Glycerol Etherification with TBA: High Yield to Poly-Ethers Using a

Article pubs.acs.org/est

Glycerol Etherification with TBA: High Yield to Poly-Ethers Using a Membrane Assisted Batch Reactor Catia Cannilla,† Giuseppe Bonura,† Leone Frusteri,‡ and Francesco Frusteri*,† †

Institute CNR-ITAE “Nicola Giordano”, Via S. Lucia 5, Messina I-98126, Italy Dipartimento di Ingegneria Elettronica, Chimica ed Ingegneria Industriale, Contrada Di Dio, Messina I-98166, Italy

‡

ABSTRACT: In this work, a novel approach to obtain high yield to poly-tert-butylglycerolethers by glycerol etherification reaction with tertbutyl alcohol (TBA) is proposed. The limit of this reaction is the production of poly-ethers, which inhibits the formation of poly-ethers potentially usable in the blend with conventional diesel for transportation. The results herein reported demonstrate that the use of a water permselective membrane offers the possibility to shift the equilibrium toward the formation of poly-ethers since the water formed during reaction is continuously and selectively removed from the reaction medium by the recirculation of the gas phase. Using a proper catalyst and optimizing the reaction conditions, in a single experiment, a total glycerol conversion can be reached with a yield to poly-ethers close to 70%, which represents data never before reached using TBA as reactant. The approach here proposed could open up new opportunities for all catalytic reactions affected by water formation.

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INTRODUCTION Recent growth in biodiesel production by transesterification reaction has produced a significant surplus of glycerol, it being the main byproduct of such process.1 As a result of this increased availability, the market price of glycerol has dropped rapidly, and consequently, research has been focused on developing new technologies to convert or use the glycerol to improve the biodiesel business and its economic viability. One of the most interesting ways to revalorize glycerol is the synthesis of oxygenated compounds, such as glycerol ethers by an etherification process with alcohols or short-chain olefins.2,3 Glycerol etherification reactions with isobutylene (IB) as reactant have largely been investigated.4−8 In the presence of homogeneous and preferentially of heterogeneous acid catalysts, a mixture of glycerol tert-butylethers is obtained as a result of three consecutive equilibrium reactions: two monotert-butylethers [1-tert-butoxypropane-2,3-diol (1-MBGE) and 2 tert-butoxypropane-1,3-diol (2-MBGE)], two di-tert-butylethers [1,3-di-tert-butoxypropan-2-ol (1,3-MBGE) and 1,2-ditert-butoxypropan-3-ol (1,2-MBGE)] and tri-tert-butylether [1,2,3-tri-tert-butoxypropane, (TBGE)]. Mono-ethers cannot be blended with diesel due to their low solubility; therefore, it is fundamental to shift the reaction toward the formation of polyethers. Di- and tri-ethers, in fact, can be considered excellent additives for diesel reformulation, being characterized by compatible physicochemical properties in terms of flash point, viscosity, cetane number, and fo forth.9 Moreover, in blend with diesel, their oxygen content enhances the burning properties of fuel, guaranteeing reduced pollutant and particulate matter emission. The use of IB as the reactant in such reactions is conditioned by its high cost and non-renewable nature. In addition, its low solubility in glycerol and its formation of side © 2014 American Chemical Society

products by oligomerization reaction represent important drawbacks to overcome.4−6 On this account, an alternative to prepare glycerol ethers is the use of tert-butyl alcohol (TBA), a byproduct of polypropylene production,10 as O-alkylant agent, instead of IB. This allows us to overcome the technological problems arising from the need to operate with solvents to dissolve glycerol or the need to use pressure to kept IB in the liquid phase. By using TBA as reactant, the etherification reaction takes place according to the following reactions: GLY + TBA ⇆ MBGEs + H 2O

(1)

MBGEs + TBA ⇆ DBGEs + H 2O

(2)

DBGEs + TBA ⇆ TBGE + H 2O

(3)

The dehydration of TBA to IB is the main side reaction, representing an independent reaction that influences tertbutylation through the undesired consumption of TBA and the formation of water:

TBA ⇆ IB + H 2O

(4)

Water is formed in each step with consequent negative effects on catalytic activity and selectivity.11 Water, in fact, inhibits the catalytic reaction, both for thermodynamic restrictions, the reactions being controlled by equilibrium, and for problems related to the stability of catalyst. Received: Revised: Accepted: Published: 6019

