Rational Optimization of Reaction Conditions for the One-Pot

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Kinetics, Catalysis, and Reaction Engineering

Rational optimization of reaction conditions for the onepot transformation of furfural to #-valerolactone over Zr-Albeta zeolite: Towards the efficient utilization of the biomass Juan A Melero, Gabriel Morales, Jose Iglesias, Marta Paniagua, and Clara López-Aguado Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02475 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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Industrial & Engineering Chemistry Research

Rational optimization of reaction conditions for the one-pot transformation of furfural to γ-valerolactone over Zr-Al-beta zeolite: Towards the efficient utilization of the biomass Juan A. Melero*, Gabriel Morales, Jose Iglesias, Marta Paniagua, Clara López-Aguado Chemical and Environmental Engineering Group, Universidad Rey Juan Carlos; C/ Tulipán s/n, E-28933 Móstoles, Madrid, Spain *To whom correspondence should be addressed E-Mail: [email protected] Tel.: +34 91 665 50 83 Fax: +34 91 488 70 68

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 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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KEYWORDS: gamma-valerolactone, GVL, furfural, optimization, Zr-Al-Beta zeolite, cascade reaction.

ABSTRACT

The optimization of the production of γ-valerolactone (GVL) from furfural (FAL) through a cascade of transformations involving hydrogen transfer and different acid-driven reactions has been tackled by using a bifunctional Zr-Al-beta zeolite as catalyst. The study involved the simultaneous evaluation of the influence of the main reaction parameters affecting the performance of the selected catalyst, including temperature, catalyst loading, furfural concentration and reaction time. An experimental design methodology was applied, aiming to maximize the performance of the catalyst in terms of GVL selectivity and efficient use of the biomass resource (minimizing the non-desired products), herein denoted as “selective productivity”. The effects of the studied reaction parameters on each response factor have been obtained and discussed. The ratio furfural/catalyst appears as the key parameter governing the performance of the catalyst system. Under the optimized reaction conditions, the maximum value achieved for GVL selective productivity is 0.61, corresponding to a SGVL of 70.0% and a productivity of 0.88 gGVL·gCAT-1.


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INTRODUCTION Lignocellulosic biomass is the cheapest and most abundant form of terrestrial biomass and, consequently, its cost-effective exploitation to obtain a wide variety of chemical products with potential use in many different industrial sectors, is very attractive. Starting from lignocellulose, there are different chemical strategies for its valorization, leading to a range of valuable platform molecules.1–8 Such compounds including furanic species coming from the hemicellulosic fraction, can be used as precursors for the production of a wide variety of so-called bio-based products. In this context, furfural (FAL), which has been considered as one of the top 30 platform chemicals derived from biomass,9,10 is a highly interesting platform molecule. It is currently produced at industrial scale from hemicelluloses through well-established methods.11–15 Among the many different valorization approaches, FAL can be reacted through combined acid and reduction pathways into value-added chemicals such as furfuryl alcohol, furfuryl alkyl ethers, levulinic acid, levulinates, α/β-angelica lactones or γ-valerolactone (GVL). Particularly, GVL arises as an appealing compound due to a remarkable combination of physicochemical properties, low toxicity and biodegradability. Actually, in the last years GVL has been identified as a versatile renewable chemical finding potential use as green solvent, fuel additive or precursor for the production of other biofuels and value-added chemicals.2,16,17 The conversion of furfural into GVL involves multiple reaction steps. One of the options includes starting from a hydrogenation step of FAL to furfuryl alcohol (FOL) which, in aqueous media, evolves by hydration to α/β-angelica lactones (ANG), and through further isomerisation to levulinic acid (LA).17 Afterwards LA can be converted into GVL via a second hydrogenation step passing through the intermediate 4-hydroxypentanoic acid, which is rapidly converted to GVL via lactonization. This cascade of reactions from FAL to GVL includes alternating acid-


