University of Groningen

Levulinic acid from lignocellulosic biomassGirisuta, Buana

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Chapter 2 Exploratory Catalyst-Screening Studies on the Conversion of 5-Hydroxymethyl-furfural and Glucose to Levulinic Acid Abstract A catalyst-screening study on the decomposition reaction of glucose and 5-hydroxymethylfurfural (HMF) to levulinic acid (LA), an interesting green platform chemical, is reported. The catalytic activities of various types of acids (homogeneous and heterogeneous Brønsted acids) were tested to determine the optimal acid catalyst with respect to activity and selectivity towards LA. When using HMF (CHMF,0 = 0.1 M, T = 98 °C, Cacid = 1.0 M and t = 60 min), H2SO4, HCl and HBr showed the highest catalytic activities and LA yields of all the homogeneous Brønsted acids tested. The HMF conversion and the LA yield are correlated with the concentration of H+ in solutions, indicating the absence of anion effects. When using glucose (CGLC,0 = 0.1 M, T = 141 °C, Cacid = 1.0 M and t = 60 min), H2SO4 and HCl showed the highest catalytic activities and LA yields. It was proven that the products LA and formic acid do not auto-catalyse the decomposition reactions of glucose or HMF to LA. Of a range of solid acid catalysts, ZSM−5 gave very promising result for the conversion of HMF to LA. Yields of LA of 70 mol % at 93 mol % conversion were obtained (CHMF,0 = 0.02 M, T = 116 °C, Cacid = 5 wt % and t = 120 min). However, when using ZSM−5 for the conversion of glucose to LA, the LA yields were very low. Other solid Brønsted acids, like Nafion® SAC-13 or Ferrierite were less active, and they profoundly catalysed the decomposition reactions of HMF and glucose to the undesired humins by-product. Keywords: Brønsted acids, levulinic acid, zeolites, dehydration reaction of glucose.

Chapter 2

2.1 Introduction Research efforts to identify attractive chemical transformations for the

conversion of biomass into alternative fuels and useful bulk chemicals have intensified considerably in the last decade [1,2]. A well-known example is the hydrolysis of lignocellulosic biomass, which is typically catalysed by enzymes or by mineral acids, to give glucose as the intermediate product. Glucose can be converted to bio-ethanol as an alternative fuel or to various other organic (bulk) chemicals. An attractive option is the conversion of glucose into levulinic acid (4-oxopentanoic acid) by acid treatment. Levulinic acid (LA) is a versatile building block for the synthesis of various organic compounds, as shown in Figure 2.1 [3,4]. Several reviews [5-9] have described the properties and the potential industrial applications of LA and its derivatives.

O CH3O CH3O

O CH3O

CH3

OH

OH

CH3

O

O

OR

CH3

O

OHOH

OH

O

O

OHNH2

CH2

O

OH

CH3

O

O

OH

CH3

O

O

OH

2-Methyl-THFγ-Valerolactone

α-Angelica lactone

1,4-Pentanediol

Levulinate esters

4,4-Bis-(4-hydroxyphenyl)valeric acid

Levulinic acid

β-Acetylacrylic acid

δ-Amino levulinic acid

Acrylic acid

Figure 2.1 Potentially interesting derivatives of LA [3,4].

The acid-catalysed dehydration reaction of glucose follows a complex reaction pathway with several intermediates (Scheme 2.1). In the main reaction, glucose (1) reacts to give 5-hydroxymethylfurfural (HMF, 2). In the presence of acid, HMF is subsequently hydrated to give LA (3) and formic acid (FA, 4) in a 1:1 mol ratio. Unfortunately, both glucose and HMF also decompose in a parallel reaction mode to produce insoluble compounds known as humins.

O

O

H

OHCH3

OH

O

O

H OH

OO

OHH

OH

OH

H

HOH

HOH

- 3 H2O

H+

+ 2 H2O

H+

+

Humins

H+

1 2 3 4

Humins

H+

Scheme 2.1 Acid-catalysed decomposition of glucose to LA.