November 29, 2013 April 7, 2014 May 5, 2014 May 5, 2014 dx.doi.org/10.1021/es4053413 | Environ. Sci. Technol. 2014, 48, 6019−6026

Environmental Science & Technology

Article

Until now, few papers deal with etherification of glycerol with TBA,11−19 in any case, if high yield to di- and tri-ethers must be obtained, regardless of the catalysts used, when TBA is the reactant, the reaction cannot be carried out without water removal from the reaction medium. Since water is a byproduct in many other catalytic reactions of industrial relevance, different technologies to remove it from reaction mediums have been already suggested: (i) reactive distillation,20 (ii) sorption,21 and (iii) membrane reactors.22 Nevertheless, as concerns the etherification reaction with TBA, very few papers propose the use of one of these technologies. One of the first approaches suggested was, for example, the use of a multifunctional reactor provided with a distillation system,23 but reactive distillation requires high-energy consumption, and is thus not economically feasible.22 Vlad et al. studied the glycerol etherification reaction in a plug flow reactor, using the A-15 resin as catalyst; the product separation was achieved by two distillation columns where high purity ethers were obtained, but a section involving extractive distillation with 1,4-butanediol was required to separate TBA from the alcohol/water azeotrope formed. Moreover, recycle of the mono-ethers was also necessary.24 Another preliminary approach to increase the poly-ethers yield was carried out by removing water from the reaction mixture using zeolites. This method allowed us to shift the equilibrium with a net increase in the DBGEs yield, but zeolites must be regenerated to be used again.14 Also Ozbay et al. obtained a significant enhancement in glycerol conversion and DBGEs selectivity by removing water produced by sorption with zeolite physically mixed with the catalyst and packed into a flow reactor.19 Actually, in literature, the in situ removal of water using membrane reactors has also been proposed and some solution strategies have been suggested to overcome the difficulties encountered by operating under different conditions, varying membrane materials or reactor configuration design, using pervaporation or vapor permeation technique.22 Nevertheless, no application in glycerol etherification reaction has yet been proposed. In this work, a novel approach to produce oxygenated additives with high yield by etherification of glycerol with TBA is proposed. Specifically, a batch reactor coupled with a water permselective membrane has been used.

Figure 1. (A) Ceramic tubular membrane and (B) PVM-035 module designed to hold the membrane.

Membrane Reactor. The etherification reaction has been carried out in liquid phase in a 300 cm3 stainless steel reactor under a stirring frequency of 1200 min−1. The temperature was maintained at 70−80 °C, under autogenous pressure. A welldefined amount of glycerol, TBA and catalyst were loaded into the reactor, under nitrogen flow to remove the air. To reduce the error in determining the exact catalytic data (conversionselectivity), each experiment was repeated three times under the same operating conditions. In order to continuously remove the water formed, the gas phase (containing TBA, H2O and products) was recirculated through the HybSi membrane. The scheme of the membranereactor system used is shown in Figure 2. The membrane and the recirculation loop were maintained at the same reaction temperature. At the permeate side of the membrane, a vacuum of approximately 10 mbar was maintained to avoid the

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MATERIALS AND METHODS Catalysts. The etherification reaction was carried out using a commercial acid ion-exchange catalyst A-15 (Rohm and Haas). Solid acid supported catalyst H730/ES70Y has been prepared using a microspherical silica (PQ Corporation) and Hyflon Ion S4X perfluorosulphic ionomer with an equivalent weight of 730 (Solvay Solexis). In particular, H730/ES70Y catalyst was prepared by impregnation method using an ethanolic solution containing about 20 wt % of Hyflon 730.4 Anhydrous glycerol (purity ≥99.5%) and tert-butyl alcohol (purity ≥99.7%), supplied by Fluka (Buchs, Switzerland), were used as reactants. Membrane Module. HybSi water permselective membrane, with hydrophilic characteristics, was furnished by Pervatech BV. It consists of a γ alumina tubular membrane (25 cm in length) shown in Figure 1A, the top layer of which is hybrid silica coated on the inside of the support tube. The membrane can operate at a maximum temperature of 150 °C and a pressure of 10 bar. In Figure 1B, the PVM-035 module which holds the membrane is shown.

