Sustainable short-chain olefin production through simultaneous dehydration of mixtures of 1-butanol and ethanol over HZSM-5 and γ-Al2O3

Arno de Reviere1,2, Dieter Gunst1,2 Maarten Sabbe1,2 and An Verberckmoes1

1Industrial Catalysis and Adsorption Technology (INCAT), Department of Materials, Textiles and Chemical Engineering, Ghent University, Valentin Vaerwyckweg 1, 9000 Ghent, Belgium

2Laboratory for Chemical Technology (LCT), Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 125, 9052 Ghent, Belgium

Supporting information:

1. Summary of the work of Gunst et al.

(1) No secondary reaction (oligomerization or cracking) products appear before full conversion of the alcohol [1]. (2) The selectivity towards the linear butene isomers approaches their thermodynamic equilibrium composition with increasing conversion [2]. At lower conversion, the deviation from equilibrium composition of the butenes is due to the high surface coverage of the catalyst by reaction intermediates (adsorbed alcohol monomer, dimer and ether), which suppresses isomerization reactions. At higher conversion, the surface coverage of reaction intermediates decreases, allowing isomerization of the butenes towards equilibrium composition [3]. (3) There is no significant formation of isobutene. (4) Dibutyl ether is an intermediate product, which can further decompose into the butenes [4, 5]. At a conversion below 0.5 mol mol-1, the selectivity towards the ether is relatively high, nevertheless, the decomposition of the ether is not the main contributor to the formation of butenes, but the direct dehydration pathways are [6]. At 513 K, all butene isomers can be formed, with the exception of isobutene, as skeletal isomerization requires higher temperatures [7]

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2. Catalyst characterization

Figure S1: N2-sorption isotherms for HZSM-5 (full lines, black) and γ-Al2O3 (dashed, blue).

Figure S2: SEM-image of the HZSM-5/15, showing aggregated crystals, clearly smaller than 1 µm.

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Figure S3: SEM-image of the γ-Al2O3 catalyst, with particle sizes ranging from 50 – 250 µm.

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Figure S4: NH3-TPD spectrum of γ-Al2O3 (black) and HZSM-5/15 (red), the spectrum consists of weakly (left), medium (middle) and strongly (right) adsorbed NH3 peaks. With increasing desorption temperature, the strength of the adsorption

increases.

Figure S5: NH3-TPD spectra of HZSM-5/15 for multiple heating rates (β), with increasing heating rates, the maximum desorption peak temperature (TM) increases. Blue = 5K/min, green = 8 K/min, red = 12 K/min, orange = 16 K/min, black = 20

K/min.

The desorption energy is calculated through equation (1). A set of desorption experiments is performed at multiple heating rates to solve this equation for both γ-Al2O3 and HZSM-5. For HZSM-5 TM is much more dependent on the heating rate than for γ-Al2O3. Therefore the NH3 desorption energy is higher on HZSM-5.

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Figure S6: XRD-spectrum of the γ-Al2O3 used in this work (top) and ICDD reference spectrum 01-074-2206 (bottom).

Figure S7: XRD-pattern of the HZSM-5 used in this work (top) and ICDD reference sample 00-044-0003 (bottom).

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3. Equilibrium considerations for the dehydration reactions

Table S1: Overall reactions and equilibrium constants at 533 K, Keq determined with values obtained from the NIST database.

Reaction Stoichiometry Keq

1 1-butanol (g) ⇋ 1-butene (g) + H2O (g) 52352 2 *1-butanol (g) ⇋ dibutyl ether (g) + H2O (g) 5.83 dibutyl ether (g) ⇋1-butanol (g)+1-butene 909.44 ethanol (g) ⇋ ethene (g) + H2O (g) 378.65 2*ethanol (g) ⇋ diethyl ether (g) + H2O 42.36 diethyl ether (g) ⇋ ethanol (g) + ethene (g) 8.97 ethanol (g) + 1-butanol (g) ⇋ ethyl butyl ether (g) + H2O (g) 39.38 ethyl butyl ether (g) ⇋ 1-butene (g) + ethanol (g) 137.89 ethyl butyl ether (g) ⇋ ethene (g) + 1-butanol 9.7

10 1-butanol (g) ⇋ trans-2-butene (g) + H2O (g) 1953711 1-butanol (g) ⇋ cis-2-butene (g) + H2O (g) 11064

The equilibrium constant for the intramolecular dehydration reactions are very high. As these reactions are dominant at every temperature for HZSM-5, there are no equilibrium considerations needed The ether formation reactions on the other hand, have a much lower equilibrium constant, i.e. the ethers are thermodynamically less stable, but contribute heavily to the product spectrum of γ-Al2O3. Therefore, the following paragraphs in this section only consider results for γ-Al2O3.

