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Reaction process of resin-catalysed methyl formate hydrolysis in biphasic continuous-flow Helena Reymond, Selin Vitas, Sergio Vernuccio, and Philipp Rudolf Von Rohr Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04820 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on February 1, 2017

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Industrial & Engineering Chemistry Research

Reaction process of resin-catalysed methyl formate hydrolysis in biphasic continuous-ow Helena Reymond, Selin Vitas, Sergio Vernuccio, and Philipp Rudolf von Rohr

Department of Process Engineering, ETH Zürich, Sonneggstrasse 3, 8092 Zürich, Switzerland E-mail: [email protected]

Abstract In the frame of process development for the generation of chemical energy carriers, we studied the synthesis of methanol and formic acid from methyl formate hydrolysis in a continuous-ow millireactor. The aim was to establish the link between kinetics and phase behaviour in the biphasic liquid regime. Reaction performance using the acidic ion-exchange resin Amberlyst 15 as catalyst was examined at various operating conditions such as space velocity, catalyst particle size, temperature and initial reactant ratio. Results revealed substantially higher yields without mass transfer impediment with feed compositions exceeding methyl formate saturation in water. The simultaneous decrease in methyl formate and increase in polar product concentrations suced to bring the initially biphasic mixture to a homogeneous system as conrmed by thermodynamic UNIFAC equilibrium calculations. The separate analysis of euent liquid phases unveiled a quasi-homogeneous catalytic process rooted in an aqueous layer at the resin surface, independent of the organic content. The fast and continuous synthesis of these chemicals constitutes a promising application for the development of direct fuel cells for portable power devices.

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Introduction Methyl formate hydrolysis to formic acid and methanol opens up access to valuable chemicals in a low-carbon economy. The hydrolysis products are indeed of practical interest in relation to the current challenges of renewable energy production and subsequent storage, in light of their high hydrogen density and liquid state at atmospheric conditions. 15 Methyl formate formation has been reported in mid-pressure hydrogenation of carbon dioxide (CO 2 ) over Pd/Cu/ZnO/Al2 O3 in presence of methanol, and during high-pressure methanol synthesis using Cu/ZnO/Al 2 O3 catalysts. 68 The development of heterogeneous CO 2 reduction catalyst selective for formate intermediate streamlines continuous methyl formate hydrolysis as potential hydrogen storage process for direct formic acid and methanol fuel cells. 912 The weakly endergonic nature of the equilibrium-limited methyl formate hydrolysis ( ∆Gr =

6.74 kJ/mol) 13 does not favour the spontaneous and fast synthesis of products. This feature, common to carboxylic ester hydrolysis, resulted in characterisation studies of similar liquidphase reversible hydrolysis-esterication systems using various acidic catalytic media. 1320 Process variations were reported to enhance reaction rate by rapid product complexation with an additive 21,22 or equilibrium conversion by furthering reaction through product separation in chromatographic reactors packed with ion-exchange resins. 2326 On a larger scale, patents described increased continuous formic acid synthesis using ester excess in a twohydrolyser process 27 and shorter reaction times by adding methanol to maintain a single reactive liquid phase to avoid mass transfer hindrance. 28 While industrial scale hydrolysers have the drawback of prolonged residence times, chromatographic reactors rely on a relative operational complexity and do not necessarily oer improved performance at high ow rates for rapid synthesis. 29 Moreover, the aforementioned studies on short-chain ester hydrolysis cover either a limited temperature or concentration range, ensuring a single homogeneous reactive phase. Although instrumental for process intensication and on-demand production, where a sustained source of reactant is primordial, reaction data in the two-phase domain are sorely missing. To this end, the present contri2


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bution reports continuous methyl formate hydrolysis at concentrations beyond saturation. Improved mass transfer between liquid phases was achieved exploiting the advantageous surface-to-volume ratio of tubular millireactors. Firstly, the theoretical background used to model phase and chemical equilibria is introduced before presenting the results of UNIFAC thermodynamic equilibrium calculations for methyl formate hydrolysis. A parametric study then investigates reaction performance over the ion-exchange resin catalyst Amberlyst 15. Finally, the theoretical and experimental knowledge gained on the reactive system are combined to develop a process and kinetic model in continuous-ow operation.