28

Exploratory Catalyst-Screening Studies …

A number of papers have appeared on the decomposition reaction of glucose using various Brønsted acid-catalysts, which can be categorized either as homogeneous catalysts (e.g., H2SO4, HCl, H3PO4, HNO3) [10-16] or solid acid catalysts [17-19]. The applications of homogeneous catalysts on the decomposition reaction of the intermediate HMF to LA have also been reported [20-24]. However, the use of solid acids for the latter reaction is unknown.

We here report an exploratory catalyst-screening study on the decomposition of HMF to LA and the decomposition of glucose to LA. Initial studies were performed with HMF, as this compound is an intermediate in the conversion of glucose to LA (Scheme 2.1). As such, the number of possible reaction pathways is limited, simplifying the discussion of the results considerably. Subsequently, the best catalysts for HMF were also tested for glucose.

2.2 Materials and methods 2.2.1 Chemicals

All chemicals used in this study were of analytical grade and used without purification. Glucose, FA and various organic acids were purchased from Merck GmbH (Darmstadt, Germany); LA 98 wt % and HMF were purchased from Acros Organics (Geel, Belgium). Deionised water was used to prepare various solutions.

2.2.2 Solid acid catalysts

Nafion® SAC−13, which is a poly-(tetrafluoro-ethylene)-sulfonic acid resin dispersed within amorphous silica, was purchased from Sigma-Aldrich. The catalyst strudates were ground into fine powder (< 250 mm) prior to the reaction to minimize the diffusional effects. Several commercial zeolites were also tested after accommodation in the H−form. Table 2.1 summarises the main characteristics of these materials. They consist of highly acidic molecular sieves with Si/Al ratio close to 10. They were acquired in the ammonium form and activated in air at 550 °C for 6 h in air. In case of Ferrierite (Na/K form), it was exchanged twice with NH4NO3 (0.5 M) and calcined afterwards at the same conditions stated above. The degree of NH4−exchange (Na and K content in Table 2.1) was confirmed by inductively coupled plasma spectroscopy (ICP). As shown, the degree of exchange was successful with remaining loadings of ca. 10 and 82 ppm for Na+ and K+ respectively. ICP was also employed to measure the actual silicon-aluminium ratio of the sample specimen employed in the catalytic tests. As these reactions are sensitive to metals, the Fe content on the samples was also measured. It was found that the ZSM−5 samples contained traces of Fe (< 500 ppm), likely from the steel templates used in industrial synthesis.

29

Chapter 2

Table 2.1 Specifications of the commercial zeolites employed in this study.

Product Ferrierite (TOSOH) 720KOA Beta Zeolyst CP8145 ZSM-5 27 Alsi Penta

Cation Type K / Na NH4 NH4

Si/Al / mol a 8.9 (9.2) 12.5 (11.1) 12-14 (12.5) Na2O / wt % 1.2 (10 ppm) b 0.05 < 0.03 K2O / wt % 5.7 (82 ppm) b — — Fe2O3 / wt % — — < 0.05 Remaining Fe (by ICP) / ppm 102 224 468

a values between parentheses are the Si/Al ratio determined by ICP; b ppms of Na/K after exchange with NH4NO3.

In order to check the effect of the mesoporosity, the ZSM−5 sample was treated with 0.2 M NaOH (80 °C for 120 min). For details of the protocol the reader is referred elsewhere [25]. This sample is labelled as ZSM−5 AW−120. Different characterization techniques such as SEM, elemental analysis (ICP) and N2 sorption experiments were applied. The reader is referred to previous work for details [25].

2.2.3 Experimental procedure for the acid-catalysed decompositions of glucose

and HMF

The reactions were carried out in two types of glass ampoules with a wall thickness of 1.5 mm and a length of 15 cm, differing in internal diameter (3 and 5 mm). The reactions catalysed by homogeneous Brønsted acids were carried out in the smaller ampoules (3 mm i.d.); the larger ampoules (5 mm i.d.) were used for the reactions catalysed by solid acids.

