Reaction kinetics and equilibrium parameters for the production of oxymethylene dimethyl ethers (OME) from methanol and formaldehyde

Accepted Manuscript Reaction kinetics and equilibrium parameters for the production of oxymethylene dimethyl ethers (OME) from methanol and formaldehyde Dorian Oestreich, Ludger Lautenschütz, Ulrich Arnold, Jörg Sauer PII: DOI: Reference:

S0009-2509(16)30691-1 http://dx.doi.org/10.1016/j.ces.2016.12.037 CES 13309

To appear in:

Chemical Engineering Science

Received Date: Revised Date: Accepted Date:

29 June 2016 3 November 2016 16 December 2016

Please cite this article as: 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, Chemical Engineering Science (2016), doi: http://dx.doi.org/10.1016/j.ces.2016.12.037

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Reaction kinetics and equilibrium parameters for the production of oxymethylene dimethyl ethers (OME) from methanol and formaldehyde

Dorian Oestreich, Ludger Lautenschütz, Ulrich Arnold*, Jörg Sauer Karlsruhe Institute of Technology (KIT), Institute of Catalysis Research and Technology (IKFT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

Highlights: Highly active catalysts for OME production from methanol and formaldehyde. Equilibrium data as well as a kinetic model have been determined. Influence of methanol, water and OME on reaction kinetics has been investigated. Ion exchange resin catalysts show stable long term performance.

Keywords: Fuel additive, soot reduction, oxymethylene ether, kinetics, equilibrium, ion exchange resin

*Corresponding author. Tel.: +49 721 608 23694; fax: +49 721 608 22244. E-mail address: [email protected] (U. Arnold).

2

Abstract A catalyst screening comprising zeolites and ion exchange resins for the synthesis of oligomeric oxymethylene dimethyl ethers (OME) from methanol (MeOH) and formaldehyde (FA) has been carried out. All catalysts led to the same product spectrum and parameters for chemical equilibrium have been determined. The ion exchange resin Dowex50Wx2 showed highest activity and reaction kinetics has been investigated employing this catalyst. The influence of the FA:MeOH ratio and water as well as refeeding of OMEs with undesired chain lengths have been considered in the kinetic model, which is based on a hyperbolic approach. Experiments have been carried out in the temperature range between 40 and 120 °C and variable FA:MeOH ratios from 0.5 to 1.5 g/g have been employed. Regarding water, up to 23 wt.% have been added to the reaction mixtures to investigate its influence on yield and reaction rate. Low water contents lead to high OME selectivities. By varying the FA:MeOH ratio, chain lengths of the OMEs can be influenced. Regarding the most active catalyst Dowex50Wx2, 90% of equilibrium conversion is reached after 5 min at 60 °C employing a catalyst loading of 1 wt.%. A study on the long term performance of the catalyst has been carried out and after 17 days the decrease of activity was below 10% while selectivity remained the same. For different Dowex catalysts a general kinetic model could be developed, which is not limited to Dowex50Wx2.

3 Nomenclature Abbreviations DME

dimethyl ether

DMM

dimethoxy methane

Equi.

experiments to determine the equilibrium parameters

FA

formaldehyde

Gly

glycol

HF

hemiformal

Kin.

experiments to determine the kinetic parameters

MeOH

methanol

MF

methyl formate

OME

oxymethylene dimethyl ether

p-FA

para-formaldehyde

THF

tetrahydrofuran

Tri

trioxane

Symbols EA

activation energy (J mol-1)

c

concentration (mol l-1)

h

inhibiting factor (l mol-1)

HR

reaction enthalpy (J mol-1)

K

equilibrium constant

k

reaction rate constant ((l/mol)n-1 s-1)

k0

pre-exponential factor or collision frequency ((l/mol)n-1 s-1)

m

mass (g)

4 n

number of formaldehyde groups in OME_n, HF_n and Gly_n

R

rate of appearance (mol l-1 s-1)

r

rate of reaction (mol l-1 s-1)

T

Temperature (K)

wt.%

weight percent (g g-1 100%)

ν

stoichiometric coefficient

Greek letter βInh

inhibition term

Subscripts Cat

catalyst

i

component

j

reaction

s

sample

5

1 Introduction A reduction of soot, hydrocarbons and nitrogen oxides is essential to prevent further environmental pollution by vehicle exhaustion and thus, political provisions are developed to set emission standards for modern transit (e.g. EURO VI). To meet the forthcoming standards, conventional exhaust aftertreatment becomes more and more complex and reaches disproportional high costs. Therefore, fuel additives or alternative fuels are in great demand to overcome these problems. Diesel fuel additives, with oxygen in the molecular structure and without carbon-carbon bonds, lead to a significant reduction of soot formation (Lahaye et al., 1983; Ren et al., 2008). Recent investigations showed that dimethoxy methane (DMM) and oligomeric oxymethylene dimethyl ethers (OMEs, CH3-[O-CH2]n-O-CH3) are outstanding diesel additives for emission reduction (Vertin et al., 1999), which can be produced from methanol (MeOH) and its oxidation product formaldehyde (FA). Since MeOH can be obtained from renewable raw materials, not only reduction of CO2 emissions, but also reduction of NOx and particulate emissions can be achieved in terms of a holistic solution. In addition, production of OMEs from oxygen-containing raw materials is particularly efficient because oxygen can substantially remain in the product and must not be removed at considerable expense. This leads to a high atom and energy efficiency in the manufacturing process. Additional benefits are their unlimited miscibility with diesel, low corrosivity, negligible toxicity and physico- as well as chemical properties similar to diesel (Lautenschütz et al., 2016; Lumpp et al., 2011). Using such additives, it is not necessary to change engines and the fuel supply infrastructure. Furthermore, OMEs exhibit higher cetane numbers (CN) compared to diesel (Lautenschütz et al., 2016) and this can lead to a higher efficiency of the diesel engine. Regarding combustion, OMEs behave similar to dimethyl ether (DME), which is another promising diesel substitute (Fleisch and Sills, 2004; Ying et al., 2008) and which is related to OMEs (CH3O-[CH2O]n-CH3, n = 0). However, due to its low boiling point of 24.8 °C, DME is a liquid gas, which affords more sophisticated modifications of engines and infrastructures, compared to OMEs. Regarding chain length, OME oligomers with n = 3-5 are preferred as additives since vapor pressures and boiling points are almost identical to standard diesel fuel (Fleisch and Sills, 2004; Lumpp et al., 2011; Marchionna et al., 2001).

6 Production of OMEs can be carried out employing different educts like MeOH, DME, DMM and FA sources like p-formaldehyde (p-FA), trioxane (Tri) or formalin (Fig. 1). Different homogeneous and heterogeneous acidic catalysts such as sulfuric acid, zeolites, ion exchange resins, metal-oxides or heteropoly acids are typically used (Burger et al., 2012; Stanonis et al., 1972; Zhang et al., 2014a; Zhao et al., 2011).

1 DMM 2 MeOH

FA

Tri

OME 3

DME Fig. 1:

Synthesis pathways for oxymethylene dimethyl ethers (OMEs).

