylene dialkyl ethers, Fuel 173 (2016) 129-137. [9]

J. Burger, M. Siegert, E. Ströfer, H. Hasse, Poly(oxymethylene) dimethyl ethers as components of tailored

ur

diesel fuel: Properties, synthesis and purification concepts, Fuel 89 (2010) 3315-3319. [10] J. Liu, P. Sun, H. Huang, J. Meng, X. Yao, Experimental investigation on performance, combustion and

Jo

emission characteristics of a common-rail diesel engine fueled with polyoxymethylene dimethyl ethers-diesel blends, Appl. Energy 202 (2017) 527-536.

[11] D. Wang, G. Zhu, Z. Li, C. Xia, Polyoxymethylene dimethyl ethers as clean diesel additives: Fuel freezing and prediction, Fuel 237 (2019) 833-839. [12] Q. Lin, K.L. Tay, D. Zhou, W. Yang, Development of a compact and robust polyoxymethylene dimethyl ether 3 reaction mechanism for internal combustion engines, Energy Conv. Manag. 185 (2019) 35-43. [13] Z. Wang, H. Liu, X. Ma., J. Wang, S. Shuai, R.D. Reitz, Homogeneous charge compression ignition (HCCI)

combustion of polyoxymethylene dimethyl ethers (PODE), Fuel 183 (2016) 206-213. [14] X. Meng, E. Hu, K.H. Yoo, A.L. Boehman, Z. Huang, Experimental and numerical study on autoignition characteristics of the polyoxymethylene dimethyl ether/diesel blends, Energy Fuels 33 (2019) 2538-2546. [15] H. Chen, J. He, H. Hua, Investigation on combustion and emission performance of a common rail diesel engine fueled with diesel/biodiesel/polyoxymethylene dimethyl ethers blends, Energy Fuels 31 (2017) 11710-11722. [16] J. Liu, J. Yang, P. Sun, W. Gao, C. Yang, J. Fang, Compound combustion and pollutant emissions characteristics of a common-rail engine with ethanol homogeneous charge and polyoxymethylene dimethyl

ro of

ethers injection, Appl. Energy 239 (2019) 1154-1162. [17] H. Huang, Q. Liu, W. Teng, M. Pan, C. Liu, Q. Wang, Improvement of combustion performance and emissions in diesel engines by fueling n-butanol/diesel/PODE3-4 mixtures, Appl. Energy 227 (2018) 38-48.

[18] H. Liu, Z. Wang, Y. Li, Y. Zheng, T. He, J. Wang, Recent progress in the application in compression ignition

-p

engines and the synthesis technologies of polyoxymethylene dimethyl ethers, Appl. Energy 233 (2019) 599-611.

re

[19] C.J. Baranowski, A.M. Bahmanpour, O. Kröcher, Catalytic synthesis of polyoxymethylene dimethyl ethers (OME): A review, Appl. Catal. B-Environ. 217 (2017) 407-420.

lP

[20] N. Schmitz, J. Burger, E. Ströfer, H. Hasse, From methanol to the oxygenated diesel fuel poly(oxymethylene) dimethyl ether: An assessment of the production costs, Fuel 185 (2016) 67-72.

na

[21] H. Li, H. Song, L. Chen, C. Xia, Designed SO42−/Fe2O3-SiO2 solid acids for polyoxymethylene dimethyl ethers synthesis: The acid sites control and reaction pathways, Appl. Catal. B-Environ. 165 (2015) 466-476. [22] R. Wang, Z. Wu, Z. Qin, C. Chen, H. Zhu, J. Wu, G. Chen, W. Fan, J. Wang, Graphene oxide: an effective

ur

acid catalyst for the synthesis of polyoxymethylene dimethyl ethers from methanol and trioxymethylene, Catal. Sci. Technol. 6 (2016) 993-997.

Jo

[23] D. Oestreich, L. Lautenschütz, U. Arnold, J. Sauer, Reaction kinetics and equilibrium parameters for the production of oxymethylene dimethyl ethers (OME) from methanol and formaldehyde, Chem. Eng. Sci. 163 (2017) 92-104.