Figure 2. Reactor-membrane separator configuration for etherification reaction with TBA. 6020

dx.doi.org/10.1021/es4053413 | Environ. Sci. Technol. 2014, 48, 6019−6026


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Table 1. Main Physico-Chemical Properties of Catalysts Employed in Glycerol Etherification with TBAa catatyst

active phase loading (wt %)

SABET (m2g−1)

PV (cm3g−1)

APD (Å)

acidity (mequivH+gcat−1)

acidity (mequivH+gion−1)

19

45 204

0.40 0.33

355 188

4.50 0.17

4.50 0.88

b

A-15 H-730/ES70Y a b

SA = surface area and PV = pore volume, determined from the nitrogen adsorption/desorption isotherms at 77 K. APD = average pore diameter. Data supplied by Rohm and Haas.

Table 2. Etherification Reaction of Glycerol with TBAa selectivity (%)

a

catalyst

Xgly (%)

1-MBGE

2-MBGE

1,3-DBGE

1,2-DBGE

TBGE

SD‑T (%)

YD‑T (%)

A-15b H-730/ES70Yb A-15c

78.9 34.1 66.9

69.6 75.1 65.2

2.0 4.7 1.7

22.9 13.9 26.8

4.8 5.4 5.1

0.5 0.9 1.2

28.2 20.2 33.1

22.3 6.9 22.1

RTBA/Gly = 4 mol/mol; Rcat/Gly = 7.5 wt %; reaction time, 6 h. bTR = 70 °C. cTR = 80 °C.

expected, irrespective of the catalyst used (A-15 or H730/ ES70Y), at 70 °C, the glycerol does not reach total conversion (A-15, Xgly = 78.9%), while, at the same time, the mono-ether is the main product formed. In particular, when A-15 is used, the 1-MBGE concentration is about 3-fold higher (69.6%) than the concentration of the main di-ether product (1,3-DBGE, 22.9%). The formation of TBGE is negligible (0.5%). The prevalent formation of 1-MBGE and 1,3-DBGE rather than 2MBGE and 1,2-DBGE is due to the electrophilic attack of the tert-butyl cation (a tertiary carbocation) preferably on the primary carbon of glycerol due to steric hindrance and electrostatic effects exerted by the −OH glycerol group.12−17 In addition, a low amount of isobutylene (∼4.5 wt % in liquid phase) was generated by TBA dehydration (eq 4). By using the H730/ES70Y catalyst, a lower Xgly has been reached (∼34%). Accordingly, such difference in the glycerol conversion value observed by using the two systems is likely ascribable to the lower acidity of H730/ES70Y (0.17 mequivH+/gcat) which is about 25 times smaller than that of A-15 (4.5 mequivH+/gcat) (see Table 1). As known di- and triethers are formed through consecutive equilibrium reaction paths; for this reason, the quite low cumulative yield to polyethers obtained with A-15 (YD‑T, 22.3%) could be justified by thermodynamic restrictions, due to the formation of water either during etherification of glycerol with TBA or by TBA dehydration to IB. Moreover, water, which is the most polar component in the reaction mixture, has higher affinity to protons and therefore it competes with TBA and glycerol adsorption on acid active sites of catalyst, thus preventing the formation of poly-ethers.12 Notwithstanding at higher reaction temperatures, the rate of glycerol conversion should increase due to the endothermic nature of the reaction,27 in the presence of A-15, at 80 °C, Xgly decreases from 78.9% to 66.9% in spite of an increasing of selectivity to poly-ethers (SD‑T) from 28.2% to 33.1%. This result, in agreement with literature data11,12 in terms of glycerol conversion, suggests that at higher temperature, water forms at a higher rate, preventing the activation of glycerol on acid sites by competitive absorption. Therefore, in order to remove the water produced during the reaction and thus to overcome the equilibrium restrictions, the use of a permselective membrane, coupled with the reactor, was exploited. Specifically, the reactor and the membrane separation layouts might be either two physically distinct units or integrated into a single unit. In such a study, it has been proposed that the first configuration (see Figure 2) and, for technological reasons, the external recirculation of the gas-

condensation of water. The recirculation of gas stream was maintained constant using a gas-pump provided with a variable potentiometer. At the end of the experiments, the reactor was cooled down by an ice-bath until the vapor pressure of the mixture came down to the atmospheric pressure, thus allowing all the gas phase compounds to condense. After opening the autoclave, the liquid mixture was analyzed off-line by a gas chromatograph, HP 6890N, provided with a capillary HP Innowax column. An automatic sampler Agilent 7683B Series was used and each data set was obtained, with an accuracy of ±3%, from an average of five independent measurements. The water content was calculated by considering the reaction stoichiometry. The reaction products were also identified by GC/MS analysis using an Agilent 5975C Instrument provided with a capillary column (DB-Waxter). Commercial solutions of MBGE compounds and 2,4,4trimethyl-1-pentene, representative of the di-isobutylene family of compounds, were used as standard references for GC analysis to obtain the corresponding response factors and then response factors of di- and tri-ethers were deduced.