The main distinction between OME synthesis from MeOH and p-FA or formalin (Pathway 1, Fig. 1) and synthesis from DMM or DME with Tri (Pathways 2 and 3, Fig. 1) is the additional formation of byproducts such as water, hemiformals (HF) and glycols (Gly) in pathway 1. However, MeOH and p-FA are low-cost educts in comparison to Tri and thus, synthesis from these two educts can become economically favorable. The anhydrous route starting from DMM and Tri has been investigated in detail and optimized to a large extent (Burger and Hasse, 2013; Burger et al., 2012; Lautenschütz, 2015). In contrast, synthesis from MeOH and p-FA still offers a large potential for optimization. Regarding the entire process chain, OMEs can be produced from biomass via synthesis gas and this approach is described in (Arnold et al., 2015; Sauer et al., 2016). In this context, Zhang et al. reported a process design starting from woody biomass with a focus on the Canadian market (Zhang et al., 2016). Regarding the involved reactions (Fig. 2), MeOH reacts with FA to HF (2). The next step can be either an etherification with MeOH to OMEs (5) or chain growth by incorporation of further FA units to HF_n (3), where n refers to the number of CH2O units in the molecule. Gly

7 can be formed from the reaction of HF with water (1). Chain growth of Gly and OMEs occurs as well and is described by (4) and (6). Furthermore, three FA units can react to Tri and vice versa (8). All reactions, except (7), are equilibrium reactions. Methyl formate (MF), which is formed according to (7), is an additional byproduct and occurs in very low quantities below 1 wt.%. (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

Fig. 2:

Reactions for the production of OMEs from methanol and formaldehyde. (FA: formaldehyde, Gly: glycols, Tri: trioxane, HF: hemiformals/hemiacetals, OME: oxymethylene dimethyl ethers, MeOH: methanol).

8 Information on the kinetics of OME synthesis from MeOH and FA is very limited. Schmitz et al. studied reactions with an Amberlyst 46 catalyst employing a catalyst loading of 1.9 wt.%. At 90 °C and a FA:MeOH ratio of 0.86 g/g 90% of the chemical equilibrium conversion is reached within 18 min (Schmitz et al., 2015a). The kinetic model is based on an exponential approach and only one rate constant for reactions (5) and (6) is used, respectively. Additionally, transacetalization reactions of different OMEs (6) have been experimentally validated only for OME_1 and no higher OMEs have been involved as starting materials. Zheng et al. developed a kinetic model for the reaction of DMM with p-FA (Zheng et al., 2015). In their model, only chain growth is considered, neglecting etherification reactions, i.e. reactions of HF with MeOH. In a further study reported by Zhang et al., MeOH and FA have been used as educts but the kinetic model is solely based on the reaction of OME_1 with FA to longer-chain OMEs and only OME_1 is formed from MeOH and FA (Zhang et al., 2014b). Within this work, the kinetic models from (Zheng et al., 2015) and (Zhang et al., 2014b) are extended regarding educts like OME_1-3 and the involved etherification reactions (5). The role of all components, i.e. MeOH, FA, water and OMEs, is considered. Furthermore, it is assumed that etherification with MeOH occurs in the case of all hemiformals and not only in the formation of OME_1. Compared to (Schmitz et al., 2015a), all reactions are considered separately and different reaction rates, depending on the chain length are used. Furthermore, it is not assumed that the non-catalyzed reactions (1)-(4) are in equilibrium at any time. Regarding the catalyst, an ion exchange resin with higher activity than previously investigated ion exchange resins has been employed. Major objective of this work is the development of a reliable kinetic model that enables an accurate prediction of product spectra over a wide temperature range and for different reaction mixtures, comprising also water and OMEs. Reaction mixtures containing OMEs are considered, to study recycling strategies for OMEs with inappropriate chain lengths, regarding fuel applications.

9

2 Materials and methods 2.1 Materials The synthesis of OMEs was carried out in a stainless steel (1.4571) batch autoclave with a volume of 180 ml. Pressure and temperature was measured using an analog pressure gauge with an accuracy of 0.05 bar and a NiCr-Ni thermocouple with an accuracy of 1 K. A valve on the bottom of the reactor was used for taking liquid phase samples during the reaction. The valve was purged with the product before sampling. Samples were taken with a syringe, filtrated with a polytetrafluoroethylene filter (0.2 µm pore width) and quenched with THF (AnalaR NORMAPUR®, 99.8 %). A schematic setup of the autoclave is shown in Fig. 3. V-1 V-2

V-3

PI

TIC

V-4

Fig. 3:

Autoclave for the synthesis of OMEs from MeOH and FA. V-1 = educt feeding, V-2 = catalyst feeding, V-3 = overflow valve, V-4 = product removal.

Reactants are methanol (Merck EMSURE®) and p-formaldehyde (Merck). Nitrobenzene (Sigma-Aldrich) was used as internal standard, THF and 1-butanol (Merck EMSURE®) were used as solvents. Different acidic catalysts have been tested (Table 1).

10 Table 1: Catalysts for OME synthesis from MeOH and p-FA.

Catalyst

Catalyst type and structure

Pore width (Å)

SiO2:Al2O3 ratio

BET surface (m²/g)

Manufacturer

Dowex50Wx21

Ion exchange resin







Dow Chemical

Dowex50Wx42

Ion exchange resin







Dow Chemical

Dowex50Wx83

Ion exchange resin







Dow Chemical

Amberlyst 36

Ion exchange resin

240



33

Rohm & Haas

IR-120

Ion exchange resin







Sigma Aldrich

H-MFI 90

Zeolite, Pentasil

6

80-100

300

Clariant

H-BEA 25

Zeolite, Beta

7

25

500

Zeolyst International

CBV 720

Zeolite, Faujasite

24

30

780

Zeolyst International

H-MFI 400

Zeolite, Pentasil

6

400

300

Clariant

H-MOR 30

Zeolite, Mordenite

7

30

400

Clariant

1

Resin crosslinked with 2% divinylbenzene.

2

Resin crosslinked with 4% divinylbenzene.

3

Resin crosslinked with 8% divinylbenzene.

2.2 Methods 2.2.1 Analysis An Agilent 6890N gas chromatograph with an Agilent DB-5MS column (length: 30 m, diameter: 0.25 mm, film: 0.25 µm) coupled with a flame ionization detector (FID) was used for the qualitative and quantitative analysis of the liquid samples. Synthesized and purified OMEs were utilized for the calibration. The relative error for methanol and methylal is less than 5%, and for OME_2-5 less than 3%. The response factors for OMEs with n > 5 were derived from those for OMEs with n < 5 by extrapolation with an assumed error of 5%. The sodium sulfite method was used to determine formaldehyde concentrations (Walker, 1944)

11 employing a Metrohm 682 Titroprocessor and a 665 Dosimat titrator. Water contents were determined by Karl Fischer titration employing a V30 Compact Volumetric KF Titrator from Mettler Toledo (Fischer, 1935). For both titrimetric methods, the indicated errors are less than 3 and 10%, respectively.