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catalyzed and reductive hydrogenating reactions. Both hydrogenation steps are commonly carried out via gas H2, using high-pressure processes and requiring noble metal catalysts. An alternative process strategy, to avoid the use of pressurized H2 and expensive catalysts, is the catalytic transfer of hydrogen, in particular a Meerwein-Ponndorf-Verley (MPV) reduction.18 In this reaction, a sacrificing alcohol, most predominantly a secondary alcohol, is used as hydrogen donor. An interesting approach for such a cascade process is the use of multifunctional catalysts, i.e. carrying different active sites on the same material. This enables the possibility of fulfilling the requirements for the individual steps to achieve a single step process in which all the chemical transformations occur. This operating mode avoids the use of intermediate separation and purification steps, which would significantly reduce the overall costs and energy demand of the process. In this way, the direct transformation of furfural into GVL in a single reaction step requires an appropriate catalyst, able to complete the whole transformation (Scheme 1) by promoting each single transformation with a high yield and selectivity. Some of the steps require the use of Brønsted acid sites, like those typically displayed by aluminum-containing acid zeolites, such as H-Beta, H-ZSM-5 or H-Y zeolites. On the other hand, MPV-type hydrogen transfer reactions are typically promoted by solid Lewis acid sites, among others, such as those in Ti-, Sn- and Zr-containing catalysts.19–23 Therefore, the simultaneous incorporation of both functionalities within the same zeolite framework would favor the sequential conversion of the intermediates in order to increase the selectivity and efficient production of the final product, GVL.24


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Scheme 1. Proposed cascade transformation of furfural to GVL through alternative acid catalyzed and MPV reactions in alcohol media (2-propanol), based on reference 17.

In this context, the one-step production of biomass-derived chemicals over bifunctional zeolites has been recently investigated by several authors. Antunes et al studied the transformation of FAL into bio-products over bifunctional (Lewis-Brønsted) Sn-Al and Zr-Al containing beta zeolite and mesoporous Zr-Al-Beta.17,25,26 However, they did not report the production of GVL over any of these materials under the studied reaction conditions. On the other hand, Winoto et al and Song et al successfully carried out the whole cascade of reactions using similar Zr-Al materials by improving the physicochemical properties of the bifunctional material and tuning the reaction conditions.27,28 In previous works, we have provided a family of bifunctional Zr- and Al-containing beta zeolite catalysts, prepared via post-synthetic modification of a commercial parent beta zeolite, able to achieve the direct production of GVL in isopropanol, not only from furfural but also from xylose.29 In this way, tuning both the catalyst synthesis and the reaction conditions we were able to maximize GVL production from xylose, providing an optimum GVL yield of 34 mol%.30 However, from an industrial point of view, the availability of isolated xylose is limited due to its


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high reactivity, being much more attractive the use of other well-established biomass-derived platform molecules, such as furfural or levulinic acid. Herein we present the optimization of the production of GVL starting from furfural, using a Zr-Al-beta zeolite as catalyst, which was prepared according to a previously optimized method.

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The optimization procedure (based on

an experimental design methodology) involved determining the influence of the main reaction variables (temperature, catalyst loading, furfural concentration and reaction time) on two different response factors: selectivity towards GVL, and efficient production of GVL (selective productivity). Thus, rather than in the yield, we have focused on the selectivity, as well as on the highest GVL production -on a catalyst weight basis- while maximizing the efficient use of the biomass precursor, i.e. minimizing the formation of non-desired by-products.

EXPERIMENTAL SECTION Catalysts preparation The preparation of the zirconium-modified beta zeolite (Zr-Al-Beta) has been previously reported in literature.30 Briefly, H-Beta zeolite (Si/Al=22) from Zeolyst International was selected as support for zirconium incorporation. Dealumination of the commercial zeolite was performed under aqueous nitric acid (6.5 M HNO3, 25 °C, 20 mL·g-1) (60% aq. HNO3, Scharlau). After washing with deionized water, the resulting material was recovered by centrifugation and dried overnight (110 °C). Thereafter, the incorporation of Zr sites was performed by bringing into contact the partially-dealuminated zeolite with zirconium nitrate (Chemical Point), used as Zr precursor. Zirconium species in the impregnation solution were equivalent to extracted Al, as determined by ICP-OES. The solid was dried under vacuum