Thermodynamic and kinetic equilibria Methyl formate hydrolysis follows the general form of reversible ester hydrolysis and esterication k+

A+B* )C+D k−

(0.1)

where A = HCOOCH3 , B = H2 O, C = HCOOH, and D = CH3 OH. The limited solubility of methyl formate in water forces the reactive system to partition into two liquid phases when saturation concentration is exceeded. This complex system can then be decoupled into simultaneously occurring chemical and phase equilibria, characterised by a minimum energy function at given conditions of pressure p, temperature T and composition. The Gibbs free energy G (Equation 0.2) dictates the direction towards which the system evolves as a measure of the energy available for phase transition and chemical reaction, where H stands for enthalpy, S for entropy, U for internal energy, and V for volume.

G = H − T S = U + pV − T S

(0.2)

Following the second law of thermodynamics, equilibrium conditions may thus be found

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by direct minimisation of Equation 0.2, expressed in terms of the experimentally measurable entities temperature, pressure and mole number ni , where µi is the chemical potential of a component as dened in Equation 0.7.

dG = −SdT + V dp +

X

µi dni = 0

(0.3)

i

In a closed reactive system, equality of temperature and pressure of phases in contact are necessary for thermal and hydrostatic equilibrium, respectively. Hence Equation 0.3 reduces to a change in mole amount, dni , which may arise through transfer of a substance between phases by diusion, as well as by chemical reaction.

Phase equilibrium

Liquid - liquid system Chemical equilibrium is reached in all phases regardless of which the reaction takes place in. Equation 0.3 is therefore valid for all phases in contact.

dGsystem = dGorganic + dGaqueous = 0

(0.4)

From mass conservation follows

dnorg = −dnaqu i i

(0.5)

Finally, uniformity of chemical potentials in each of the individual phases between which substances can freely diuse is derived for equilibrium condition

µorg = µaqu i i

(0.6)

where the chemical potential of a component µi is related to its standard state and to its thermodynamic activity ai according to Equation 0.7. The procedure for the estimation of 4


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activity coecients is presented in the following section.

µi = µ◦i + RT ln(ai )

(0.7)

Solvent - polymer system The interaction between resin catalysts and solvents and its impact on the reaction course is presented in this section in relation to phase equilibria. 30,31 Herein, the macroreticular-type cation-exchange resin Amberlyst 15 consists of an insoluble and elastic copolymer backbone of styrene-divinylbenzene functionalysed by sulfonic groups ( −SO3 H). When in contact with a solvent, the resin expands as the former is incorporated in the elastic polymer matrix until counter-acting forces equilibrate. An equilibrium state is reached as the osmotic and eletrostatic forces forcing the solvent into the matrix are balanced out by the resistance to expansion, conferred by the crosslink density in the gelular matrix. The magnitude of osmotic and electrostatic forces depends on the polarity and dieletric constant of the solvent in contact with the ionic sites of the polymeric backbone. The degree of swelling relies thus largely on the properties and nature of the reactive mixture: water and methyl formate exert dierent inuences on the resin. This dierence dictates their equilibrium distribution between the bulk liquid phase and the one trapped in the resin pores. Consequently, they inuence reaction performance by aecting not only the level of catalyst-reactant contact, but also the accessibility in and out of the matrix proportionally to the latter's macropore dimensions. In addition, the nature of the catalytic species is determined by the intensity of the interaction between the solvent and the acid groups of the polymer backbone. 31 Type A reactions are characterised by aqueous or polar feeds: water hydrates the protons of the sulfonic groups, which become mobile within the resin pores and act as catalytic species. This class is further divided into A1-type catalysis by fully swollen resins in aqueous systems, where total dissociation of the polymer-bound −SO3 H groups is achieved, and A2-type

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catalysis by mixed organo-water solvents with variable extent of proton hydration. Type B reactions are dened by anhydrous feeds, where the sulfonic groups remain undissociated and act as catalytic species in absence of solvating medium. They are not discussed here as they do not pertain to ester hydrolysis.

Chemical equilibrium

Reaction thermodynamics In a reactive system the variation in molar amount of each component dni in Equation 0.2 are derived from the reaction stoichiometry and the extent of reaction ζ (single reaction), where νi stands for the stoichiometric coecient of component i.

dG|T,p =

X

µi dni =

X

i

(0.8)

νi µi dζ = 0

i

In other terms, Equation 0.8 requires the euent's net chemical potential to be equal to the chemical potential of the feed at equilibrium.