2.2.2 Catalytic activity Different catalysts (Table 1) were tested regarding activity and selectivity in OME synthesis. All catalysts were dried under vacuum (3 mbar) for 24 h at 100 °C and stored under argon before use. Pelletized zeolites were ground in a mortar before drying. Reactions were carried out at 80 °C using 60 g of p-FA and 40 g of MeOH with a catalyst loading of 1 g. Samples were taken sequentially until chemical equilibrium was reached.

2.2.3 Procedures for the determination of equilibrium and kinetic parameters To obtain a homogeneous phase before reaction, p-FA and MeOH were mixed for two days at a temperature of 80 °C under reflux. A maximum p-FA:MeOH ratio of 1.5 g/g was used to avoid precipitation of constituents from the reaction mixtures. Initial compositions of the reaction mixtures and reaction temperatures for the determination of equilibrium parameters as well as the corresponding data for the measurement of kinetic data are summarized in the supplementary material. Reaction mixtures (100 g) were heated to the respective temperature and reactions were started by addition of the dried catalyst (0.3-1 g) using V-2 (Fig. 3). Reactions were monitored by taking samples before addition of the catalyst (t = 0) and during reaction. Samples were taken with a syringe, filtrated and quenched with THF. The reaction time (tR) was corrected according to Eq. (9). Thus, the normalized time tN was determined, which implies the loss of catalyst and reaction solution (mSample) during sampling (mCat = mass of initial catalyst, mMixture = mass of initial reaction mixture). The term mSample is zero, if the catalyst is discharged during sampling, because in this case the catalyst to mixture ratio stays constant.

12

tN 

t R  mCat n

mMixture   mSample,s

 10 2 in (min gCat/100 gMixture)

(9)

s 1

Samples were taken until chemical equilibrium was reached. A reaction progress of over 99% of maximum equilibrium conversion, is reached in all experiments within 3 h. To ensure to get the right composition of the chemical equilibrium, an additional sample was taken after 24 h for the calculation of the equilibrium parameters. For the analysis and numerical integration of the rate equations, the software Presto Kinetics v4.8 (Telgmann and Wulkow, 2005) was used. The unit mol/l was used for the implementation of the components to the kinetic model. An exponential approach was not sufficient to describe the catalyzed reactions. Therefore, a hyperbolic approach was used to optimize the model. The kinetic coefficients were adjusted to the experimental data using the internal solver. In the first step, the embedded differential equations (1) to (8) were solved with the predefined starting conditions. Appropriate starting conditions simplify the following optimization of the parameters or reduce the required computational effort. Therefore, the simulated annealing method for finding the optimal starting conditions was used. This method makes it possible to remove a local minimum and to find the global optimum.

13

3 Results and discussion 3.1 Catalytic activity For the comparison of different catalysts, reaction times from catalyst addition until chemical equilibrium were determined. Reactions were run at 80 °C, employing a p-FA:MeOH ratio of 1.5 g/g. The time until the OME_2 content in the reaction mixture reached 9.0 wt.% was determined to compare the activities of the different catalysts (Table 2). The equilibrium content of OME_2 is 9.9 wt.%, so that data refer to the reaction period before equilibrium is reached. In equilibrium, the same OME contents are observed for all catalysts (OME_1: 15.8 wt.%, OME_2: 9.9 wt.%, OME_3: 5.6 wt.%, OME_4: 3.1 wt.%, OME_5: 1.6 wt.%, OME_6: 0.8 wt.%). Thus, it is ensured that the thermodynamic equilibrium is reached and not an intermediary stage, depending on the catalyst.

Table 2: Catalytic activities and OME contents in the reaction solutions.1

OME selectivity Catalyst2

tOME_2 = 9 wt.% (min)

OME_1 (wt.%)

OME_2 (wt.%)

OME_3 (wt.%)

OME_4 (wt.%)

OME_5 (wt.%)

Dowex50Wx2

1.4

11.5

9.0

5.0

2.4

1.1

Dowex50Wx4

1.8

13.4

9.0

4.4

2.0

0.9

Dowex50Wx8

2.8

13.3

9.0

4.4

1.8

0.8

Amberlyst 36

12.5

13.1

9.0

4.4

2.0

0.8

IR 120

35.0

14.0

9.0

4.5

1.9

0.8

H-BEA 25

6.5

11.5

9.0

4.5

2.7

1.6

H-MFI 90

8.0

10.6

9.0

5.0

3.2

1.8

CBV 720

86.0

10.2

9.0

5.4

3.0

1.4

H-MFI 400

> 100











H-MOR 30

> 100











1 2

Reaction conditions: 60 g p-FA, 40 g MeOH, 80 °C, 1 wt.% catalyst. Particle size: < 74 µm.

In general, the ion exchange resins Dowex50Wx2, Dowex50Wx4 and Dowex50Wx8 showed the highest activity followed by the acidic zeolites H-BEA 25 and H-MFI 90. The zeolites

14 H-MFI 400, CBV 720 and H-MOR 30 showed the lowest activity. The resin Amberlyst 36 exhibited a slightly lower activity than H-MFI 90, while resin IR 120 exhibited the lowest activity among the ion exchange resins. In case of the highly active catalysts Dowex50Wx2, Dowex50Wx4 and Dowex50Wx8 various particle sizes (37-74, 74-149 and 150-300 µm) have also been studied to estimate the influence of pore diffusion at different temperatures (40, 60 and 80 °C, Fig. 4). For these experiments the same FA-MeOH-H2O mixture as in experiment Kin.-1 (see supplementary material, Table 2) has been used.

(a)

0.12

Particle size 150 - 300 µm 74 - 149 µm 37 - 74 µm

Mass fraction / g g-1

0.10 0.08 0.06 0.04 0.02 0.00 0

5

10

Time / min

15

15 (b)

0.12 Particle size 150 - 300 µm 74 - 149 µm 37 - 74 µm

Mass fraction / g g-1

0.10 0.08 0.06 0.04 0.02 0.00 0

10

20

30

40

Time / min

(c)

0.12

Particle size 150 - 300 µm 74 - 149 µm 37 - 74 µm

Mass fraction / g g-1

0.10 0.08 0.06 0.04 0.02 0.00 0

20

40

60

80

Time / min

Fig. 4:

Influence of catalyst particle size on OME_2 formation at 80 °C (a), 60 °C (b) and 40 °C (c). Reaction conditions: 55.6 g p-FA, 40.7 g MeOH, 3.7 g H2O, catalyst: 1 g Dowex50Wx2.

Activities of Dowex catalysts can be considerably enhanced by decreasing the particle sizes. This is exemplarily shown for the most active catalyst Dowex50Wx2 (Fig. 4) and it can be seen that this effect is particularly pronounced at temperatures higher than 60 °C. No significant dependence is found for particle sizes below 149 µm and temperatures below 60 °C. Thus, Dowex50Wx2 catalysts can be used with particle sizes smaller than 149 µm without pore diffusion effects for temperatures up to 60 °C.