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overnight and calcined in air, first at 200 °C (6 h, 3 °C·min-1) and then at 550 °C (6 h, 3 °C·min1

). Characterization of the catalyst By means of Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) metal

contents (Zr and Al) were determined, using a Varian Vista AX spectrophotometer. Textural properties were estimated from Ar adsorption‐desorption isotherm (87 K), using an AutoSorb AS1 equipment (Quantachrome Instruments). The estimation of mean pore size was done by using the non-local DFT calculation, assuming a kernel model of Ar at -186 °C on a zeolite/silica (spherical and cylindrical pores, NLDFT adsorption model), total pore volume was calculated at P/P0 = 0.98. X-ray powder diffraction (XRD) pattern was acquired on a Philips X‘pert diffractometer using the CuKα line (2θ angle range 5° to 65°, step size 0.04°). A Philips Tecnai20 electronic microscope operating at 200kV was used to obtain transmission electron microscopy (TEM). Acid sites properties of the catalyst was evaluated by TemperatureProgrammed Desorption (TPD) of NH3 in a Micromeritics 2910 apparatus with TCD detector. Likewise, acid sites characterization was completed by using Diffuse Reflectance Infra-red Fourier Transform (DRIFT) spectroscopy of chemically-adsorbed pyridine, as molecular probe, using a ThermoScientific Nicolet iS50 FT-IR spectrometer. Table 1 summarizes the most relevant physicochemical properties for the parent Beta zeolite and the Zr-Al-Beta zeolite prepared under the optimal synthesis conditions reported in our previous work.30 Structural and spectroscopic characterization of the catalyst corroborated the preservation of the zeolite network as well as the incorporation of zirconium correlated to aluminum vacancies. No evidence of large domains of the zirconium oxide could be detected (Table 1 and Figures S1-S3). The leaching of Al species from the BEA structure, combined with


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the incorporation of Zr species, decrease the acidity of the material. Likewise, changing Al species by Zr species decreases the Brønsted to Lewis (B/L) acid sites ratio in the material (Figure S4), as the Brønsted sites are directly related to the Al content, whereas Zr provides Lewis acidity. Noteworthy, though Lewis acidity is clearly predominant in the resultant Zr-AlBeta, the modified zeolite still contains enough Brønsted sites to catalyze the acid steps involved in the cascade of reactions, as previously shown for the conversion of xylose into GVL.30 Table 1. Properties of parent and modified zeolite. Compositiona Catalyst

BET areab (m2 g-1)

Total pore volumec (cm3 g-1)

(mmol H+g-1)d

B/L ratioe

Acidity

% Al

% Zr

Si/Al

Si/Zr

Al/Zr

2.0

0.0

22

-

-

623

0.36

0.41

0.56

0.27

4.5

156

32

0.20

685

0.38

0.29

0.05

Beta (commercial) Zr-Al-Beta (synthesized) a

Aluminium and zirconium content (% w/w); Si/Al, Si/Zr, Al/Zr (atomic ratios) as determined by ICP-OES. b B.E.T. surface area. c Total pore volume at P/P0 = 0.98. d Acid sites concentration by NH3-TPD. e Brønsted/Lewis acid sites ratio determined by FTIR using pyridine.

Catalytic tests Catalytic runs were performed in a stainless steel stirred autoclave (200 mL) fitted with a temperature controller and a pressure transducer. Typically, the selected amount of furfural (FAL, Sigma Aldrich, 99%) and 100 mL of 2-propanol (Scharlau, 98%) were loaded together, followed by the addition of the corresponding amount of catalyst. n-Decane (Acros Organics >99%) was also added as internal standard (1 g·L-1). In order to avoid mass transfer limitations, stirring rate was fixed at 1000 rpm, and the reaction mixture was heated to the desired temperature (approx. 30 min heating time). Pressure was allowed to evolve under autogenous conditions, depending on the reaction temperature. Reaction aliquots were periodically


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withdrawn and filtered prior to their analysis. A factorial design of experiments was carried out for the study of the following variables and experimental ranges: temperature (130-170 °C), catalyst loading (5-15 g·L-1) and starting FAL concentration (6-44 g·L-1).