[24] J. Burger, M. Siegert, E. Ströfer, H. Hasse, Chemical equilibrium and reaction kinetics of the heterogeneously catalyzed formation of poly (oxymethylene) dimethyl ethers from methylal and trioxane[J]. Ind. Eng. Chem. Res. 51 (2012) 12751-12761. [25] C.J. Baranowski, A.M. Bahmanpour, F. Héroguel, J.S. Luterbacher, O. Kröcher, Insights into the nature of

the active sites of Tin-Montmorillonite for the synthesis of polyoxymethylene dimethyl ethers (OME), ChemCatChem 11 (2019) 3010-3021. [26] J. Weitkamp, Zeolites and catalysis, Solid State Ion. 131 (2000) 175-188. [27] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts, Nature 461 (2009) 246-249. [28] M. Dusselier, P. Van Wouwe, A. Dewaele, P.A. Jacobs, B.F. Sels, Shape-selective zeolite catalysis for bioplastics production, Science 349 (2015) 78-80. [29] L. Lautenschütz, D. Oestreich, P. Haltenort, U. Arnold, E. Dinjus, J. Sauer, Efficient synthesis of

ro of

oxymethylene dimethyl ethers (OME) from dimethoxymethane and trioxane over zeolites, Fuel Process. Technol. 165 (2017) 27-33.

[30] J. Wu, H. Zhu, Z. Wu, Z. Qin, L. Yan, B. Du, W. Fan, J. Wang, High Si/Al ratio HZSM-5 zeolite: an efficient

catalyst for the synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxymethylene,

-p

Green Chem. 17 (2015) 2353-2357.

[31] C.J. Baranowski, A.M. Bahmanpour, F. Héroguel, J.S. Luterbacher, O. Kröcher, Prominent role of mesopore

re

surface area and external acid sites for the synthesis of polyoxymethylene dimethyl ethers (OME) on a hierarchical H-ZSM-5 zeolite, Catal. Sci. Technol. 9 (2019) 366-376

lP

[32] W.H. Fu, X.M. Liang, H. Zhang, Y.M. Wang, M.Y. He, Shape selectivity extending to ordered supermicroporous aluminosilicates, Chem. Commun. 51 (2015) 1449-1452.

na

[33] Z. Xue, H. Shang, Z. Zhang, C. Xiong, C. Lu, G. An, Efficient synthesis of polyoxymethylene dimethyl ethers on Al-SBA-15 catalysts with different si/al ratios and pore sizes, Energy Fuels 31 (2017) 279-286. [34] A.H. Janssen, A.J. Koster, K.P. de Jong, Three-dimensional transmission electron microscopic observations

ur

of mesopores in dealuminated zeolite Y, Angew. Chem. Int. Edit. 40 (2001) 1102–1104. [35] I. Graça, M.C. Bacariza, A. Fernandes, D. Chadwick, Desilicated NaY zeolites impregnated with magnesium

Jo

as catalysts for glucose isomerisation into fructose, Appl. Catal. B-Environ. 224 (2018) 660-670.

[36] J.C. Groen, W. Zhu, S. Brouwer, S.J. Huynink, F. Kaptejin, J.A. Moulijn, J. Pérez-Ramírez, Direct demonstration of enhanced diffusion in mesoporous ZSM-5 zeolite obtained via controlled desilication, J. Am. Chem. Soc. 129 (2007) 355-360. [37] M. Rutkowska, D. Macina, N. Mirocha-Kubień, Z. Piwowarska, L. Chmielarz, Hierarchically structured ZSM-5 obtained by desilication as new catalyst for DME synthesis from methanol, Appl. Catal. B-Environ. 174 (2015) 336-343.

[38] C. Selzer, T. Biemelt, A. Werner, S. Kaskel, Hierarchical zeolite ZSM-58 as shape selective catalyst for methanol-to-olefins reaction, Micropor. Mesopor. Mat. 261 (2018) 51-57. [39] W. Gac, M. Greluk, G. Słowik, Y. Millot, L. Valentin, S. Dzwigaj, Effects of dealumination on the performance of Ni-containing BEA catalysts in bioethanol steam reforming, Appl. Catal. B-Environ. 237 (2018) 94-109. [40] C. Chen, Z. Hu, J. Ren, S. Zhang, Z. Wang, Z.Y. Yuan, ZnO nanoclusters supported on dealuminated zeolite β as a novel catalyst for direct dehydrogenation of propane to propylene, ChemCatChem 11 (2019) 868-877. [41] J.C. Groen, J.C. Jansen, J.A. Moulijn, J. Pérez-Ramírez, Optimal aluminum-assisted mesoporosity

ro of

development in MFI zeolites by desilication, J. Phys. Chem. B 108 (2004) 13062-13065. [42] S.M. Maier, A. Jentys, J.A. Lercher, Steaming of zeolite BEA and its effect on acidity: A comparative NMR and IR spectroscopic study, J. Phys. Chem. C 115 (2011) 8005-8013. [43] J. Pérez-Pariente, J. Sanz, V. Fornés, A. Corma,