16 In this context, different stirrer speeds (250, 500, 750, 1000 and 1250 rpm) were tested to identify possible film diffusion phenomena. Fig. 5 shows OME_2 formation as a function of time for different stirrer speeds. The stirrer speed did not affect the experiments in the range from 250 to 1250 rpm and the stirrer speed was set to 750 rpm for all experiments.

Mass fraction / g g-1

0.12

Average 250 rpm 500 rpm 750 rpm 1000 rpm 1250 rpm

0.08

0.04

0.00 0

5

10

15

20

25

Time / min Fig. 5:

Time dependence of OME_2 formation at different stirrer speeds. Reaction conditions: T = 60 °C, 55.6 g p-FA, 40.7 g MeOH, 3.7 g H2O, catalyst: 1 g Dowex50Wx2.

Major byproducts are in all cases glycols and hemiacetals as well as minor amounts of trioxane and methyl formate. Formation of the latter is pronounced in the case of zeolite catalysts and its content can exceed 1 wt.% after reaction times of about 20 h, at temperatures of 80 °C and higher and a catalyst loading of 1 wt.%. In the case of the ion exchange resins, methyl formate contents are far below 1 wt.% even after longer reaction times.

17

3.2 Chemical equilibrium Temperature-dependent equilibrium constants Kj(T) of reaction j, based on mole fractions were determined employing Eq. (10), where xi is the mole fraction and νi is the stoichiometric coefficient of component i. Concentrations of FA, HF and Gly cannot be determined directly by the employed GC methods. Thus, FA and H2O concentrations were determined by titration while mass fractions of MeOH were determined by GC-FID. From these data, concentrations of FA, HF and Gly have been calculated using equilibrium data published by Hahnenstein et al. (Hahnenstein et al., 1995a; Hahnenstein et al., 1994) and Siling et al. (Siling and Aksel'rod, 1968). From these data and the directly determinable OME concentrations, equilibrium constants for reactions (5) and (6) can be calculated. The temperature dependence of Kj(T) is calculated by the integrated van’t Hoff equation with parameters A and B (11). The chain length of HF, Gly and OME is limited to n = 10. An extension to n = 20 does not improve the results in terms of accuracy. Equilibrium constants have been determined for different reaction mixtures (see supplementary material, Table 1) and the values have been averaged for each temperature. Logarithmic plotting of the equilibrium constants against inverse temperatures leads to the independent parameters A and B (Fig. 6 and Table 3). Since the equilibrium constant of reaction (6) is independent of the chain length, the molar ratio OME_n/OME_n+1 is constant and can be described by a Schulz-Flory distribution. Regarding reaction (5), the equilibrium constant for n = 1 is sufficient for the description of equilibria for all OMEs. Constants for n > 1 are linearly dependent from the correspondent reactions. Deviations of these results from recently published data by Schmitz et al. are below 10% (Schmitz et al., 2015b).

n

K j (T )   x ivi

(10)

i 1

lnK j (T )  A 

B T /(K)

(11)

18 (a)

8

(b) 2.5

ln KOME n>0

ln KOME n=1

7 2.0

5

1.0 2.6

2.8

1000 / (T/K)

Fig. 6:

6

1.5

3.0

3.2

2.6

2.8

3.0

3.2

1000 / (T/K)

Mole fraction based equilibrium constants for OME_1 (a) (Fig. 2, reaction (5)) and OME_n>0 (b) (Fig. 2, reaction (6)) at different temperatures. The squares represent experiments Equi.-1 to Equi.-5 (see supplementary material, Table 1), results from the other experiments (Equi.-6 to Equi.-21) are within the error bars. The model corresponds to the continuous line.

19 Table 3: Mole fraction based equilibrium parameters for the synthesis of OMEs from formaldehyde, methanol and water.

Reaction (see Fig. 2)

Chain length

A

B

Reference

(1)

-

-2.325

2579

(Siling and Aksel'rod, 1968)

(2)

-

-1.902

3512

(Hahnenstein et al., 1995a)

(3)

n>0

-0.3476

-503.2

(Hahnenstein et al., 1994)

(4)

n=1

0.01449

560.9

(Hahnenstein et al., 1994)

(4)

n>1

-0.1084

460.4

(Hahnenstein et al., 1994)

(5)

n=1

-0.7576

875.6

this work

(5)

n=2

-0.9705

908.3

this work

(5)

n=3

-1.1832

941.0

this work

(5)

n=4

-1.3961

973.8

this work

(5)

n=5

-1.6088

1006.4

this work

(5)

n=6

-1.8217

1039.1

this work

(5)

n=7

-2.0345

1071.8

this work

(5)

n=8

-2.2472

1104.5

this work

(5)

n=9

-2.4600

1137.2

this work

(5)

n = 10

-2.6728

1169.9

this work

(6)

n>0

-2.4624

3041.5

this work

(8)

-

-6.8289

8252.0

this work

Experimental and simulated data are in good agreement and a slight influence of temperature on OME yield is visible (Fig. 7). The reaction is slightly exothermic (ΔHR = 25.3 kJ/mol) and thus, low temperatures promote OME formation. ΔHR can be calculated via Eq. (12) from the ideal gas constant (R) and the van’t Hoff Parameter B of reaction (6).

HR  R  B

(12)

20

Exp. 40 °C Sim. 40 °C Exp. 80 °C Sim. 80 °C Exp. 120 °C Sim. 120 °C

Mass fraction / g g-1

0.15

0.10

0.05

0.00 1

2

3

4

5

6

OME_n Fig. 7:

Comparison between experimental and simulated OME contents in chemical equilibrium at different temperatures (reaction conditions: see supplementary material, Table 1, Equi.-1, -3, and -5).

3.3 Reaction kinetics Regarding reaction kinetics, syntheses of HF and Gly without acidic catalysts have already been investigated (Drunsel et al., 2012; Hahnenstein et al., 1995b; Rudnev, 1977; Schecker and Schulz, 1969) and a pH-dependent reaction rate has been described. For the description of OME formation from MeOH and p-FA, literature data on the formation of HF and Gly can be used (Hahnenstein et al., 1995a; Rudnev, 1977; Schecker and Schulz, 1969). In these previous studies it is pointed out, that OME formation does not influence the kinetics of the simultaneously occurring formation of HF and Gly. Furthermore, these non-catalyzed reactions are orders of magnitude faster than OME formation and it can be assumed that these reactions are in equilibrium (Drunsel et al., 2012; Schmitz et al., 2015a). Within this work, literature data on the kinetics of reactions (1) to (4) have been implemented (Table 4) to describe the kinetics of OME formation. The pH value (1.8) and the density of the reaction mixture (1 g/cm³) are considered as constant throughout the reaction. The ion exchange resin Dowex50Wx2 (mesh 200-400), which exhibited highest activity (Table 2), was used as catalyst in all experiments.