For reactions with homogeneous Brønsted acids, an ampoule was filled with a certain amount of reaction mixture (between 0.2–0.5 cm3), consisting of reactant (either glucose or HMF) and catalyst at a certain concentration. For the reactions with the solid acid catalyst, an ampoule was loaded with a predetermined amount of solid acid (typically 5 wt %). Subsequently, about 1 cm3 of either glucose or HMF solution in water was added. The glass ampoules were then sealed using a torch and placed in a constant-temperature oven (± 1 °C). To enhance the mixing process between the reactant and the solid acid catalysts, the large ampoules were arranged in a rotating aluminium plate (10 rpm). At various reaction times, ampoules were taken from the oven and quenched in an ice-water bath (4 °C) to stop the reaction. The ampoule was opened, and the liquid was separated from the solids using a micro-centrifuge (Omnilabo International BV) for approximately 15−20 min at 1200 rpm. A certain amount of the clear solution was taken (100−200 µL) and diluted with water (2 cm3). The composition of the solution was determined using high performance liquid chromatography (HPLC).

30

Exploratory Catalyst-Screening Studies …

2.2.4 Adsorption phenomena of LA and FA on the ZSM−5 zeolites

Standard solutions of LA and FA in water were prepared at a certain concentration. Several large ampoules were loaded with a predetermined amount of ZSM−5 zeolite catalysts (typically 5 wt %). About 1 cm3 of either LA or FA standard solution was added into these ampoules, which were then sealed using a torch. Subsequently, the same experimental procedure described in subsection 2.2.3 was applied. The concentrations of LA and FA in the bulk-liquid phase at various reaction times were measured using HPLC and were used to calculate the partitioning coefficients of LA (yLA) and FA (yFA) using the following equations:

1bulkLA,

total

bulkLA,

bulkLA,total

bulkLA,

solidLA,LA −=

−==

nn

nnn

nn

y (2.1)

1bulkFA,

total

bulkFA,

bulkFA,total

bulkFA,

solidFA,FA −=

−==

nn

nnn

nn

y (2.2)

In equations (2.1)−(2.2), nLA,solid, nLA,bulk, nFA,solid, nFA,bulk and ntotal represent the moles of LA adsorbed on the solid surface, the moles of LA in the bulk-liquid phase, the moles of FA adsorbed on the solid surface, the moles of FA in the bulk-liquid phase and total moles in standard solutions, respectively.

2.2.5 Analytical methods

The composition of the liquid phase was determined using an HPLC system consisting of a Hewlett Packard 1050 pump, a Bio-Rad organic acids column Aminex HPX-87H and a Waters 410 refractive index detector. The mobile phase was a diluted solution of sulphuric acid (5 mM) at a flow rate of 0.55 cm3 min-1. The column was operated at 60 °C. A typical chromatogram is given in previous reports [26,27]. The concentrations of each compound in the liquid-phase mixture were determined using calibration curves obtained by analysing standard solutions with known concentrations.

2.2.6 Definitions

Catalyst performance for the HMF hydration reaction to LA was quantified in terms of conversion of HMF (XHMF), the yields of LA (YLA/HMF) and FA (YFA/HMF):

HMF,0

HMFHMF 1

CCX −= (2.3)

HMF,0

LALA/HMF C

CY = (2.4)

HMF,0

FAFA/HMF C

CY = (2.5)

31

Chapter 2

32

In equations (2.3)–(2.5), CHMF, CHMF,0, CLA and CFA are the concentration of HMF, the initial concentration of HMF, the concentration of LA and the concentration of FA at a certain reaction time, respectively.

Similar definitions were applied for the dehydration reaction of glucose to LA:

GLC,0

GLCGLC 1

CCX −= (2.6)

GLC,0

HMFHMF C

CY = (2.7)

GLC,0

LALA/GLC C

CY = (2.8)

GLC,0

FAFA/GLC C

CY = (2.9)

Here, CGLC and CGLC,0 are the concentration of glucose after a certain reaction time and the initial concentration of glucose, respectively.