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RESULTS AND DISCUSSION Preliminary glycerol etherification experiments with TBA have been carried out at 70 °C and for 6 h using a TBA/Gly molar ratio of 4 and an amount of catalyst of 7.5 wt % with respect to the glycerol weight.4 Two experiments were performed using the commercial A-15 resin as a reference catalyst and our own production H730/ES70Y catalyst. Specifically, A-15 is a macroreticular strongly acidic resin containing sulfonic groups and it is one of the main catalysts employed in literature for such reaction.11,12,14,21,23 The use of Hyflon-based catalyst in this reaction, indeed, was due to its promising catalytic behavior shown in the glycerol etherification reaction with isobutene.4 In particular, under the same reaction conditions, but using IB as the reactant, H730/ES70Y resulted in much more efficient ability, compared to A-15, to obtain a cumulative yield of polyethers of 97%, with total glycerol conversion, at 70 °C after 6 h of reaction . The Hyflon-based system has been developed by combining the hydrophobic/hydrophilic and acid properties of the perfluorosulfonic ionomer Hyflon and the structural features of microspherical silica used as carrier. The main physicochemical properties of catalysts used are reported in Table 1. Some results obtained in the etherification of glycerol with TBA in terms of glycerol conversion and product selectivity, without the use of membrane, are reported in Table 2. As 6021



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Table 3. Etherification Reaction of Glycerol with TBA in the Presence of the Ceramic Membranea selectivity (%)

a

TR (°C)

Xgly (%)

1-MBGE

2-MBGE

1,3-DBGE

1,2-DBGE

TBGE

SD‑T (%)

YD‑T (%)

70 80

79.2 80.1

61.9 61.9

1.8 1.7

29.2 29.6

6.1 5.6

0.9 1.1

36.2 36.3

28.8 29.1

Catalyst, A-15; RTBA/Gly = 4 mol/mol; Rcat/Gly = 7.5 wt %; reaction time, 6 h.

phase was preferred to the recirculation of the liquid phase.22,25,26 The gas phase leaving the catalytic reactor, containing both the unconverted TBA, the reaction products, and very likely IB formed by the TBA dehydration, enters the membrane for selective and continuous water removal, while the retentate is recirculated to the catalytic reactor. The advantage of this technique lies in the fact that the liquid phase, where the reaction takes place, is not influenced, and furthermore, a low energy consumption to perform the gas recirculation by gas-pump is required. On the contrary, the recirculation of a liquid phase could make things more complex and could compromise the life of the catalyst. The first results obtained with the membrane under the same reaction conditions (TR, 70 °C, RTBA/Gl y= 4 mol/mol; Rcat/Gly = 7.5 wt %; reaction time, 6 h) using A-15 as catalyst are reported in Table 3. At first glance, the results did not seem very interesting, since the increase in YD‑T was quite low, from 22.3% to 28.8% with an increasing in selectivity to 1,3-DBGE and 1,2DBGE from 22.9% to 29.2% and from 4.8% to 6.1%, respectively. Moreover, in spite of the low increase in SD‑T (from 28.2% to 36.2%), the glycerol conversion does not increase, remaining at a value of about 79%. On the contrary, results obtained at 80 °C with the membrane indicate that water removal promotes the glycerol conversion, which increases from 66.9% to 80.1%, and the cumulative yield, which raises from 22.1% to 29.1%. At 80 °C with the membrane, a significant change in the product selectivity was not observed with respect to the results obtained at 70 °C; so in order to exclude that the problem could be ascribable to the low concentration of water in the gas phase which could require a long permeation time for its removal, experiments at different reaction times have been carried out. The results obtained are shown in Figure 3. The increasing reaction time only initially positively reflects on glycerol conversion, which reaches an higher value (85.3%) after 17 h, but, as the reaction proceeds, the Xgly decreases to 72.5% and 56.6% after 27 h and 40 h, respectively. It has also been seen that, although a relatively high glycerol conversion was achieved, the liquid phase recovered after 27 h of reaction was separated in two phases, the upper one mainly containing an ethers mixture, and the lower one containing unreacted glycerol. Moreover, after 40 h, two phases were recovered because of the lower glycerol conversion. However, by increasing the time, a positive effect was observed in terms of poly-ether selectivity, which increased at the expense of monoethers. Particularly, 1-MBGE selectivity decreased, linearly with time reaction, from 61.9 at 6 h to 18.3% after 40 h. At the same time, 1,3-DBGE increased from 29.6% to 43.8% by reaching a plateau after 27 h of the reaction. On the whole, a significant exponential increase in TBGE selectivity from 1.1% after 6 h to 31.7% after 40 h was recorded. By considering that after 27 h, the glycerol conversion decreases, apart from what was expected and from what occurs in glycerol etherification with TBA without the membrane,14 it has been suspected that, for a long reaction time, side reactions