X

(0.9)

νi µ i = 0

i

For a reversible reaction like hydrolysis, the equilibrium constant Keq (Equation 0.10) states the relative concentrations at equilibrium.

Keq

k+ = = k−

aC aD aA aB

= eq

xC xD xA xB

eq

γC γD γA γB

= Kx K γ

(0.10)

eq

where k + and k − are the rate constants for the forward and reverse reactions, xi are the reactant molar fractions and γi their activity coecients. The equilibrium constant is also related to the standard Gibbs free energy as

ln Keq = − 6

∆G◦r RT


(0.11)

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Combining Equations 0.2 and 0.11 leads to the dierential form of the van't Ho equation, describing the dependence of the equilibrium constant on reaction temperature.

∆Hr◦ dln Keq = dT RT 2

(0.12)

Upon integration if ∆Hr◦ is assumed constant for small temperature intervals, Keq yields

ln (Keq ) = ln

◦ Keq

∆Hr◦ − R

1 1 − ◦ T T

(0.13)

Reaction kinetics Ester hydrolysis and esterication are accelerated under acidic conditions. In addition to the autocatalytic eect induced by the acid product of hydrolysis, acidic conditions are herein enhanced by means of an acidic catalyst. An ideal expression of the kinetic behaviour of equilibrium-limited resin-catalysed reactions can be written as follows. Incidentally, the non-ideality of the liquid phase can be corrected for by replacing concentrations ci by the component activities ai . 1719

r=

cA cB − cC cD /Keq 1 dni P = mcat k+ νi dt (1 + i ki ci )n

(0.14)

where mcat is the mass of catalyst, ki are the component adsorption constants and n is a parameter depending on the kinetic model. It denes the catalytic mechanism following

n = 0 for a quasi-homogeneous model (QH), n = 1 for a Eley-Rideal mechanism, and n = 2 for a Langmuir-Hinshelwood type mechanism. The temperature dependence of the rate constant k+ is expressed by the Arrhenius law.

k+ =

◦ k+ exp

−Ea RT

(0.15)

The idealised homogeneous frame (referred to as n = 0 in Equation 0.14) requires a complete swelling of the polymer matrix and an extensive dissociation of the sulfonic groups 7



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as in type A1 reactions. Although the packed bed comprises discrete particles, the mobile hydrated protons catalyse the reaction within the particle pores which act as conned reaction vessels. In the frame of hydrolysis, such a quasi-homogeneous model of type A1 is safely assumed when the reaction is carried out with an excess water to drive the reaction forward. In the case of an signicant volumetric ester excess, the solvation capacity is greatly reduced and catalysis may occur from a combination of solvated protons and direct interaction with the substrate as in type A2. As undissociated sulfonic groups react dierently, an adsorption-based heterogeneous (single- or dual-site mechanism, n = 1 or 2, respectively) may be considered in type A2.

Materials and methods Thermodynamic modelling

Thermodynamic equilibrium calculations were performed using the activity coecient property method UNIFAC 3234 implemented in the Aspen Plus simulation tool for its ability to simulate non-ideal mixtures at moderate pressures (< 10 bar) over a wide temperature range (17-147◦ C ). The calculations were performed by minimisation of the Gibbs free energy with respect to mole amounts at xed reaction pressure and temperature (Equation 0.3). The existence of a vapour phase was discarded as experiments were carried out under pressure and the gas volume caused by the evaporation of methyl formate was assumed negligible in comparison to the liquid volume. The activities of the components are dened in terms of the auxiliary activity coecients γi related to operating conditions, where ζi stands for any measure of concentration in the gas or liquid phase. We have used component mole fractions

xi .

ai = γi ζi = γi xi 8


(0.16)

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Activity coecients in UNIFAC entail a combinatorial and a residual part to account for the entropic contribution and the enthalpic interactions.

(0.17)

lnγi = lnγiC + lnγiR

The group interaction parameters anm required for the estimation of activity coecients were obtained from UNIFAC group specications database (Supporting Information Tables S1-S2). The standard thermodynamic parameters for methyl formate hydrolysis used for the simulations were calculated from the components' formation enthalpies (Supporting Information Tables S3-S4).