2.3 Results and discussions 2.3.1 Acid-catalysed hydration reaction of HMF to LA 2.3.1.1 Homogeneous Brønsted acid catalysts

A range of Brønsted acids varying in acid strengths were tested at a CHMF,0 of 0.1 M, a reaction temperature (T) of 98 °C, an acid concentration (Cacid) of 1 M and a reaction time (t) of 60 min. The results are provided in Table 2.2.

Table 2.2 Catalytic activity of Brønsted acids on the HMF hydration reaction to LA. a

Acid catalyst XHMF (mol %)

YLA/HMF

(mol %) YFA/HMF

(mol %) Acidity b

LA, C5H8O3 0 0 0 pKa = 4.59 b FA, HCOOH 1 0 0 pKa = 3.74 H3PO4 6 5 6 pKa1 = 2.15, pKa2 = 7.2, pKa3 = 12.4Oxalic acid, (COOH)2 24 9 9 pKa1 = 1.25, pKa2 = 3.77 HNO3 46 0 12 pKa < - 2 HI 76 10 46 pKa < - 2 HCl 52 48 49 pKa < - 2 HBr 54 51 51 pKa < - 2 H2SO4 57 53 54 pKa,1 < - 2, pKa2 = 1.96

a CHMF,0 = 0.1 M, T = 98 °C, Cacid = 1.0 M and t = 60 min; b All acid ionisation constants were taken from [28], except for LA [29,30].

Exploratory Catalyst-Screening Studies …

Acids with a pKa > 2 (H3PO4, LA and FA) are poor catalysts for the reaction and the XHMF for all these catalysts was less than 6 mol %. The near absence of catalytic activity at this condition for both LA and FA imply that the reaction of HMF to LA is not auto-catalysed by the products. Oxalic acid (pKa1 = 1.25) was considerably more active and the XHMF was 24 mol %. However, the selectively to LA formation was low (9 mol %) and significant amounts of insoluble humins were formed.

Significant higher activities were found for catalysts with pKa values below 1. For common mineral acids like HCl, HBr and H2SO4, the HMF conversions (52-57 mol %) and LA yields (48-53 mol %) were in the same range. These data indicate that the reaction is highly selective under these conditions and that insoluble humins by-products are formed in only very minor amounts. LA and FA are also formed in the theoretical 1 : 1 molar ratio.

The highest HMF conversion was obtained with HI (76 mol %); however, the LA yield was only 10 mol %. LA and FA were not present in the reaction mixture after reaction in the theoretical 1 : 1 molar ratio. Instead, the FA concentration is consistently higher and even in the same range as for HCl, HBr and H2SO4. This suggests that the LA formed may be subsequently decomposed by HI, supported by the presence of several other peaks in the HPLC chromatograms.

For HNO3, a different reactivity pattern was observed. The conversion of HMF was close to the values of HCl, HBr and H2SO4. However, the yield of LA was close to zero, and that of FA was only 12 mol %. The main products in this case are unidentified gaseous products and insoluble humins. On the basis of this data, we can conclude that amongst the homogeneous Brønsted acids tested, HCl, HBr and H2SO4 are the best catalyst for LA formation with respect to activity and selectivity.

The question arises whether only the proton concentration in solution or also the anion plays a role and affects the activity-selectivity patterns. To study this effect, additional experiments with all acids, except HNO3 and HI, were performed at different acid concentrations (0.5 and 2 M). The concentration of H+ in solution was calculated for each experiment using the pKa values given in Table 2.2. The HMF conversions and LA yields were determined and the results are provided in Figure 2.2. It clearly shows that both the XHMF and YLA/HMF are directly proportional to the CH+. It implies that the anion does not play an important role and that activity/selectivity is solely depending on the CH+.