Figure 3. Etherification reaction of glycerol with TBA in the presence of the ceramic membrane: influence of reaction time. Catalyst, A-15; RTBA/Gly = 4 mol/mol; TR = 80 °C; Rcat/Gly = 7.5 wt % with respect to glycerol weight.

could take place, thus causing a substantial modification in the reaction medium. Specifically, as already discussed, in the presence of a solid-acid catalyst (like A-15), formation of isobutene is expected as a result of the dehydration of TBA.28 Likely, the presence of the permselective membrane which removes the water could also promote such reaction, shifting the equilibrium reaction versus the IB formation. Then IB, in its turn, can form oligomers, mainly DIB dimers (C8H16, i.e., a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4,-trimethyl-2pentene) or further TRIB trimer (C12H24, i.e., a mixture of heptane, 2,2,6,6-tetramethyl-4−4methylene or 3 heptene,2,2,6,6,pentamethyl) compounds. According to ref 28, the surface of sulfonated catalysts are very active in the dimerization of IB, and such reaction is considered to be of second-order with respect to the acid capacity. Even if the presence of water generally strongly reduces the rate of IB dimerization reaction,29,30 in such a case, the presence of the permselective membrane could favor and promote this reaction too. Indeed, as shown in Figure 4, it is easy to observe that, after 6 h of reaction in the presence of A-15, only 0.6 wt % of DIB has been formed by IB dimerization, and the unreacted TBA is still present in the reaction medium (36.4 wt %), favoring the unconverted glycerol to dissolve. As the reaction time increases, the TBA decreases (15.1 wt % after 17 h), but glycerol is still 6022



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likely prefers to react with itself. Therefore, in this condition, the oligomerization reaction prevails on glycerol etherification, and the continuous IB conversion to DIB further promotes the TBA dehydration, thus reaching the total consumption of TBA and justifying the “byproduct” distribution observed after 27 h. In addition, when the concentration of TBA is low, a long reaction time could also favor other secondary reactions, e.g., disproportionation of glycerol ethers [DBGEs to MBGEs and TBGE (eq 5)] and, at the same time, the formation of glycerol by splitting of ethers (eq 6).8,11,12 2 DBGE ⇆ TBGE + MBGE

(5)

2 MBGE ⇆ GLY + DBGE

(6)

These reactions could rationalize both the formal “decrease” in glycerol conversion after 27 h and the trend of ethers selectivity observed in Figure 3. In fact, the high selectivity to TBGE is counterbalanced by a linear decrease in 1-MBGE, which likely reconverts in glycerol and di-ethers. At the same time, the almost constant amount of DBGEs compounds observed after 27 h could suggest that the rate at which they are formed is proportional to the rate at which they are converted, showing the typical behavior of intermediate products. The high selectivity to TBGE, observed after 40 h, could also be related to the change in viscosity of the reaction medium. In fact, as the viscosity increases, the mobility of the reactants decreases, thus the isobutene (which after 27 h represents the main O-alkylant agent) dissolved into the ether mixture could further react with the ethers still adsorbed on the catalyst surface to further form tri-ether, according to the following reactions:29

Figure 4. Etherification reaction of glycerol with TBA in the presence of the ceramic membrane: influence of reaction time on “byproducts” distribution. Catalyst, A-15; RTBA/Gly = 4 mol/mol; TR = 80 °C; Rcat/Gly = 7.5 wt % with respect to glycerol weight.