Kinetic modelling

A system of ordinary dierential equations was constructed from Equation 0.14 for each single species involved and numerically solved to calculate concentrations. The estimation of kinetic parameters was performed by minimising the sum of the squared residuals between experimental and calculated concentrations through the objective function F (Equation 0.18), while the prediction accuracy of the models was evalutated in terms of the standard deviation

σk (Equation 0.20). q m n X X 1 X (yi,j,k − ci,j,k )2 F = w j,k i=1 j=1 k=1

! (0.18)

The symbols yi,j.k and ci,j,k are respectively the experimental and calculated concentrations, m is the number of experimental points recorded during each run, n the number of detected species, and q the number of experiments involved in the optimisation procedure. The weighting factors wj,k are dened as m

wj,k =

1 X yi,j,k m i=1

9


(0.19)


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σk = 100 ×

n X



s

 1 wj,k j=1

Pm

i=1

2

Page 10 of 31



(yi,j,k − ci,j,k )  m

(0.20)

Catalyst and chemicals

Methyl formate (Sigma-Aldrich, 97%), formic acid (Sigma-Aldrich, 95%) and methanol (Sigma-Aldrich, 99.9%) were used as received from the manufacturer without purication. The macroreticular resin Amberlyst 15 in hydrogen form (Sigma-Aldrich) was used as catalyst. The physico-chemical properties of the reactive compounds are listed in Table S5 in Supporting Information.

Reactor system

The experimental setup for continuous high-pressure operation is shown in Figure 1. Two water-cooled (Huber CC-308) syringe pumps (Teledyne ISCO, 260D, accuracy of ± 5%) allowed the individual adjustment of reactant ow rates for the delivery of distilled water and methyl formate. The reactants were contacted in a T-junction (stainless steel, inner =

1 mm) before owing through a xed-bed reactor (stainless steel, inner = 1.52 mm). The system pressure was controlled by an automated needle valve back-pressure regulator (Jasco, BP-2080, accuracy ± 2 bar) while temperature was adjusted by tting the reactor between two copper bodies heated by a PID controller (Eurotherm 2132) connected to a K-type thermocouple and two resistive cartridges (Wisag, 200W, 230 V). Pressure at the packed-bed inlet and outlet were monitored (Endress and Hauser, accuracy ± 1 bar) online via LabView software. 35 The system can be operated in continuous mode up to 150 bar from 25 ◦ C to 350◦ C. The catalyst beads were crushed and sieved, then separated into fractions of 71-90

µm, 90-125 µm and 450-500 µm. A precise mass of fresh catalyst (75 mg of dried catalyst corresponding to 5 cm packed-bed were used for base operating conditions) was loaded in the reactor without preliminary purication. The pressure drop across the xed-bed, depending on reaction temperature, did not exceed 10% for isobaric conditions. The system 10


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was allowed time to reach swelling equilibrium before heating to reaction temperature. At each temperature, the system achieved chemical and physical steady-state within 15 minutes on stream at a total liquid ow rate of 1 mL ·min−1 and samples were collected for analysis. Reaction quenching was achieved by rapid cooling of the reactor outlet tubing with ice.

FIC

PI

4

2

PI

FIC

3

PI

5 Organic

Methyl formate

Aqueous

1

6

Water

Figure 1: Representation of the reaction setup. (1,2) Water-cooled high-pressure syringe pumps. (3) Packed packed-bed reactor. (4) Reactor bypass for blank measurements. (5) Back pressure regulator. (6) Sample vial.

Swelling and sorption experiments

Swelling and phase equilibrium partitioning experiments were carried out by contacting a known amount of dried resin Amberlyst 15 with either single chemical species or non-reactive binary mixtures and left to equilibrate in a thermostated incubator (Minitron INFORS HT) at 23◦ C 500 rpm. Precisely 1 mL of dried resin was placed in a 5 ± 0.08 mL sealed graduated cylinder with pure solvent ( ∼4 mL). The equilibrated volume of the swollen resin was measured by eye after 24 hours. Binary mixtures ( ∼40 mL) were prepared in 50-mL glass asks containing a xed mass of dried catalyst ( 3 ± 0.01 g). Liquid samples were collected after 48 hours for gas chromatography analysis.