For the dehydration of pure 1-butanol, the quotient of the partial pressures of the products and reagents of reaction 2 (QDBE) are shown in Figure S8, to illustrate that at 533 K reaction 2 is far from equilibrium, at 553 K reaction 2 reaches equilibrium, and at 573 K the reaction is far from equilibrium due to the shift in dominant reaction pathways. At 573 K, both kinetics and thermodynamics favor reaction 1 and 3, hence why reaction 2 is far from equilibrium throughout the conversion range.

Figure S8: Evolution of QDBE versus conversion for the dehydration of 1-butanol over γ-Al2O3 at 533 (left), 553 (middle) and 573 K (right).

For the dehydration of 6/1 1-butanol/ethanol mixtures, we obtained similar results as for the dehydration of pure 1-butanol: at low temperature and low conversion, the Q’s are far from equilibrium and the selectivity towards ethers is dominant, i.e. the process is kinetically controlled. At higher temperatures, 573 to 613 K, and high conversion (> 0.9 mol mol -1) QDBE, QDEE and QEBE

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approach equilibrium (Figure S9). Therefore, at these conditions the process is under thermodynamic control.

Figure S9: Evolution of QDBE (top), DDEE (middle) and QEBE (bottom) versus conversion for the dehydration of a 6/1 1-butanol/ethanol mixture over γ-Al2O3 at 573 (left), 593 (middle) and 613 K (right). Due to the large uncertainty of Q

beyond 0.95 mol mol-1, the conversion is capped at 0.95 mol mol-1.

4. Apparent activation energy calculationsThe apparent activation energies for the formation of 1-butene and for dibutyl ether are calculated by assuming that the formation rates of 1-butene(r1-butene) and dibutyl ether (rDBE) are:

r1−butene=k1−butenePBuOH (S1)

rDBE=kDBE PBuOH2 (S2)

With k1-butene and kDBE the reaction rate constants for the formation of 1-butene and dibutyl ether. Furthermore, the formation rates of 1-butene and dibutyl ether can be calculated from

r1−butene=F1−buteneW Ca

(S3)

rDBE=FDBEW Ca

(S4)

With F1-butene and FDBE (mol s-1) the flowrate of respectively 1-butene and dibutyl ether in the reactor effluent, W (kg) the weight of the catalyst and Ca (mol kg-1) the active site density of the catalyst.

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By using the Arrhenius equation, the apparent activation energy (Ea,app) and pre-exponential factor (A) are determined by plotting ln(k) for both k1-butene and kDBE as function of T-1.

k=Ae−E a,appRT (S5)

The apparent activation energies are: 169.0 kJ mol-1 for the formation of 1-butene and 154.9 kJ mol-1 for the formation of dibutyl ether.

5. ROE comparison of pure 1-butanol and 1-butanol/ethanol mixtures

Table S2: ROE for the dehydration of 1-butanol and ethanol over HZSM-5 at different temperatures and conversions, the ROE is compared at identical conversion of both alcohols, however these conversions are obtained at different site times due to the difference in reactivity.

T (K)BuOH EtOH

X (mol mol-1) ROE,HZSM-5

X (mol mol-1) ROE,HZSM-5

513 0.3 1.87 0.3 1.6533 0.5 6.85 0.5 1.5553 0.8 >100 0.8 7.5

Figure S10: ROE,HZSM-5 vs XBuOH for mixture as feed (red dots) and a single 1-butanol feed (blue dots) at 240 °C.

The ROE,HZSM-5 is defined as the selectivity towards butenes over the selectivity towards dibutyl ether, thus allowing to assess the relative selectivity towards alkenes over ether for both the pure feed as

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the mixture of 1-butanol and ethanol. The “mixture” is a feed composed of 1-butanol and ethanol in a 6/1 mass-ratio, as found in the ABE fermentation.

Figure S11: ROE;Al2O3 versus XBuOH for single 1-butanol feed (blue circles) and a mixture (red circles) at 260 °C.

The ROE,Al2O3 is defined as the selectivity towards butenes over the selectivity towards dibutyl ether, thus allowing to assess the relative selectivity towards alkenes over ether for both the pure feed as the mixture of 1-butanol and ethanol. The “mixture” is a feed composed of 1-butanol and ethanol in a 6/1 mass-ratio, as found in the ABE fermentation.