21 Table 4: Kinetic parameters for the synthesis of HF and Gly from FA and MeOH.

Kinetic parameter1

Chain length

A

B

C1

C2

C3

Reference

k1

-

8.962

1913

870

6.3 x 10-8

0

(Schecker and Schulz, 1969)

k-2

-

10.90

4939

0

0

0

(Rudnev, 1977)

k3

n>0

21.79 10190

1413

-5.574

2228

(Hahnenstein et al., 1995a)

k4

n>0

23.12

247.3

-0.838

3102

(Hahnenstein et al., 1995a)

1

-1

-1

-1

-1

-pH

k1/s , k-2/s = (1 + C110

-pH

k3/s , k4/s = (1 + C110

+pH

+ C210

8551

) exp(A – B/(T/K)), +pH

+ exp(C2 - C3/(T/K))10

) exp(A - B/(T/K)).

The rate of appearance Ri of all components i can be calculated via Eq. (13) where ci is the concentration of component i in mol/l, t is the time in seconds, m is the total number of reactions, νi,j is the stoichiometric coefficient of component i in reaction j and rj is the reaction term. The non-catalyzed reactions (1) to (4) are described by Eqs. (15) to (18) where kj(T) is the rate constant and Kj(T) is the equilibrium constant of reaction j. All reactions, except the formation of MF, are equilibrium reactions und thus, back reactions can be described using equilibrium constants. In the case of the catalyzed reactions (5) to (8), the reaction term has to be extended by two additional coefficients, expressing the catalyst concentration (cCat, in gCat/lMixture) and the inhibition term (βInh) to describe the hyperbolic reaction approach. Both variables are multiplied by the corresponding reaction term and the result represents the effective reaction rate. The reaction rate depends on the composition of the reaction mixture. The presence of H2O, MeOH and to a lower extent also OMEs leads to a reversible inhibition depending on the concentrations of these components. This phenomenon can be explained by a dynamic swelling behavior due to the different polar components (Oktar et al., 1999; Ziyang et al., 2001). As a result, not all catalytically active centers of the acidic ion exchange resin are steadily accessible. Furthermore, microspheric diffusion due to solvatation effects can affect the reaction rate (Helfferich and Plesset, 1958). In such a case, the reaction rate not only depends on macrokinetic parameters like film and pore diffusion

22 but also on mass transfer inside the swollen ion exchange resin. The upper part in Fig. 8 describes the non-catalytic reactions, whereas the lower part represents the catalytic reactions. The diffusion of the educts to the active sites is influenced by the swelling behavior. Depending on the composition of the reaction mixture, diffusion influences kinetics in different ways. To describe this inhibition, Eq. (14) is implemented. It consists of the adjusted temperature-independent inhibiting factors (hi) and the concentrations (ci) of water, MeOH and OME. The highest impact comes from H2O since it is the most polar component in the investigated reaction system (inhibiting factor hH2O = 1) followed by MeOH (hMeOH = 0.9) and OME (hOME = 0.2). Since HF and Gly are directly related to MeOH and H2O, no separate coefficient is needed. In Eq. (14), hi describes the inhibiting factor, which is temperature-independent and ci represents the concentration of component i. The independence of hi from temperature is proven by experiments Kin.-6, -8, -19 and -20 (see supplementary material, Table 2). These experiments were started with an enhanced MeOH or H2O content. Furthermore, reactions were carried out at 60 and 80 °C and hi were identical at both temperatures.

Fig. 8:

Catalytically active centers, bulk phase and the involved reactions.

23 For simplification, a hyperbolic description is deduced from the inhibiting term (14) and the respective reaction term (Eqs. (15)-(22)), analogous to a Langmuir-Hinshelwood description. Both, the hyperbolic as well as an exponential approach have been examined, but the hyperbolic approach fits more accurate to all experimental data. Similar models to describe the kinetics of reactions catalyzed by ion exchange resins were used by (Beránek, 1977; Gangadwala et al., 2003; Gates and Johanson, 1971). The correlation of simulation and experimental data is shown in the supplementary material (Figs. 1-22). The temperature dependence of rate constants kj(T) is expressed by the Arrhenius equation (23), which comprises the pre-exponential factor k0, the activation energy EA, the universal gas constant R and the temperature T. The experiments for kinetics determination are summarized in the supplementary material (Table 2). The software Presto-Kinetics v4.8 (Telgmann and Wulkow, 2005) was employed for data analysis and numerical integration of the reaction equations. The maximum chain length for HF, Gly and OME has been set to n = 10. The rate constants have been determined for all experiments separately. From the differences in k5 at the same temperatures but different compositions and without OME as an educt the parameters hi have been determined. After adaption of hi, the preliminary constants k5 have to be determined again and this iteration has been repeated for all compositions and temperatures until no further significant changes of hi und k5 could be observed. The consideration of reaction (6), which is the main reaction in non-aqueous reaction mixtures (Burger et al., 2012; Lautenschütz et al., 2016; Lautenschütz, 2015), has been realized by adaption of k6 from experiments with OME_1, OME_2 and OME_3 as educts (Kin.-10-18, -21 and -22, see supplementary material, Table 2). Overall kinetics can only be described successfully by considering not only the educts MeOH and p-FA but also the OME products as reactants and this is mandatory for the development of an efficient process, which requires separation and refeeding of undesired OMEs, i.e. OME_1, OME_2 and OMEs with n > 6, which do not match the specifications for standard diesel fuel. Due to the implementation of reaction (6), k5 and hi have been fitted again by an iteration algorithm.

Ri 

dc i m   ν i , j  rj dt j 1

(13)

24

βInh 

1 10

(1  hH2O  cH2O  hMeOH  cMeOH  hOME   cOME_n )2

(14)

n1

r1  k1 (T )  cH2O  cFA 

k1 (T )  cGly_1 K1 (T )

(15)

k2 (T )  cHF_1 K2 (T )

(16)

r2  k2 (T )  cMeOH  cFA 

r3_n  k3_n (T )  cHF_n  cHF_1 

k3_n (T ) K 3_n (T )

r4_n  k4_n (T )  cGly_n  cGly_1 

 cHF_n 1  cMeOH

k4_n (T ) K 4_n (T )

 cGly_n 1  cH2O

(17)

(18)

  k 5_n (T ) r5_n  c Cat  βInh  k 5_n (T )  cHF_n  cMeOH   c OME_n  cH2O  K 5_n (T )  

(19)

  k6_n (T ) r6_n  c Cat  βInh  k6_n (T )  c OME_n  cFA   c OME_n 1  K 6_n (T )  

(20)



2 r7  cCat  βInh  k7 (T )  cFA



  k (T ) 3 r8  c Cat  βInh  k8 (T )  c FA  8  c Tri  K 8 (T )  

(21)

(22)

25

EA

k j (T )  k 0  e RT

(23)

Rate constants k5 versus inverse temperatures for OMEs with n = 1-6 are shown in Fig. 9. All reaction rates increase with increasing temperature. Rate constants, activation energies and pre-exponential factors for OMEs with n > 6 have not been fitted to experimental data via the software, since yields are low and the resulting error would be high. Thus, data are deduced from the corresponding data for the short-chain OMEs by extrapolation. Activation energies and pre-exponential factors as a function of chain length are shown in Fig. 10. OME_2-6 show a linear dependence of activation energies on chain length and thus, EA for OMEs with n > 6 can be estimated accurately by extrapolation. However, OME_1 behaves different and is not considered for extrapolation. This exceptional case of OME_1 and its analog HF_1 is also described in literature (Hahnenstein et al., 1994). Regarding the preexponential factor, data have been fitted by an exponential function and data for OMEs with n > 6 have been calculated from k0 values for OME_2-6 while values for OME_1 have been neglected.