Products analysis Reaction samples were analyzed by means of gas chromatography (GC), using a Varian 3900 gas chromatograph fitted with an Agilent CP-WAX 52 CB column (30 m x 0.25 mm, DF=0.25 µm) equipped with a FID detector. The following commercial materials were used to obtain GC calibration curves: furfural (FAL, Sigma Aldrich, 99), furfuryl alcohol (FOL, Sigma Aldrich, 98%), isopropyl furfuryl ether (FE, Manchester Organics, 97%), levulinic acid (LA, Sigma Aldrich 98%), α/β-angelica lactone (ANG, Sigma Aldrich, 98%) and γ-valerolactone (GVL, Sigma Aldrich, 99%). Additionally, isopropyl levulinate (ILEV), non-commercially available, was synthetized by esterification of LA with 2-propanol using sulfuric acid as catalyst in 98% purity.31 Quantification of reaction samples was performed by using the calibration of standard solutions of each compound with n-decane (internal standard). The performance of the catalyst is presented in terms of furfural conversion (XFAL) or selectivity to the different products (Si). The following formulas include the mathematical definitions of these parameters: =

× 100

(1)

= × 100

(2)


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Optimization of the reaction conditions In order to optimize the operating conditions to maximize GVL production using Zr-Al-Beta zeolite, a response-surface methodology was applied. As independent reaction variables (factors) the following were selected: temperature (X1), catalyst loading (X2), and FAL concentration (X3). Table 2 lists the studied experimental ranges of each factor, based on previous reported results,29 and the corresponding levels. We performed a face-centered composite design of experiments consisting of 18 reaction runs, with 8 factorial points, 6 star points, and 4 cube center replicas. Table 2. Factors and levels corresponding to the selected full factorial face-centered composite design utilized for the optimization of GVL production over Zr-Al-Beta. Coded levels Factor -1

0

+1

130

150

170

X1

Reaction temperature (°C)

X2

Catalyst loading (g·L-1)

5

10

15

X3

Starting FAL concentration (g·L-1)

6

25

44

The experiments were randomized to avoid unexplained variability in the observed response. Experimental results were evaluated by using response-surface methodology, applying a secondorder polynomial equation:

= + ∑

!

· + ∑

!

· " + ∑

! ∑#

· ·

(3)

where S is the response (e.g., GVL selectivity, % mol) and β0, βn, βnn and βnm are the coefficients of the mathematical model, representing the intercept, and the interactions (linear,


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quadratic, and binary), respectively. Xn and Xm are the independent factors. In order to calculate the regression coefficients an analysis of variance (ANOVA) was carried out. A total error criteria with a confidence level of 95.0% was applied.

RESULTS AND DISCUSSION Optimizing the reaction system i) GVL selectivity: Maximizing the transformation of furfural to GVL Within this work, the optimization of the reaction conditions to reach the highest selectivity towards GVL in the transformation of furfural with a bifunctional Zr-Al-Beta zeolite has been tackled. With this purpose, it is essential to understand the influence of the main reaction conditions, so that three operation factors have been identified as the most significant reaction variables affecting the selectivity to GVL: reaction temperature, catalyst loading, and starting furfural concentration. These variables have been modified according to a factorial design of experiments to obtain a mathematical model correlating the GVL selectivity with the reaction conditions in the evaluated experimental field. Besides providing a predictive model representing the system, this study shows the influence of each factor on the catalytic performance of the zeolite. Table 3 lists the experimental design and the obtained selectivity towards GVL after 24 h for each set of values of the experimental factors.