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Figure S12: ROE;Al2O3 versus XBuOH for single 1-butanol feed at 280 °C (blue circle) and 300 °C (grey diamond) and a mixture at 280 (red circle) and 300 °C (yellow diamond).

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6. Proposed reaction mechanism for the dehydration of BuOH/EtOH mixtures

Table S3: Elementary reaction steps and reaction mechanisms for B/E dehydration over HZSM-5, subscript B is used for 1-butanol, subscript E is used for ethanol.

Step PathAB AE B BE CB CE DB DE

(1) BuOH(g) + * ↔ M1B 1 0 1 0 1 0 0 0(2) M1B ↔ M2B 1 0 0 0 0 0 0 0(3) M2B ↔ 1-butene* + H2O 1 0 0 0 0 0 0 0(4) 1-butene* ↔ 1-butene(g) + * 1 0 0 0 0 0 1 0(5) M1B + BuOH(g) ↔ DB1 0 0 1 0 0 0 0 0(6) D1B ↔ D2B 0 0 1 0 0 0 0 0(7) D2B ↔ DBE* 0 0 1 0 0 0 0 0(8) DBE* ↔ DBE(g) 0 0 1 0 0 0 -1 0(9) DBE* ↔ C1B 0 0 0 0 0 0 1 0

(10) C1B ↔ 1-butene* + BuOH(g) 0 0 0 0 0 0 1 0(11) EtOH(g) ↔ M1E 0 1 0 1 0 1 0 0(12) M1E ↔ M2E 0 1 0 0 0 0 0 0(13) M2E ↔ Ethoxy 0 1 0 0 0 0 0 0(14) Ethoxy ↔ Ethene* 0 1 0 0 0 0 0 0(15) Ethene* ↔ Ethene(g) 0 1 0 0 0 0 0 1(16) M1E ↔ D1E 0 0 0 1 0 0 0 0(17) D1E ↔ D2E 0 0 0 1 0 0 0 0(18) D2E ↔ DEE* 0 0 0 1 0 0 0 0(19) DEE* ↔ DEE(g) 0 0 0 1 0 0 0 -1(20) DEE* ↔ C1E 0 0 0 0 0 0 0 1(21) C1E ↔ Ethene* 0 0 0 0 0 0 0 1(22) M1B ↔ D1BE 0 0 0 0 1 0 0 0(23) D1BE ↔ D2BE 0 0 0 0 1 0 0 0(24) D2BE ↔ EBE* 0 0 0 0 1 0 0 0(25) M1E ↔ D1EB 0 0 0 0 0 1 0 0(26) D1EB ↔ D2EB 0 0 0 0 0 1 0 0(27) D2EB ↔ EBE* 0 0 0 0 0 1 0 0(28) EBE* ↔ EBE(g) 0 0 0 0 1 1 -1 -1(29) EBE* ↔ C2BE 0 0 0 0 0 0 1 0(30) EBE* ↔ C2EB 0 0 0 0 0 0 1 1(31) C2BE ↔ 1-butene* + EtOH(g) 0 0 0 0 0 0 1 1(32) C2EB ↔ Ethene* + BuOH(g) 0 0 0 0 0 0 1 1

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7. Proton affinity and carbenium ion stability definitions

The proton affinity (PA) and carbenium ion stability (CIS), calculated by Kostestkyy et al. were defined as[8]:

PA = |H protonatedalcohol−Halcohol| (S6)

CIS = |H carbeniumion−Halkene| (S7)

Where H is the total enthalpy of the system, so either the total enthalpy of alcohol, and the corresponding protonated alcohol, or the total enthalpy of an alkene or the corresponding protonated alkene (i.e. carbenium ion).

The proton affinity is a parameter that quantifies how easily the alcohol is protonated. With increasing PA or CIS, the reactivity of the alcohol increases. Both parameters can be used interchangeably as reactivity descriptors for alcohol dehydration.

To allow the reader to obtain a better understanding of both PA and CIS and why they can be used as reactivity descriptors, we describe in this section what these parameters mean: the PA gives insight in how easily the alcohol is protonated, which is an important step in the dehydration mechanism. The carbenium ion stability quantifies how stable the carbenium ion of the corresponding alkene is, this essentially hints at how stable intermediates with carbenium ion character are. If these intermediates are more stable, the activation energy to form these species is lower, which enhances the reactivity.

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