26 (a)

(b)

-3

-4

-4

-5

-5

ln k5_2

ln k5_1

-3

-6

-6

-7

-7

2.6

2.8

3.0

2.6

3.2

2.8

1000 / (T/K)

(c)

3.0

3.2

1000 / (T/K)

(d)

-3

-4 -4

ln k5_4

ln k5_3

-5 -5

-6

-6 -7 -7 2.6

2.8

3.0

-8 2.6

3.2

2.8

1000 / (T/K)

(e)

(f)

-4

3.2

-4 -5

ln k5_6

-5

ln k5_5

3.0

1000 / (T/K)

-6

-6 -7

-7 -8 -8 2.6

2.8

3.0

1000 / (T/K)

Fig. 9:

3.2

-9 2.6

2.8

3.0

3.2

1000 / (T/K)

Rate constant k5 for OME_1 (a), OME_2 (b), OME_3 (c), OME_4 (d), OME_5 (e) and OME_6 (f) versus inverse temperature (squares represent experimental data and the line corresponds to the kinetic model).

27 (a)

k0 / 106 (l / mol)n-1 s-1

EA / (kJ/mol)

3

(b)

70

60

50

2

1

0

40 1

2

3

4

5

6

7

8

9

1

10

OME_n

2

3

4

5

6

7

8

OME_n

Fig. 10: Activation energy EA (a) and pre-exponential factor k0 (b) versus OME chain length with respect to reaction (5). Squares represent experimental data and the line corresponds to the kinetic model (OME_1 is not considered for extrapolation up to OME_10).

Regarding reaction (6), EA and k0 are determined by adding OMEs to the reaction mixtures before starting the reactions (experiments Kin.-10-18, -21 and -22, see supplementary material, Table 2). The rate constant k6 is fitted analogous to k5 to minimize the error. The Arrhenius plots for OME_1-3 are shown in Fig. 11 while EA and k0 as functions of chain length are shown in Fig. 12.

(a)

(b)

-2.6

-3.2 -3.4

ln k6_2

ln k6_1

-2.8

-3.0

-3.6 -3.8

-3.2

-4.0

-3.4 2.8

2.9

-4.2 2.8

3.0

1000 / (T/K)

(c)

2.9

3.0

1000 / (T/K)

-4.0

ln k6_3

-4.5

-5.0

-5.5

-6.0 2.8

2.9

3.0

3.1

1000 / (T/K)

Fig. 11: Rate constant k6 for OME_1 (a), OME_2 (b) and OME_3 (c) versus inverse temperature (squares represent experimental data and the line corresponds to the kinetic model).

28 (a)

(b)

50

1200

k0 / (l / mol)n-1 s-1

EA / (kJ/mol)

40 30 20

800

400

10 0

0 1

2

3

OME_n

4

1

2

3

4

OME_n

Fig. 12: Activation energy EA (a) and pre-exponential factor k0 (b) versus OME chain length with respect to reaction (6). Squares represent experimental data and the line corresponds to the kinetic model.

The rate constants of reaction (6), which describe transacetalization reactions of OMEs among each other, influence the overall reaction system in the case of n > 3 only slightly. Even if k6 is not considered for n > 3 in the simulation, differences between experiment and simulation are below the standard deviation and not quantifiable. Thus, implementation of k6 for n = 1-3 is sufficient for an accurate description. Pre-exponential factors and activation energies for reactions (5) and (6) are summarized in Table 5. Without considering the inhibiting factors in the case of reaction (6), discrepancy between experiment and model becomes significant. Thus, the hyperbolic description is appropriate for all OME syntheses from MeOH and p-FA in aqueous media, which are catalyzed by Dowex50Wx2.

29 Table 5: Kinetic data for OME synthesis from MeOH, FA and H2O catalyzed by Dowex50Wx2.

Rate constant

Chain length

k0 ((l/mol)n-1 s-1)

EA (J/mol)

k5_1

n=1

8.06 · 104

4.73 · 104

k5_2

n=2

6.58 · 104

4.63 · 104

k5_3

n=3

1.53 · 105

4.93 · 104

k5_4

n=4

3.57 · 105

5.23 · 104

k5_5

n=5

8.32 · 105

5.54 · 104

k5_6

n=6

1.94 · 106

5.84 · 104

k5_7

n=7

4.51 · 106

6.14 · 104

k5_8

n=8

1.05 · 107

6.45 · 104

k5_9

n=9

2.45 · 107

6.75 · 104

k5_10

n = 10

5.71 · 107

7.05 · 104

k6_1

n=1

3.99 · 10-1

5.72 · 103

k6_2

n=2

2.29 · 101

1.98 · 104

k6_3

n=3

1.31 · 103

3.40 · 104

k6_4

n=4

7.53 · 104

4.81 · 104

k6_5

n=5

4.32 · 106

6.22 · 104

k6_6

n=6

2.48 · 108

7.63 · 104

k6_7

n=7

1.42 · 1010

9.04 · 104

k6_8

n=8

8.15 · 1011

1.05 · 105

k6_9

n=9

4.68 · 1013

1.19 · 105

k6_10

n = 10

2.68 · 1015

1.33 · 105

3.4 Comparison of different Dowex catalysts Different types of Dowex catalysts are available, which differ in their degree of crosslinking (Table 6). According to the manufacturer, different amounts of divinylbenzene (DVB) have been employed for crosslinking and the catalysts are designated according to the amount of DVB. Thus, catalysts Dowex50Wx2, Dowex50Wx4 and Dowex50Wx8 have been prepared

30 employing 2, 4 and 8% DVB, respectively. As can be seen, capacity of the resins, i.e. their H + loading, increases with increasing crosslinking.

Table 6: Properties of Dowex50W ion exchange resins.

Capacity1 (wet) (meq/ml)

Capacity2 (wet) (mmol H+/g)

Capacity2 (dry) (mmol H+/g)

Moisture1 (%)

Density1 (wet) (g/ml)

Dowex50Wx23

0.6

0.81

3.70

78

0.74

Dowex50Wx43

1.1

1.43

4.47

68

0.77

Dowex50Wx83

1.7

2.12

4.61

54

0.80

Catalyst

1

Manufacturer information (www.dow.com).

2

Derived from manufacturer information.

3

Dowex50Wx2: 2% divinylbenzene as crosslinking agent, Dowex50Wx4: 4% divinylbenzene, Dowex50Wx8: 8% divinylbenzene.