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Table 3. Full factorial face-centered composite design matrix of three variables and the observed response after 24h of reaction in the optimization of GVL selectivity over Zr-Al-Beta zeolite. Experimental factors a Run

a

Response

X1

Ta (°C)

X2

CCAT(g·L-1)

X3

CFAL(g·L-1)

SGVL b

1

0

150

0

10

0

25

17.9

2

0

150

0

10

0

25

17.4

3

0

150

0

10

0

25

19.3

4

0

150

0

10

0

25

17.0

5

-1

130

-1

5

-1

6

42.5

6

-1

130

-1

5

1

44

2.2

7

-1

130

1

15

-1

6

57.6

8

-1

130

1

15

1

44

4.0

9

1

170

-1

5

-1

6

70.0

10

1

170

-1

5

1

44

1.0

11

1

170

1

15

-1

6

72.1

12

1

170

1

15

1

44

20.3

13

-1

130

0

10

0

25

15.8

14

1

170

0

10

0

25

39.9

15

0

150

-1

5

0

25

5.8

16

0

150

1

15

0

25

43.4

17

0

150

0

10

-1

6

68.0

18

0

150

0

10

1

44

5.9

Coded factors shown as Xi. b Selectivity to GVL at 24h.

An important issue in an experimental design is to check the quality and dispersion of the obtained data. Thus, variability of the catalytic test and analytical procedure was assessed, comparing the results obtained in four replicas of the central point (Runs #1-4). Furthermore, the progress of GVL selectivity with time in the catalytic assays under the reaction conditions


12

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corresponding to the center of the experimental field is represented in Figure S5. Repeatability of the catalytic tests is considered to be good, as far as the average standard deviation of SGVL from the four replicas for every given reaction time is relatively small (time-averaged SD = ± 0.92%). To predict the value of GVL selectivity as a function of the operation conditions, the experimental values at 24 h were fitted to a quadratic model. The following mathematical equation is the result of the data reconciliation with the model at 95% confidence level:

$% &%( = 21.7 + 8.1 · ! + 7.6 · " − 27.7 · + 2.4 · !" − 3.4! · + 11.4 · " (4)

In this mathematical expression, SGVL represents the GVL selectivity (mol%) and Xi refers to the independent coded factors, which adopt values between -1 and +1 (Table 2). The value of the overall regression coefficient, r2 = 0.937, indicated a relatively high degree of correlation between the experimentally observed and model-predicted values for GVL selectivity (Figure S6). Attending to the statistical analysis (Pareto Chart, Figure S7), only the significant effects and interactions have been considered in Eq. (4). It must be noted, however, that this mathematical model and its accuracy can only be guaranteed within the explored experimental region. Statistical analysis of the model identifies the starting FAL concentration (X3) as the most important variable having a remarkable negative effect on the GVL selectivity. This means that an enhancement of the starting FAL concentration produces an important decrease in the selectivity towards GVL, regardless the rest of studied variables. This effect can be attributed, at least partially, to the competition of FAL with the solvent isopropanol, since it is also part of the reaction sequence as hydrogen donor for the MPV reduction. Indeed, in the two catalytic


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hydrogen transfer steps (production of FOL-FE and GVL), the Zr active sites need to have isopropanol adsorbed to be able to do the intermolecular hydrogen transfer. An increase in FAL concentration would thus interfere in the reaction progress by reducing the availability of free active sites for isopropanol adsorption. In contrast, the temperature (X1) and the catalyst loading (X2) both positively influence the GVL selectivity, though these effects are less important than the starting FAL concentration. In addition, the quadratic effect of starting FAL concentration has a significant positive influence, indicating that the increase in this operating variable does not produce a constant drop in GVL selectivity. The analysis of model-derived response surfaces has also been used to evaluate the influence of the multiple operation parameters. Figure 1 represents the response surfaces and contour plot corresponding to Eq. (4), showing the predicted influence of the selected factors (temperature, catalyst loading and furfural concentration) on the obtained selectivity towards GVL. Figure 1A clearly shows the above-commented predominant effect of the initial concentration of furfural: the lower the starting FAL concentration, the higher the SGVL. Setting the starting concentration of furfural at the lowest level (6 g·L-1), the effect of the other two variables can be studied (contour plot in Figure 1B). As commented, both factors, temperature (X1) and catalyst loading (X2), have a positive contribution in the model. Thus, the higher the temperature and the catalyst loading, the higher the SGVL. Nevertheless, this contribution is significantly lower than the opposing effect coming from furfural concentration.