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Analytical method

At reaction steady-state, 5 mL euent samples were collected in a graduated cylinder. The respective volumes of the organic and the aqueous phase in the sample were measured visually before separation by decantation. For biphasic euents, the two phases were analysed oline separately with a gas chromatograph (Bruker GC-450) equipped with a VF-WAXms separation column (25 m X 0.25 mm, coating thickness = 0.25 µm) and a ame ionisation detector (FID). The injection port and FID temperatures were 250 and 220 ◦ C, respectively. The oven temperature was increased from 45 to 80 ◦ C at a rate of 10◦ C·min−1 and further to 170◦ C at 25◦ C·min−1 to remove trace of higher-boiling compounds. A 2 vol.% solution of 1-hexanol (Sigma-Aldrich, ≥ 99.9%) was used as internal calibration standard for the analysis of both phases from which the respective amounts of methyl alcohol and methyl formate were calculated according to their relative response factors. Formic acid was traced qualitatively in the GC chromatograms. An analysis cycle lasted 13 minutes in total which allowed, during the acquisition of the organic phase, the preparation of a second sample for the direct analysis of a freshly-acquired aqueous phase minimising artefacts related to products re-esterication. The pH of the euent's two phases were measured (Hamilton Polilyte Plus H S8 120). The analysis of homogeneous euents followed the same analytical procedures for the single phase.

Results and discussion Methyl formate hydrolysis phase equilibrium

Liquid-liquid equilibrium Figure 2 presents the phase diagrams for the ternary system methanol-methyl formate-water at 10 bar and 20, 60 and 110 ◦ C. As expected for condensed phases, pressure was found not to aect the curved area delimiting the immiscibility gap at low methanol fraction, while

12


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0.

.9

0.1 5

5 0.8

Industrial & Engineering Chemistry Research 0.8

0.2

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0.2 5

5 0.7

0.0 5 0.1

0.3 5

0.1 5

0.4

0.2

0.4 5

0.2 5

MF 0.5

0.3

0.5 5

0.0 5

0.6

0.1

0.3 5

0.3 5

0.6

0.3

0.5 5

0.2 5

MF 0.5

0.2

0.4 5

0.1 5

0.4

0.6 5

0.7

0.7 5 0.8

0.8 5

0.6 5

0.4 0.7

0.4 5

0. 05

0.9 0.9 5

MF 0.5

0.7 5

0.5 0.55 0.50 WATER

0.6

0.60

0.65

0.7

0.70

0.5 5

Water

0.75

0.8

0.85

0.80

0.9

Helena Reymond | 16.12.2015 | 15

0.9

0.6 5

0.9 5

0.7

0.3

0. 45 0. 4

0.7 5

0.1

0. 7

0.25

0.3

0.30

0.35

0.4

0.40

0.6

0.60

0.65

0.7

0.70

0.75

0.8

0.80

0.85

0.9

0.90

0.95


0.8 5

0.1 5 0.0

0. 1

0.9 5

0. 15

0. 85

0.5 0.55 0.50 WATER

Water

0.9

0. 2

0. 9

0.45

5 0.1

0. 25

0. 75

0.2

0.20

0.2

0. 3

0. 8

0.15

0.8

0. 65

0. 35

0.10

(d) 110°C

5 0.2

0.05

5 0.0

5 0.3

OH ME 5 0.

0. 55

0.1

0.4

0.6

0.95

0.90

5 0.1

0.8 5

0.45

0.2

0.8

0.4

0.40

5 0.0

0.35

0.1

0. 1

5 0.1

0.3

0.30

5 0.2

0.25

5 0.4

0. 6

0. 55

0.2

0.3

0.2

0.20

OH ME .5 0

0.15

5 0.5

0. 65

0. 6

5 0.2

5 0.3

0.6

0. 15

0.3

0.4

5 0.6

0. 2

5 0.3

5 0.4

0.7

0. 25

0.4

OH ME .5 0

5 0.5

5 0.7

0. 7

0. 3

5 0.4

0.6

0.8

0.1

0.10

0. 75

0. 35

OH ME .5 0

5 0.5

5 0.6

5 0.8

0. 8

0. 4

0.6

0.7

0.9

0. 85

0. 45

5 0.6

5 0.7

0. 9

0.05

(c) 60°C

MF 0. 5

0.7

0.8

5 0.9

(b) 20°C

T e r na ry M a p ( M o le B a s is )