Regarding catalytic activity, the influence of DVB content on reaction rate has been investigated and, exemplarily, the depencence of rate constants for reaction (5) on DVB content is shown in Fig. 13. Data are normalized to the H+ ion concentration and refer to a reaction temperature of 60 °C. Despite a higher H+ loading, a higher DVB content, i.e. higher crosslinking, decreases catalytic activity and thus, accessibility of catalytically active sites seems to be crucial rather than the number of Brønsted acidic sites. The linear dependence can be used to describe reactions (5) and (6) by equation (24) where rj represents the 

H modified reaction rate, c Cat. is the molar concentration of H+-Ions per g catalyst and cDVB is

the concentration of DVB in %. rj is the original reaction rate for the catalyst Dowex50Wx2 

H with the known molar concentration of H+-Ions ( cDowex50Wx2 ). The arithmetic mean from the

linear gradients for all k values allows for the calculation of the coefficient a. Thereby the rate law becomes independent of the used Dowex catalyst.

31 

 j

r  rj 



H cCat.

H cDowex50Wx2

a = -6.5·10-2 ± 1.0·10-2,

 a  (cDVB  cDVB, Dowex50Wx2 )  1, j = 5, 6 

H = 3.7·10-3 mol/g, cDowex50Wx2

(24)

cDVB, Dowex50Wx2 = 2%

k5_2

1.0

k5_3 k5_4

k5 / cH+

0.8

k5_5 k5_6

0.6

0.4

0.2 2

4

6

8

DVB / % Fig. 13: Influence of the crosslinking degree on reaction rates k5_2-6 (reaction conditions: T = 60 °C, p-FA:MeOH = 1.5 g/g, catalyst loading = 0.1 wt.%).

3.5 Long term stability of catalyst Dowex50Wx2 With respect to application of catalyst Dowex50Wx2 in a continuously operating process its long term stability was also investigated. Therefore, the catalyst was employed for about 17 days nonstop in a laboratory plant at 40 °C (Fig. 14). The educts p-FA and MeOH were employed in a mass ratio of 1.5 and the feed flow was set to a conversion of about 90% with respect to equilibrium conversion.

32 After a short initiating period lasting for about 10 h, catalytic activity is widely constant. After 17 days activity decreases by about 10% but this could be reversed by a reduction of feed or by increasing the temperature. OME selectivity remained unchanged. Time / d 0.0

1.4

2.8

4.2

17.4

OME_1 OME_2 OME_3 OME_4 OME_5

Mass fraction / g g-1

0.15

0.10

0.05

0.00 0

2000

4000

6000

25000

Time / min Fig. 14: Long term stability of catalyst Dowex50Wx2. Reaction conditions: T = 40 °C, p-FA:MeOH = 1.5 g/g.

33

4 Conclusions Synthesis of OMEs from MeOH and p-FA can be catalyzed by a series of Brønsted acidic catalysts. Among the tested catalysts the ion exchange resin Dowex50Wx2 exhibited highest activity followed by Dowex50Wx4 and Dowex50Wx8. The decrease of activity correlates with an increasing crosslinking degree of the resins, which affects accessibility of the catalytically active sites. In the case of the Dowex catalysts, rate laws have been extended and a general kinetic model could be developed, which is independent of the respective catalyst. Due to the low reaction enthalpy of -25.3 kJ/mol, OME yield only slightly depends on temperature. A decrease of temperature from 120 to 40 °C increases OME yield by about 8%. In contrast, reaction rate strongly depends on reaction temperature and an increase from 40 to 120 °C reduces the time until equilibrium is reached by a factor of 20. The presence of H2O significantly reduces the reaction rate, which is reflected by the corresponding inhibiting factor (hH2O = 1.0). The swelling behavior of Dowex50Wx2 is largely influenced by polar compounds and thus, the H2O content in the reaction solution should be minimized to ensure a high reaction rate. The same applies to MeOH, albeit to a lower extent. Due to its polar character, MeOH also reduces the reaction rate (hMeOH = 0.9) and shifts the product spectrum to short-chain OMEs. By increasing the FA:MeOH ratio, OME chain length shifts to higher OMEs. The kinetic model is based on a hyperbolic approach and it is suitable for FA:MeOH ratios from 0.5 to 1.5 g/g with up to 23 wt.% of H2O. The model can also describe the decomposition of undesired OMEs. An OME synthesis plant operates only efficiently if all undesired OMEs are fed back to the process. OMEs with n < 3 and n > 5 can be fed back completely to the reactor and the reaction model is only slightly influenced by this. Even the hi-constants remain unchanged and can be employed for all reactions catalyzed by Dowex50Wx2 within systems comprising MeOH, FA, H2O and OMEs. Regarding the long term activity of Dowex50Wx2 a stable performance lasting for 17 days has been proven.

34

Acknowledgements Financial support from the Helmholtz Research School Energy-Related Catalysis and Fachagentur Nachwachsende Rohstoffe/BMEL (Joint research project: Oxymethylene ethers (OME): Eco-friendly diesel additives from renewables, FKZ 22403814) is gratefully acknowledged.

35

References Arnold, U., Lautenschütz, L., Oestreich, D., Sauer, J., 2015. Production of Oxygenate Fuels from Biomass-derived Synthesis Gas. DGMK-Tagungsbericht 2015-2, 127-136. Beránek, L., 1977. An examination of the Langmuir-Hinshelwood Model using ion exchange catalysts. Catalysis Reviews Science and Engineering 16, 1-35. Burger, J., Hasse, H., 2013. Multi-objective optimization using reduced models in conceptual design of a fuel additive production process. Chemical Engineering Science 99, 118-126. Burger, J., Ströfer, E., Hasse, H., 2012. Chemical Equilibrium and Reaction Kinetics of the Heterogeneously Catalyzed Formation of Poly(oxymethylene) Dimethyl Ethers from Methylal and Trioxane. Industrial & Engineering Chemistry Research 51, 12751-12761. Drunsel, J.-O., Renner, M., Hasse, H., 2012. Experimental study and model of reaction kinetics of heterogeneously catalyzed methylal synthesis. Chemical Engineering Research and Design 90, 696-703. Fischer, K., 1935. Neues Verfahren zur maßanalytischen Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern. Angewandte Chemie 48, 394-396. Fleisch, T.H., Sills, R.A., 2004. Large-scale gas conversion through oxygenates: beyond GTLFT. Natural Gas Conversion Vii 147, 31-36. Gangadwala, J., Mankar, S., Mahajani, S., Kienle, A., Stein, E., 2003. Esterification of acetic acid with butanol in the presence of ion-exchange resins as catalysts. Industrial & Engineering Chemistry Research 42, 2146-2155. Gates, B., Johanson, L., 1971. Langmuir‐hinshelwood kinetics of the dehydration of methanol catalyzed by cation exchange resin. AIChE Journal 17, 981-983. Hahnenstein, I., Albert, M., Hasse, H., Kreiter, C.G., Maurer, G., 1995a. NMR spectroscopic and densimetric study of reaction kinetics of formaldehyde polymer formation in water, deuterium oxide, and methanol. Industrial & Engineering Chemistry Research 34, 440-450. Hahnenstein, I., Albert, M., Hasse, H., Kreiter, C.G., Maurer, G., 1995b. NMR Spectroscopic and Densimetric Study of Reaction Kinetics of Formaldehyde Polymer Formation in Water, Deuterium Oxide, and Methanol. Ind. Eng. Chem. Res. 34, 440-450. Hahnenstein, I., Hasse, H., Kreiter, C.G., Maurer, G., 1994. 1H-and 13C-NMR-spectroscopic study of chemical equilibria in solutions of formaldehyde in water, deuterium oxide, and methanol. Industrial & Engineering Chemistry Research 33, 1022-1029. Helfferich, F., Plesset, M.S., 1958. Ion exchange kinetics. A nonlinear diffusion problem. The Journal of Chemical Physics 28, 418-424. Lahaye, J., Prado, G., North Atlantic Treaty Organization. Scientific Affairs Division., 1983. Soot in combustion systems and its toxic properties. Published in cooperation with NATO Scientific Affairs Division by Plenum Press, New York. Lautenschütz, L., Oestreich, D., Seidenspinner, P., Arnold, U., Dinjus, E., Sauer, J., 2016. Physico-chemical properties and fuel characteristics of oxymethylene dialkyl ethers. Fuel 173, 129-137. Lautenschütz, L.P., 2015. Neue Erkenntnisse in der Syntheseoptimierung oligomerer Oxymethylendimethylether aus Dimethoxymethan und Trioxan. Heidelberg.