33

Chapter 2

0.0 0.5 1.0 1.5 2.0 2.5

0

20

40

60

80

100

X H

MF /

%-m

ol

CH+ / M

H2SO4

HCl HBr H3PO4

(COOH)2

FA LA

0.0 0.5 1.0 1.5 2.0 2.5

0

20

40

60

80

100

H2SO4

HCl HBr H3PO4

(COOH)2

FA LA

Y LA

/HM

F / %

-mol

CH+ / M (a) (b)

Figure 2.2 Effects of CH+ on the XHMF (a) and YLA/HMF (b) for the acid-catalysed decomposition of HMF to LA (CHMF,0 = 0.1 M, T = 98 °C and t = 60 min). 2.3.1.2 Solid acid catalysts

Several solid acid catalysts (Table 2.3) were tested for the conversion of HMF to LA. Their catalytic activities were evaluated at CHMF,0 = 0.02 M , T = 116 °C and t = 120 min, and were compared with sulphuric acid. The solid acid concentration was 5 wt % for all experiments and is defined as the ratio between the mass of solid acid catalyst and the total mass of the reaction mixture. The experimental results are given in Table 2.4.

Nafion SAC-13®, a strong solid acid catalyst based on (tetrafluoro-ethylene)-sulfonic polymer with an acid strength comparable to sulphuric acid, shows poor performance and only 16 mol % conversion was observed (c.f. 88 mol % for sulphuric acid under these conditions). Additional experiments were performed at longer reaction times, and after 18 h XHMF and YLA/HMF were 75 and 32 mol %, respectively. Thus, the catalyst is stable under the reaction conditions; however, catalytic activity is reduced considerably compared to sulphuric acid. In addition, the yield is also considerably lower as a result of excessive humins formation. Speculatively, these results could either be due to the lower acid concentration in the reaction mixture compared to H2SO4 or intra-particle diffusion limitations of HMF. For parallel reactions (Scheme 2.1), such mass transfer effects generally lower the selectivity and the yield of the desired products [31].

Disappointing results were obtained with Ferrierite, Beta and ZSM−5 AW−120 zeolites. Although the conversions were higher than for Nafion SAC-13®, and for ZSM−5 AW−120 even higher than for sulphuric acid, the product yield was low (< 8 mol %). Insights in the factors related to the low LA yields may be obtained by comparing the mol ratio of LA and FA after reaction. For zeolites Beta and ZSM-5, the FA content was considerably higher than LA, indicating excessive LA decomposition or strong adsorption of the LA formed on the zeolites (vide infra).

34

Exploratory Catalyst-Screening Studies …

Table 2.3 Properties of the solid-acid catalysts tested.

Sample Si/Al a Grain size / µm b SBET / m2 g-1 Vtotal / cm3 g-1 Vµpores / cm3 g-1

Nafion SAC-13® 0.15 c n.a. d 200 e 0.70 e − Ferrierite 9.2 1-3 331 0.23 0.12 Beta 11.1 < 0.1 636 0.89 0.18 ZSM-5-AW120 f 10.9 1-2 364 (135) g 0.38 0.09 ZSM-5 12.5 3-5 405 (35) g 0.21 0.16

a determined by ICP; b visualized by SEM; c acidity density as meq H+ per gram; d this material consists of nano particles of H-nafion resins with quadrulobe-extruded silica matrix (5-10 mm length); e from [32]; f samples derived by applying controlled alkaline desilication; g values between brackets are mesopores and are derived from t-plot analysis. Table 2.4 Catalytic activity of solid-acid catalyst on the HMF hydration reaction to LA. a

Solid-acid catalyst XHMF (mol %) YLA/HMF (mol %) YFA/HMF (mol %) Nafion SAC-13® 16 4 6 Ferrierite 43 0 0 Beta 85 0 7 ZSM-5 AW-120 90 8 29 ZSM-5 93 22 52 ZSM-5 corrected for adsorption 93 70 71 Sulphuric acid 88 77 79

a CHMF,0 = 0.02 M, T = 116 °C, Cacid = 5 wt % and t = 120 min

The most promising results were obtained using ZSM−5 zeolite, which gave the highest YLA/HMF among all solid-acid catalysts. However, again the LA and FA were not formed in the theoretical 1 to 1 mol ratio. To gain more insights in this phenomenon, the reaction was followed in time between 0 and 180 min (Figure 2.3).