29Si

and

MAS NMR study of zeolite β with different

-p

Si/Al ratios, J. Catal. 124 (1990) 217-223.

27Al

[44] J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. van Bekkum, A.P.M. Kentgens, Stepwise

Chem. Soc. 122 (2000) 12842-12847.

re

dealumination of zeolite beta at specific T-sites observed with 27Al MAS and 27Al MQ MAS NMR, J. Am.

lP

[45] K. Góra-Marek, M. Derewiński, P. Sarv, J. Datka, IR and NMR studies of mesoporous alumina and related aluminosilicates, Catal. Today 101 (2005) 131-138.

na

[46] J. Klinowski, Solid-state NMR studies of molecular sieve catalysts, Chem. Rev. 91 (1991) 1459-1479. [47] G. Garbarino, R.P.P. Vijayakumar, P. Riani, E. Finocchio, G. Busca, Ethanol and diethyl ether catalytic conversion over commercial alumina and lanthanum-doped alumina: Reaction paths, catalyst structure and

ur

coking, Appl. Catal. B-Environ. 236 (2018) 490-500. [48] Z. Xie, J. Bao, Y. Yang, Q. Chen, C. Zhang, Effect of treatment with NaAlO2 solution on the surface acid

Jo

properties of zeolite β, J. Catal. 205 (2002) 58-66.

[49] C.A. Emeis, Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts, J. Catal. 141 (1993) 347-354.

[50] K. Góra-Marek, K. Tarach, M. Choi, 2,6-Di-tert-butylpyridine sorption approach to quantify the external acidity in hierarchical zeolites, J. Phys. Chem. C 118 (2014) 12266-12274. [51] Y. Liao, M. d’Halluin, E. Makshina, D. Verboekend, B.F. Sels, Shape selectivity vapor-phase conversion of lignin-derived 4-ethylphenol to phenol and ethylene over acidic aluminosilicates: Impact of acid properties

and pore constraint, Appl. Catal. B-Environ. 234 (2018) 117-129. [52] L.A. Bivona, A. Vivian, L. Fusaro, S. Fiorilli, C. Aprile, Design and catalytic applications of 1D tubular nanostructures: Improving efficiency in glycerol conversion, Appl. Catal. B-Environ. 247 (2019) 182-190. [53] S. Telalović, J.F. Ng, R. Maheswari, A. Ramanathan, G.K. Chuah, U. Hanefeld, Synergy between Brønsted acid sites and Lewis acid sites, Chem. Commun. 38 (2008) 4631-4633. [54] C.J. Baranowski, M. Roger, A.M. Bahmanpour, O. Kröcher, Nature of the synergy between Brønsted and Lewis acid sites in Sn-Beta zeolites for polyoxymethylene dimethyl ethers synthesis, ChemSusChem, 12 (2019) 4421-4431.

ro of

[55] F. Wang, G. Zhu, Z. Li, F. Zhao, C. Xia, J. Chen, Mechanistic study for the formation of polyoxymethylene dimethyl ethers promoted by sulfonic acid-functionalized ionic liquids. J. Mol. Catal. A-Chem. 408 (2015) 228-236.

[56] T.J. Goncalves, U. Arnold, P.N. Plessow, F. Studt, Theoretical investigation of the acid catalyzed formation

-p

of oxymethylene dimethyl ethers from trioxane and dimethoxymethane, ACS Catal. 7 (2017) 3615-3621.

[57] L.K. Brice, L.P. Lindsay, Temperature and salt effects on the rate of depolymerization of trioxane in

Jo

ur

na

lP

re

concentrated hydrochloric acid solutions, J. Am. Chem. Soc. 82 (1960) 3538-3540.