dissolved. After 27 h, TBA almost disappears (1.4 wt %), with a parallel increase in DIB and a small amount of TRIB is also formed. Then, after 40 h, the DIB and TRIB amount reached 39.5 and 2.5 wt %, respectively, showing that under such conditions, the oligomerization of IB does not stop at dimers, but rathergoes on to trimer compounds. The glycerol, still present after 27 h and 40 h, separates from the ether mixture, thus two liquid phases are formed. Really, di-isobutenes have high octane numbers (RON/MON = 100/89) so, potentially, they could be used in gasoline as such, or hydrogenated to isooctane;30 on the contrary, the higher oligomers are not useful components in gasoline because of their high molecular weight and low volatility.31 However, in diesel engines, the presence of IB could lead to the formation of undesirable deposits, therefore they must be removed from the ether mixture before their use5 along with mono-ethers compounds, with consequent negative effects on the economics of the global process. During the etherification reaction of glycerol, the progressive decrease in TBA amount, which represents both the reactant and the solvent of the reaction mixture, likely reflects in an increase in the viscosity of the solution, which is accompanied by mass transfer limitation between liquid−liquid and liquid− solid phases. It should also be considered that, since the reaction is under an autogenous atmosphere, at 80 °C, IB formed by TBA dehydration is only partially dissolved in the reaction mixture in the gas phase. Moreover, IB itself is sparingly soluble in glycerol; thus, at some stage, if the concentration of TBA-IB-GLY and ethers formed are not proper for mutual dissolution, more phases form and then, as the reaction proceeds, the products mainly accumulate in one or more phases, according to their solubility.32 As IB forms, it could react with glycerol, but, because of the higher viscosity of the reaction medium, the contact with the polar glycerol, adsorbed on the catalyst surface, becomes harder so that it

MBGEs + IB ⇆ DBGEs

(7)

DBGEs + IB ⇆ TBGE

(8)

It is noteworthy to point out this increase in tri-ether selectivity by glycerol etherification with TBA, which diverges from the usual trends observed without the use of the membrane, thus allowing it to reach a value (31.7%) which was not reported before in literature under similar reaction conditions. Previous results14 showed that, with A-15 catalyst, only a trace of TBGE (≤2%) formed after 30 h of reaction. The use of zeolite for the water removal between two steps of etherification reactions14 or the in situ water removal by sorption during the reaction, proposed by Ozbay et al., allowed an enhancement in di-ether selectivity but not in tri-ether formation.19 Gonçalves et al. obtained a yield to TBGE of about 19% with the sugar cane bagasse-based catalyst after 4 h at 120 °C; however, also in this case, the cumulative YD‑T still does not reach 46%.17 At this point, the results obtained by increasing the reaction time suggested that the decrease of TBA from reaction medium and consequently the increasing in viscosity of the liquid phase was decisive to control the product distribution. So, in the attempt to avoid such increase in viscosity, an experiment with higher molar ratio between TBA and glycerol (RTBA/Gly) was carried out. The reaction was performed for no more than 27 h, and the results obtained in the presence of A-15, in terms of glycerol conversion, yield to cumulative poly-ethers and products distribution are shown in Figure 5. First of all, at an RTBA/Gly molar ratio of 8, a significant increasing in Xgly from 72.5% to 93.5% was observed with a correspondent increase in the cumulative YD‑T from 43.0% to 53.2%. In addition, the doubling of the amount of TBA has not been counterbalanced by a remarkable increase in DIB production (14.8 wt % @ 4 6023