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A) ity (%) GVL Selectiv

90

Ca ta

B)

75 60 45 30 15 0 5.0

lys t

170 160

7.5

150

0.0

1 loa 5 din 12. g( gL -1 )

140 0 15.

130

m Te

pe

re tu a r

) (ºC

170 GVL Yield (mol %)

165 160

Temperature (ºC)

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


44.10

155

48.88 53.65

150

58.42 63.20

145

67.97 72.75

140

77.53

135 130 5.0

82.30

7.5

10.0

12.5

15.0

-1

Catalyst loading (gL )

Figure 1. A) Response surfaces obtained for the predicted influence of the selected factors on the selectivity to GVL at 24 h, as a function of the starting furfural concentration (green: 6 g·L-1; purple: 25 g·L-1; orange: 44 g·L-1). B) Contour plot obtained for the predicted influence of the selected factors on the selectivity to GVL at 24 h, as a function of the temperature and the catalyst loading (starting furfural concentration 6 g·L-1).


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On the other hand, the analysis of the evolution with time of the distribution of products as a function of FAL concentration (Figure S8) evidences that lower furfural loadings lead to a faster progression of the proposed cascade of reactions from FAL to GVL, clearly providing higher yields and selectivities towards GVL. Indeed, at higher starting FAL concentrations, the main final product, even after long reaction times (24 h), is the isopropyl furfuryl ether (FE), an intermediate product. This suggests that under such reaction conditions, the limiting step is the acid-catalyzed reaction of furfuryl ether into levulinic acid/levulinates. A feasible explanation for this behavior is that the amount of Brønsted acid sites (Al centers) in the optimized bifunctional Al-Zr-Beta zeolite used as catalyst is relatively small (Table 1), especially as compared to Zr sites (Zr/Al = 0.20). The deactivation of such a small quantity of Al sites in a more concentrated FAL medium is more likely to occur than in the most diluted system (via deposition of carbonaceous compounds, such as humins, whose formation is usually linked to strong acid catalysis). In such a scenario, under high FAL concentrations, the limited availability of nondeactivated Al sites on the catalyst would slow the transformation of furfuryl ether (FE) into levulinic acid/levulinate (LA/ILEV), leading to lower GVL yields. Additionally, the abovecommented effect of reactants competition would be more significant at high FAL concentration. Applying a mathematical optimization to the model [Eq. (4)], the optimum value for the selectivity to GVL at 24 h, in the experimental region, corresponds to a reaction temperature of 170 °C, a catalyst loading of 15 g·L-1, and a starting furfural concentration of 6 g·L-1 (X1 = +1, X2 = +1, X3 = -1, in coded values). It must be noted that under these reaction conditions, the highest value of GVL selectivity, slightly over 70 mol%, is already reached after 4 h of reaction (Figure S9). From this experiment, it is also concluded that the produced GVL is stable in the catalytic


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system, not decaying its concentration in the period from 4h to 24h in the reaction media at the highest temperature (170 °C). As shown, the optimization of selectivity towards GVL requires, for a given catalyst loading, the use of low furfural concentrations. However, from a point of view of productivity at industrial scale, and aiming to an optimal use of the catalyst, it is important to check the effect of the ratio FAL/catalyst (wt/wt) on the selectivity to GVL (Figure 2). This analysis evidences that the lower the FAL/catalyst mass ratio, the higher the selectivity to GVL. Noteworthy, the highest values of selectivities are achieved at the highest temperature (170 °C), in contrast with the typical behavior of furfural chemistries where high temperature usually leads to non-desired side reactions, reducing the selectivity towards the target compound. This is an evidence of the leading role of FAL/catalyst ratio as paramount parameter affecting the selectivity to GVL in this system. On the other hand, in spite of the high selectivity achieved under such conditions, working at very low FAL/catalyst ratios provides a catalytic system with low specific productivity of the desired product (g of GVL/ g of catalyst). PS