36 Lumpp, B., Rothe, D., Pastoetter, C., Laemmermann, R., Jacob, E., 2011. Oxymethylenether als Dieselkraftstoffzusaetze der Zukunft. Motortechnische Zeitschrift 72, 198-202. Marchionna, M., Patrini, R., Sanfilippo, D., Paggini, A., Giavazzi, F., Pellegrini, L., 2001. From Natural Gas to Oxygenates for Cleaner Diesel Fuels, in: E. Iglesia, J.J.S., Fleisch, T.H. (Eds.), Studies in surface science and catalysis. Elsevier, pp. 489-494. Oktar, N., Murtezaoglu, K., Dogu, T., Dogu, G., 1999. Dynamic analysis of adsorption equilibrium and rate parameters of reactants and products in MTBE, ETBE and TAME production. Canadian Journal of Chemical Engineering 77, 406-412. Ren, Y., Huang, Z., Miao, H., Di, Y., Jiang, D., Zeng, K., Liu, B., Wang, X., 2008. Combustion and emissions of a DI diesel engine fuelled with diesel-oxygenate blends. Fuel 87, 2691-2697. Rudnev, A., 1977. Formaldehyde 2: Kinetics of the Desolvation of Formaldehyde in Aqueous and Methanolic Solutions. Russion Journal of Physical Chemistry 51, 1519-1521. Sauer, J., Arnold, U., Dahmen, N., 2016. Synthetic fuels from biomass: Potentials and viability, Internationaler Motorenkongress 2016. Springer, pp. 489-504. Schecker, H.-G., Schulz, G., 1969. Untersuchungen zur Hydratationskinetik von Formaldehyd in wäßriger Lösung. Zeitschrift für Physikalische Chemie 65, 221-224. Schmitz, N., Burger, J., Hasse, H., 2015a. Reaction Kinetics of the Formation of Poly(oxymethylene) Dimethyl Ethers from Formaldehyde and Methanol in Aqueous Solutions. Industrial & Engineering Chemistry Research 54, 12553-12560. Schmitz, N., Homberg, F., Berje, J., Burger, J., Hasse, H., 2015b. Chemical equilibrium of the synthesis of poly(oxymethylene) dimethyl ethers from formaldehyde and methanol in aqueous solutions. Industrial & Engineering Chemistry Research 54, 6409-6417. Siling, M.I., Aksel'rod, B.Y., 1968. Determination of the equilibrium constants of hydration and protonation of formaldehyde by a spectrophotometric method. Zhurnal Fizicheskoi Khimii 42, 2780-2786. Stanonis, D.J., King, W.D., Vail, S.L., 1972. Influence of Chain Length on the Rate of Hydrolysis of Polyoxymethylene Ethers. Journal of Applied Polymer Science 16, 1447-1456. Telgmann, R., Wulkow, M., 2005. Presto-Kinetics-Simulations of Kinetic Models. Computing in Technology GmBH: Rastede, Germany. Vertin, K.D., Ohi, J.M., Naegeli, D.W., Childress, K.H., Hagen, G.P., McCarthy, C.I., Cheng, A.S., Dibble, R.W., 1999. Methylal and methylal-diesel blended fuels for use in compressionignition engines. Society of Automotive Engineers SP-1458, 29-41. Walker, J.F., 1944. Formaldehyde. Reinhold Publishing Corporation, 467-510. Ying, W., Genbao, L., Wei, Z., Longbao, Z., 2008. Study on the application of DME/diesel blends in a diesel engine. Fuel Processing Technology 89, 1272-1280. Zhang, J., Fang, D., Liu, D., 2014a. Evaluation of Zr–Alumina in Production of Polyoxymethylene Dimethyl Ethers from Methanol and Formaldehyde: Performance Tests and Kinetic Investigations. Industrial & Engineering Chemistry Research 53, 13589-13597. Zhang, J., Shi, M., Fang, D., Liu, D., 2014b. Reaction kinetics of the production of polyoxymethylene dimethyl ethers from methanol and formaldehyde with acid cation exchange resin catalyst. Reaction Kinetics, Mechanisms and Catalysis 113, 459-470.

37 Zhang, X., Oyedun, A.O., Kumar, A., Oestreich, D., Arnold, U., Sauer, J., 2016. An optimized process design for oxymethylene ether production from woody-biomass-derived syngas. Biomass and Bioenergy 90, 7-14. Zhao, Q., Wang, H., Qin, Z.-f., Wu, Z.-w., Wu, J.-b., Fan, W.-b., Wang, J.-g., 2011. Synthesis of polyoxymethylene dimethyl ethers from methanol and trioxymethylene with molecular sieves as catalysts. Journal of Fuel Chemistry and Technology 39, 918-923. Zheng, Y., Tang, Q., Wang, T., Wang, J., 2015. Kinetics of synthesis of polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane catalyzed by ion-exchange resin. Chemical Engineering Science 134, 758-766. Ziyang, Z., Hidajat, K., Ray, A.K., 2001. Determination of adsorption and kinetic parameters for methyl tert-butyl ether synthesis from tert-butyl alcohol and methanol. Journal of Catalysis 200, 209-221.

Gly_ n

HF_n+1

HF_n + FA

Gly_1

+ FA Gly_ n

A O+F

H2

MeOH + FA

+1

HF_1

H2O

FA

HF

Gly

MeOH

OME

Tri

MF

H2O

FA

HF

Gly

MeOH

OME

Tri

MF

OME_n + FA

H+

OME_n+1 +

SO3-

H

OH + Me HF_n

SO32 FA

H+

MF

-O S 3

SO33F A

H+

Tr i

Bulk

O n + H2 OME_

SO3-

Catalyst environment

38

Highlights:

Highly active catalysts for OME production from methanol and formaldehyde Equilibrium data as well as a kinetic model have been determined Influence of methanol, water and OME on reaction kinetics has been investigated Ion exchange resin catalysts show stable long term performance