Clearly, throughout the reaction, the CFA is always higher than the CLA. Furthermore, the amount of HMF drops rapidly at the start of the reaction, without a concurrent increase in product formation. This suggests that HMF adsorbs strongly to the ZSM−5 zeolite. Such adsorption phenomena could also explain the deviation of the molar ratio of LA to FA from the theoretical value of 1.

35

Chapter 2

0 30 60 90 120 150 180

0.000

0.005

0.010

0.015

0.020

0.025 CFA

CLA

CHMF

C /

M

t / min

Figure 2.3 Concentration profiles for the ZSM-5-zeolite-catalysed reaction of HMF (CHMF,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %).

To gain insights in the extent of adsorption of LA and FA on ZSM−5, a number of adsorption experiments were performed using the products LA and FA and the ZSM−5 zeolite. Standard solutions of LA and FA (0.023 M) were mixed with ZSM−5 zeolite (5 wt %) at T = 116 °C. The concentrations of LA and FA were measured at various reaction times between 0 and 180 min. By substituting the concentration values of LA and FA into equations (2.1)−(2.2), the partition coefficients of LA (yLA) and FA (yFA) during the reaction course were obtained and are given in Figure 2.4. It is clearly shown in Figure 2.4 that LA has higher partition coefficients, and thus LA adsorbs more strongly to the ZSM−5 zeolite than FA.

0 30 60 90 120 150 180

0.1

1

yLA yFA

y LA, y

FA /

mol

mol

-1

t / min

Figure 2.4 Partition coefficients of LA and FA on ZSM-5 zeolite as function of reaction time (CLA,0 = CFA,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %).

36

Exploratory Catalyst-Screening Studies …

The amounts of LA (nLA,corrected) and FA (nFA,corrected) formed by chemical reaction distribute between the bulk-liquid phase and the solid-phase and may be calculated using the following equations:

( )LAbulkLA,bulkLA,LAbulkLA,solidLA,bulkLA,correctedLA, 1 ynnynnnn +=+=+= (2.10)

( )FAbulkFA,bulkFA,FAbulkFA,solidFA,bulkFA,correctedFA, 1 ynnynnnn +=+=+= (2.11)

By combining the experimental results given in Figures 2.3−2.4 and equation (2.10)−(2.11), the amount of LA and FA formed by chemical reactions can be calculated and the results are shown in Figure 2.5. With this information, the yields may be recalculated and are 70 mol % for YLA/HMF and 71 mol % for YFA/HMF after 180 min reaction time. These values are only slightly lower than for sulphuric acid. Thus the relatively low yields and the deviation from a 1 : 1 mol ratio of LA and FA are due to preferential adsorption of LA on ZSM−5.

These results imply that ZSM−5 is a promising solid acid catalyst for the conversion of HMF to LA. Further optimisation studies will be required to assess the full potential and to identify whether it could be a replacement for sulphuric acid. With ZSM−5, catalyst recycle may be more facile than with sulphuric acids, leading to simplified catalyst recycle strategies.

0 30 60 90 120 150 180

0.000

0.005

0.010

0.015

0.020

0.025 nFA

nLA

n LA, n

FA /

mm

ol

t / min

Figure 2.5 The total amount of LA and FA formed in the HMF hydration reaction using ZSM-5 zeolite as catalyst (CHMF,0 = 0.023 M, T = 116 °C, CZSM-5 = 5 wt %). 2.3.2 Acid-catalysed dehydration reaction of glucose to LA 2.3.2.1 Homogeneous Brønsted acid catalysts

The decomposition of glucose to LA using a variety of homogeneous Brønsted acid catalysts has been studied extensively [10-16]. To investigate possible auto-catalytic effects, both LA and FA were tested as catalysts for the reaction and the results were compared with strong acids like sulphuric acid and hydrochloric acid. All the experiments were conducted at a temperature of 141 °C and a

37

Chapter 2

reaction time of 60 min. The initial concentration of glucose and the acid concentration were kept constant at 0.1 M and 1 M, respectively. The experimental results are provided in Table 2.5.