80 130 ºC

150 ºC

170 ºC

70 60 50

SGVL(%)

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


40 30 20 10 0 0

1

2

3

4

5

6

7

8

9

gFAL/gCAT


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Figure 2. Selectivity towards GVL (SGVL) as a function of starting FAL/catalyst mass ratio in the transformation of FAL into GVL at the three temperature levels. Reaction time 24h. ii) Selective productivity of GVL: Maximizing the selective production of GVL per gram of catalyst The most efficient catalytic system for the transformation of furfural into γ-valerolactone will be that maximizing simultaneously GVL selectivity and productivity (defined as the amount of GVL produced per gram of catalyst, gGVL·gCAT-1). In order to combine both responses, we have defined a new parameter named selective productivity of GVL (ΨGVL), mathematically defined as follows:

Ψ$% =

1234 &%( !

5 6 $% &7(

× 8 9: 7 &7(

(5)

This parameter has been calculated for all the experiments carried out in the above-described experimental design (Table S1). The optimal conditions predicted in the previous model for GVL selectivity (170 °C, catalyst loading 15 g·L-1, and a starting furfural concentration of 6 g·L-1) provide a ΨGVL value of 0.22. However, as shown in the table, this parameter displays higher values under other reaction conditions, being thus susceptible to further optimization. Figure 3 shows the correlation of ΨGVL with the FAL/catalyst mass ratio at the three temperatures evaluated (130, 150 and 170 °C). The resultant plots show a clear maximum, indicating that there is an optimal value of FAL to catalyst mass ratio to provide the highest selective productivity. Furthermore, the temperature exerts a greater influence on the selective productivity of GVL than on the selectivity (Figure 2).


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0.7 130 ºC

150 ºC

170 ºC

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gFAL/gCAT Figure 3. Selective productivity of GVL (ΨGVL) as a function of starting FAL/catalyst mass ratio in the transformation of FAL into GVL at the three temperature levels. Reaction time 24h.

In this way, the optimum value of selective productivity of GVL is 0.61 (corresponding to a SGVL of 70.0% and a productivity of 0.88 gGVL·gCAT-1), obtained for a ratio FAL/catalyst of 1.2 gFAL·gCAT-1 under the following reaction conditions: 170 °C, catalyst loading 5 g·L-1, FAL concentration 6 g·L-1 (X1 = +1, X2 = -1, X3 = -1, in coded values). For the sake of comparison, the optimum ΨGVL of our system is noticeably higher than the best results recently reported in literature for the same transformation of FAL into GVL with other catalytic systems: 0.38 in Song et al28; 0.50 in Winoto et al. 27 On the other hand, analyzing the kinetics of transformation under the optimal conditions for selective productivity (Figure S10), the evolution of GVL in the media is slightly slower than in the optimum conditions for GVL selectivity (Figure S9), but using the catalyst much more efficiently (5 g·L-1 vs 15 g·L-1). Aiming to improve the kinetics, and looking for taking


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advantage of the positive influence of the temperature on the selective productivity, an additional experiment was carried out at 190 °C (Figure 4). As expected, such an increase in the reaction temperature allows the cascade of reactions to accelerate, achieving a ΨGVL close to the maximum of 0.61 already after 6 h. However, the lower the temperature, the better the mass balance, i.e. the sum of selectivities to the desired compounds is closer to the conversion of furfural (Figure S11). This indicates that the increase of temperature improves not only the kinetics of the cascade of reaction, but also the kinetics of undesired side-reactions, such as polymerizations and formation of humins. In this way, while at 130 °C a mass balance over 95% can be achieved even after 24h, at 170 °C and 190 °C the mass balances remain around 70%. This would be interesting from the point of view of the most efficient use of the biomass. However, due to the limited reaction rate, the catalytic test at the lowest temperature does not achieve high GVL selectivities even after 24h.