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7.0%). The correspondent increase in poly-ethers is almost doubled, reaching a value of 53.2%. This result clearly demonstrates that water formed during the reaction selectively permeates through the membrane allowing excellent results to be reached in terms of productivity to poly-ethers. Nevertheless, in the absence of the membrane, the DIB amount formed was very low (0.9 wt %) and the amount of unconverted TBA still remained high, being 58.0 wt %. However, it is noteworthy to underline that the catalyst properties also play a fundamental role in promoting the formation of poly-ethers. In fact, by using H730/ES70Y catalyst further to reach a Xgly close to 100%, the YD‑T reached a very high value, close to 70%, with a selectivity to di- and tri-ethers of 59% and 11.6%, respectively. Moreover, with H730/ES70Y system, also in the presence of the membrane, the formation of DIB by IB dimerization is strongly inhibited with respect to the A-15 catalyst. In fact, as shown in Figure 6B, no presence of oligomerization products has been detected even after 27 h of reaction. On the whole, A-15 is more active than H730-ES70Y without the use of a membrane (see Table 2) because of its higher total acidity, but in the presence of the membrane, H730/ES70Y performance is better. With such a catalyst, by using the membrane, both the etherification of glycerol and the dehydration of TBA reaction take place with higher rates, glycerol easily reaches the total conversion and the cumulative yield to di- and tri-ethers also increases without formation of undesired di-isobutenes. This result differs from that obtained with A-15 catalyst, and the reason could be searched in the surface properties. Very probably, the best performance of Hyflon-based catalyst in promoting the etherification reaction, rather than the side-reactions, could be found in its hydrophobic character and the active site accessibility that could be the determinant result in etherification of glycerol with TBA. The results described in this work clearly demonstrate that the use of a permselective membrane represents an effective way to overcome thermodynamics constraints and that the equilibrium composition could be significantly shifted toward the formation of the final product. In the example reported here, in particular, the membrane was found to be very efficient and selective toward the

Figure 5. Etherification reaction of glycerol with TBA: influence of TBA/Gly molar ratio. Catalyst, A-15; TR = 80 °C; Rcat/Gly = 7.5 wt % with respect to glycerol weight, reaction time, 27 h.

mol/mol vs 17 wt % @ 8 mol/mol). This result clearly suggests that by assuring an high solubility of reactant in the liquid phase, glycerol conversion and poly-ethers formation are favored with respect to the oligomerization reaction. Considering the benefits observed by operating at high a RTBA/Gly molar ratio, an experiment was carried out using the H730/ES70Y catalyst. The results obtained are compared with those obtained by using A-15, both with and without the membrane (see Figure 6). First of all, it is possible to observe that, under such reaction conditions, the use of the membrane was crucial in the etherification of glycerol with TBA to overcome the thermodynamic constraints. Indeed, the glycerol tends to be completely converted, Xgly passing from 84% (without the membrane) to 93.5% (with the membrane) and simultaneously, as the reaction goes on, the selectivity of MBGEs decreases from 64.5% to 43.1% in favor of the formation of DBGEs (33.9% → 49.9%) and TBGE (1.7% →

Figure 6. Comparison of catalytic activity of A-15 and H730/ES70Y catalysts in glycerol etherification reaction: TR = 80 °C; Rcat/Gly = 7.5 wt % with respect to glycerol weight, RTBA/Gly = 8 mol/mol, reaction time, 27 h. A-15* indicated an etherification reaction carried out without the membrane. 6024



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production of poly-ethers in the etherification of glycerol with TBA. Without the membrane, the main product is the monoether; indeed, by recirculating the gas phase through the membrane, the water is continuously and selectively removed and the reaction shifts towards polyethers reaching total conversion of glycerol.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +39 090 624 233; fax: +39 090 624 247; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Italian Ministry of Agriculture, Food, and Forestry for partial funding of this work through the “TERVEG”Project (National funding call “bando bioenergetico DM 246/2007, GU No. 94 del 27/11/2007” and the Ministry of Education, Universities, and Research for partial funding of this work through the “RITMARE” Project (Research National Program 2011-2013)

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ABBREVIATIONS 1-MBGE 1-mono-tert-butyl glyceryl ether 2-MBGE 2-mono-tert-butyl glyceryl ether 1,3-DBGE 1,3-di-tert-butyl glyceryl ether 1,2-DBGE 1,2-di-tert-butyl glyceryl ether DBGEs 1,3-di-tert-butyl glyceryl ether and 1,2-di-tert-butyl glyceryl ether DIB di-isobutylene dimers Gly glycerol IB isobutylene MBGEs 1-mono-tert-butyl glyceryl ether and 2-mono-tertbutyl glyceryl ether SD‑T selectivity to DBGEs and TBGE calculated as moles of glycerol reacted to form DBGEs and TBGE, related to total moles of reacted glycerol TBA tert-butyl alcohol TBGE tri-tert-butyl glyceryl ether TRIB tri-isobutylene trimes Xgly glycerol conversion YD‑T yield to DBGEs and TBGE

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REFERENCES

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Glycerol Etherification with TBA: High Yield to Poly-Ethers Using a

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