5 0.8

(a) 10 bar, 20°C

0.9


temperature greatly aected the mutual miscibilities. 0. 95

0. 95

0. 05

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5 0.9

0.3


0.05

0.1

0.10

0.15

0.2

0.20

0.25

0.3

0.30

0.35

0.4

0.40

0.45

0.5 0.55 0.50 WATER

Water

0.6

0.60

0.65

0.7

0.70

0.75

0.8

0.80

0.85

0.9

0.90

0.95

0.05

0.1

0.10

0.15


0.2

0.20

0.25

0.3

0.30

0.35

0.4

0.40

0.45

0.5 0.55 0.50 WATER

0.6

0.60

0.65

0.7

0.70

0.75

0.8

0.80

0.85

0.9

0.90

0.95

Water Helena Reymond | 16.12.2015 | 17

Figure 2: (a) Ternary map (mole basis) of the system methanol-methyl formate-water at 10 bar and 20◦ C. Close up of immiscibility gap at 10 bar and (b) 20◦ C, (c) 60◦ C (d) 110◦ C. Red markers represent the molar feed compositions used in the parametric study. The tie-lines connect the L-L compositions at equilibrium for each feed composition.

The additional diusive resistance implied by the permeable liquid-liquid (L-L) interface was alleviated by the use of a milliscale reactor with high surface-to-volume ratio, achieving improved mass transfer and advantageous performance compared to its macroscopic counterparts. In such reactors where surface forces prevail, various ow patterns may be observed depending on the volume ratio of both phases, their supercial velocities, and intrinsic physical properties. 36 The partial solubility of methyl formate in water (Supporting Information Table S5) induces a very low interfacial tension, thereby increasing the likelihood of parallel or annular ow. As such ow patterns expose only limited surface area for the solubilisation of methyl formate, two modications of the setup were tested in order to ensure that water reached saturation before entering the catalyst packed bed. Modications improved diusion via increased recirculation (i) by breaking up the ow pattern by co-feeding an inert gas phase through a section of smaller diameter (Modication 1: inner = 0.5 mm, Lcoiled = 50 mm + N2 ), and (ii) by thorough mixing over an inert silicon particle packed-bed (Modication 2: Si-beads 70 − 110 µm). According to Figure 2(b), a feed of molar composition

H2 O : MF : MeOH = 77 : 22 : 1 splits into aqueous and organic phases of methyl formate content 8 and 80 mol%, respectively. Although slightly lower than calculated as saturation 13



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reference, both phases contained identical methyl formate and methanol contents whether the standard setup from Figure 1 ( inner = 1 mm, L = 20 mm) was used or any of the above-mentioned modications. The theoretical and experimental saturation concentrations are tabulated in Supporting Information Table S6.

Solvent-polymer equilibrium Non-reactive experiments were carried out contacting the resin with the reaction species to evaluate the volume increase by swelling, and the partitioning ratio of each compound between bulk and polymer phase. The strong anity of water toward this resin was reported for a similar chemical system in the frame of liquid-phase esterication. 2325 Preferential sorption of water and methanol was reported compared to the markedly lower resin anity for the less polar carboxylic acids and their esters. Although dierent chemical species are involved herein, the ndings can be generalised to methyl formate hydrolysis and the following decreasing order of anity concluded: water > methanol > formic acid > methyl formate. This hypothesis is consistent with the degree of swelling measured in pure solvents reported in Table 1. Binary sorption experiments remained inconclusive due to negligible concentration dierences. Table 1: Experimental swelling ratios in pure solvent at 23 ◦ C. Chemical species Water Methanol Formic acid Methyl formate

Swelling Ratio 1.583 1.516 1.455 1.417

The propensity of the components to sorb in the reaction locus controls the reaction yield and selectivity as the polymeric structure selectively removes the components of weakest anity into the bulk phase. Under continuous ow conditions, the strong selectivity of the resin with respect to the two educts implies a preferential water uptake at the reaction locus susceptible of driving the chemical equilibrium more in favour of the products. 14


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Ion-exchange catalysed reaction

A parametric study was carried out for the hydrolysis of methyl formate catalysed by the resin Amberlyst 15. The eects of operating conditions such as temperature, pressure, feed composition, and weight hourly space velocity (WHSV) on reaction performance are reported in the following sections. The reliability of the results was ascertained by repeating a minimum of three runs representative sets of experiments in order to ensure the stability of the experimental method. Methanol content served as basis for the calculation of methyl formate conversion. Material loss by ash evaporation was found negligible from the constant liquid ow rates obtained under all conditions of pressure and temperature, and from the closed carbon balance. No side-reactions were observed in the temperature range investigated, as secondary decomposition of formic acid to methanol and carbon monoxide, as well as catalyst degradation, are prone to happen only at temperatures above 120◦ C. Although euent compositions were proved stable over several hours, re-esterication artefacts were prevented by direct analysis after sampling. Reaction conversion and yield translate the overall reaction performance summing up concentrations weighted by the volume fractions of each individual liquid phase.