0.7 130 ºC

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Figure 4. Selective productivity of GVL (ΨGVL) as a function of reaction time at different temperatures. Catalyst loading 5 g·L-1; FAL concentration 6 g·L-1. Reusability was evaluated by reutilization experiments of the zirconium-modified zeolite, Zr-AlBeta. Catalyst was recovered by filtration after a first reaction cycle of 24 h at 170ºC under the optimum conditions for selective productivity. The solid, once dried, presented dark color, indicating the possible adsorption of reaction products and non-identified compounds, i.e. humins. Recovered catalyst was used directly in a second reaction cycle, without any further treatment for regeneration. The yield to GVL dropped dramatically over the spent catalyst, from 71% to 24%, indicating strong deactivation phenomena. Thereafter, it was activated by thermal treatment in air (5 h at 550ºC), and used again in a third identical reaction cycle, totally recovering the initial activity. After a new thermal treatment the same catalyst was used again in a fourth cycle. Figure 5 includes the results in terms of yield to GVL. As shown, the catalyst displays a very good reusability after regeneration, evidencing that the active sites keep their activity essentially intact.


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80 70 60 50 GVL yield (%)

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1s t Re ge ne ra t. 2 nd

Re ge ne ra t.

Us ed

es

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0 Fr

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Figure 5. Reutilization of Zr-Al-Beta. Yield to GVL starting from furfural with fresh catalyst; used catalyst directly recovered from reaction medium; and two consecutive uses with intermediate thermal activation of the catalyst at 550ºC. Reaction conditions: 24 h, 170 ºC, catalyst loading 5 g·L-1, FAL concentration 6 g·L-1.

Conclusions Within this work we show the rational optimization of the catalytic process from furfural (FAL) to gamma-valerolactone (GVL) over a bifunctional Zr-Al-Beta zeolite and using 2propanol as hydrogen donor and solvent. The reaction conditions maximizing the selectivity to GVL were 170 °C, a catalyst loading of 15 g·L-1, and a starting furfural concentration of 6 g·L-1, leading to a maximum value of GVL selectivity slightly over 70 mol% after just 4h of reaction. On the other hand, aiming to the most efficient use of the biomass resource on a catalyst weight basis, the conditions maximizing the selective productivity of GVL per gram of catalyst were


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170 °C and a ratio FAL/catalyst of 1.2 gFAL·gCAT-1. This ratio appears as the key parameter governing the performance of the catalyst system. Under such optimized reaction conditions, the maximum value achieved for GVL selective productivity is 0.61, corresponding to a SGVL of 70.0% and a productivity of 0.88 gGVL·gCAT-1.

ACKNOWLEDGMENT The authors thank the financial support from the projects CTQ2014-52907-R and CTQ201568844-REDT (Spanish Ministry of Economy and Competitiveness), and through the project S2013-MAE-2882 (Regional Government of Madrid). C. López-Aguado acknowledges a FPI grant (BES-2015-072709) from the Spanish Government.

Supporting information Figures: Catalyst characterization: Ar adsorption isotherms and pore sizes distribution; XDR patterns; TEM images; DRIFT signals of adsorbed pyridine. Reaction results: Repeatability of the catalytic tests in the design of experiments; Fitting goodness of the mathematical model; Pareto Chart of standardized effects and interactions of the experimental design; Reaction products distribution as a function of the starting furfural concentration; Furfural conversion and product selectivities under the optimal conditions predicted for GVL selectivity. Furfural conversion and product selectivities under the optimal conditions calculated for selective productivity of GVL. Influence of the reaction temperature in the transformation of FAL to GVL over Zr-Al-Beta at different reaction times. Table: Observed responses after 24h of reaction in


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the optimization of GVL production over Zr-Al-Beta zeolite (selectivity, productivity and selective productivity.


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0.7 130 ºC 150 ºC 170 ºC

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Towards the efficient utilization of lignocellulosic biomass: optimization of the catalytic transformation of furfural into the green solvent γ-valerolactone ACS Paragon Plus Environment

Rational Optimization of Reaction Conditions for the One-Pot

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