Eects of weight hourly space velocity and catalyst particle size The dierent mass transfer resistances in the packed bed were proofed to ensure kinetic control of the process in the following reaction data. The negligible diusion barrier at the L-L interface was previously shown from the fast solubilisation rate of methyl formate even in presence of laminar parallel ows on the 20-mm distance separating the T-junction from the packed bed entrance. The observation was conrmed by performing blank tests over a packed-bed of inert silicon beads ( 70 − 110 µm) at 110◦ C. The increased turbulence and solubility caused little evolution in the aqueous and organic exiting stream compositions. The low conversion (< 1%) proved not only the fast diusion rates, but also the necessity of an external catalyst. 15



100 90

100 WHSV = 760 h-1 at 1 ml·min-1 WHSV = 760 h-1 at 2 ml·min-1 WHSV = 380 h-1 at 1 ml·min-1

(a)

80

90

70 60 50 40 30

70 60 50 40 30

20

20

10

10

0 20

40

60

80

100

120

Temperature [°C]

Figure 3: Eect of

71-90 µm 90-125 µm 450-500 µm

(b)

80

XMethyl Formate [%]

YMethanol [mmol min-1 g -1 ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 20

40

60

80

100

120

Temperature [°C]

(a) WHSV and (b) catalyst particle size on reaction performance.

Film diusion limitation at the catalyst surface was minimised by high space velocities forcing intensive mixing in the packed-bed. 37 The inuence of residence time on reaction performance was studied by doubling the catalyst loading and/or changing the total feed owrate to achieve WHSV ranging from 380 to 760 h −1 . Results in Figure 3(a) show increasing yields reaching faster equilibrium at longer contact times and/or lower space velocities due to the larger amount of acid sites available for reaction. However, a two-fold increase in supercial velocity and catalyst loading at a xed WHSV of 760 h −1 revealed identical net methanol yields establishing the absence of external mass transfer hindrance. WHSV of 760 h−1 at 1 mL/min was hence used for the subsequent parameter studies. Internal diusion limitation was limited using catalyst particles of 90 − 125 µm, in light of the lower conversions presented in Figure 3(b) with markedly larger particles of 450 − 500 µm. Smaller particles than the former however reached same conversions at the cost of a higher pressure drop. The eect of catalyst swelling on intra-particle resistance was disregarded from the constant swelling extent observed irrespective of the reactant feed composition. Despite the 12% lower swelling ability in pure methyl formate than in water reducing accessibility to the catalytically active sites (Table 1), the water fraction suced to improve swelling under reaction conditions and to dismiss pore diusion hindrance in the case of reactions with methyl formate as continuous carrier phase ( i.e. in volumetric excess). 16


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Eects of temperature and pressure As pressure was changed from 10 bar to 20 bar, no dierence was observed in reaction performance due to minimal liquid density change. A pressure of 10 bar was selected for all further experiments to avoid methyl formate vaporisation, while remaining within the validity frame of the UNIFAC property method. The temperature study spanned 20 − 120◦ C, where the maximum temperature was set to prevent the gradual desulfonisation of the resin and investigate the eect of a gas phase in reaction performance. Methyl formate conversions and methanol yields for the base conditions (10 bar, WHSW = 760 h−1 , dp = 90 − 125 µm) are presented in Figure 4 together with the theoretical equilibrium conversions for various feed compositions. Feed compositions are described as the water:ester ratio in mole basis Rn =

nW /nE and corresponding volume basis Rv = vW /vE . Table 2 reports euent compositions, expressed as volume fraction of aqueous phase collected at each reaction temperature for various feed compositions. 60

100

80

(a)

Water molar excess

(b) 50

14.2

YMethanol [mmol g -1 min-1 ]

90

XMethyl Formate [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30

5.3 3.6 2.7 1.8 0.9

20

40

30

20

10

10 0 20

40

60

80

100

120

Temperature [°C]

0 20

40

60

80

100

120

Temperature [°C]

Figure 4: Reaction performance as a function of temperature and molar feed composition Rn . (a) Equilibrated theoretical (solid lines) and experimental (dashed lines) methyl formate conversions, (b) experimental methanol yields. The vertical line at 55◦ C represents the transition from dual- to single phase euent. Colours stand for dierent feed compositions quantied in terms of molar water excess reported above the corresponding equilibrium conversion line.

17



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Table 2: Euent liquid-liquid volumetric composition at basic conditions.

FEED Water excess Rn

14.2 5.3 3.6 2.7 1.8 0.9

Rv

4.0 1.57 1.0 0.75 0.52 0.25

EFFLUENT Aqueous phase volume fraction [%] 23◦ C 30◦ C 40◦ C 50◦ C 60◦ C 70◦ C 80◦ C 90◦ C 100◦ C 110◦ C 120◦ C 80 68 58 46 20

80 68 58 46 20

84 74 61 46 12

88 92 74 46

HOMOGENEOUS AQUEOUS HOMOGENEOUS ORGANIC

The vertical lines in Figure 4 mark the transition from a two liquid-phase system observed until 50◦ C into a single homogeneous phase from 60−120◦ C. The increasing conversions with temperature resulted in the gradual depletion of methyl formate, and increase in methanol and formic acid concentrations driving the system out of the immiscibility zone. The onset temperature of the single-phase regime did not shift signicantly within the feed compositions studied as visible from Table 2. This eect is attributed to the small variation in phase-equilibrium concentrations dictated by the binodal curve and tie-lines of Figure 2, despite a stark dierence in initial feed compositions. The strong eect of temperature on the extent of reaction associated with the steady evolution at the two- and single-phase regime transition inferred an identical reaction process and conrmed again the absence of mass transfer limitation. The conversions at 50◦ C and 60◦ C, extrapolated to ternary mixtures water-methanol-methyl formate, laid on each side of the binodal curve connecting the phases in equilibrium at room temperature (Figure 2(b)). A gradual shift from kinetic to thermodynamic regime took place as temperature approached 100◦ C for all feed compositions; only the largest educt excess reached equilibrium sooner, at 90◦ C. pH analysis of the organic and aqueous phases conrmed equilibrium status as their pH-values decreased monotonously until stabilising at the same temperatures. Figure 5 traces the general pH evolution in the aqueous phase, although both pHs generally converged towards the same value if not coinciding. As expected from theoretical calculations (solid lines in Figure 4(a)), equilibrium compositions varied only slightly with temperature due to the small equilibrium

18


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constant combined with the weakly endothermic reaction enthalpy ∆Hr◦ (Equation 0.12). The slight drop in activity at 120◦ C is imputed to methyl formate vaporisation at 111 ◦ C causing slower solubilisation in water (c.f. supporting information Table S5). This temperature was included in order to verify the impact of vapourisation in reaction performance, such a decline was indeed not observed at 20 bar. 3 R n=1.8

2.8

R n=2.7 R n=3.6

2.6 2.4 2.2 pH [-]

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2 1.8 1.6 1.4 1.2 1 20

40

60 80 Temperature [°C]

100

120

Figure 5: pH of aqueous phase for the characteristic feed compositions Rn = 1.8 (blue), 2.7 (green), 3.6 (yellow). Same colour scheme as in Figure 4. Both phases pH generally converged towards the same value if not identical.

Eect of initial feed composition Running the hydrolysis reaction in a plug ow mode does not exploit the separator properties of the resin to react beyond the equilibrium by segregating products and prevent re-esterication. Nonetheless, a reactant excess displaces the equilibrium towards product formation as predicted by Le Chatelier principle. The eect of feed composition on reaction equilibrium was investigated by varying the water:ester molar ratio Rn from 0.9 to 14.2. The solubility limit of methyl formate in water at 20◦ C would correspond to a molar ratio of 11, thus the largest ratio investigated in this work, 14.2, speaks for a single-aqueous phase reaction. The lowest value of Rn = 0.9 represents a two-phase reaction regime where the organic uid is in molar excess in contrary to all other ratios. The initial molar ratio compositions are represented by red dots in the ternary diagrams of Figure 2. As depicted on 19



Figure 4(a), except for the fact that a larger excess of one of the educts increased reaction rate, all conversion curves present the same evolution trend, whether homogeneous or biphasic, pointing towards an identical reaction process. Methyl formate equilibrium conversions increased with the magnitude of water excess from 26% at Rn = 0.9 to 81% at Rn = 14.2. The theoretical equilibrium conversions showed a satisfactory t (

Reaction Process of Resin-Catalyzed Methyl ... - ACS Publications

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