Table 2.5 Catalytic activities of Brønsted acid catalysts on the glucose dehydration reaction to LA. a

Acid catalyst XGLC (mol %) YHMF (mol %) YLA/GLC (mol %) YFA/GLC (mol %) No catalysts 5 0 0 0 LA 6 3 0 0 FA 8 0 0 0 HCl 67 1 45 48 H2SO4 69 1 45 44

a CGLC,0 = 0.1 M, T = 141 °C, Cacid = 1 M and t = 60 min.

Both LA and FA gave low catalytic-activities and the results were not significantly different from the blank/un-catalysed experiment. Based on these results, it can be concluded that both LA and FA do not auto-catalyse the reaction.

2.3.2.2 Solid acid catalysts

For the acid-catalysed HMF hydration reaction to LA, positive results were only obtained for ZSM−5; therefore this catalyst was tested for the dehydration of glucose to LA. The reaction was conducted at CGLC,0 = 0.05 M, T = 141°C and Cacid = 1 wt %. The reaction was followed in time (between 0 and 420 min), and the results were compared with that of sulphuric acid (see Figure 2.6).

0 60 120 180 240 300 360 420

0

10

20

30

40

50

60

0 60 120 180 240 300 360 420

0

10

20

30

40

50

60

X GLC

/ m

ol %

t / min

ZSM-5 H2SO4

Y LA

/GLC

/ m

ol %

t / min

ZSM-5 H2SO4

(a) (b)

Figure 2.6 Comparison of the catalytic activities of ZSM-5 and sulphuric acid expressed as XGLC (a) and as YLA/GLC (b) for the acid-catalysed dehydration reaction of glucose to LA (CGLC,0 = 0.05 M, T = 141 °C and CZSM−5 = 1 wt %).

38

Exploratory Catalyst-Screening Studies …

ZSM−5 is considerably less active than sulphuric acid and the selectivity to LA is very low as shown in Figure 2.6 (b). The yields of LA were considerably higher when using HMF instead of glucose as the reactant (vide supra). When considering the proposed kinetic pathway for the conversion of glucose to LA (Scheme 2.1), it appears that the reaction of glucose to HMF is not favoured by ZSM-5, resulting in the formation of insoluble humins, whereas the subsequent step (HMF to LA) is well possible with ZSM−5. It is well possible that glucose diffusion into the small pores of ZSM−5 is hindered considerably and has a negative effect on the selectivity and activity. Further experiments with acidic meso-porous materials are in progress to test this hypothesis and will be reported in due course.

2.4 Conclusions

Homogeneous and heterogeneous Brønsted acids have been tested as catalysts for the reaction of HMF or glucose to LA. From the screening results with liquid Brønsted acids, it may be concluded that inorganic strong acids like sulphuric acid or hydrochloric acid give the best performance. The products LA and FA do not auto-catalyse the decomposition reactions of glucose or HMF to LA.

Of a range of solid acid catalysts tested, ZSM−5 gave very promising result for the conversion of HMF to LA, though further optimisation studies will be required to identify whether it could be used as a replacement for sulphuric acid catalyst. With ZSM−5, catalyst recycle may be more facile than with sulphuric acid, leading to simplified catalyst recycle strategies.

2.5 Nomenclature

CGLC : Concentration of glucose (M) CGLC,0 : Initial concentration of glucose (M) CHMF : Concentration of HMF (M) CHMF,0 : Initial concentration of HMF (M) CFA : Concentration of FA (M) CLA : Concentration of LA (M) nFA,bulk : Moles of FA in the bulk-liquid phase (mol) nFA,corrected : Total moles of FA both in bulk-liquid and on the solid surface (mol) nFA,solid : Moles of FA adsorbed on the solid surface (mol) nLA,bulk : Moles of LA in the bulk-liquid phase (mol) nLA,corrected : Total moles of LA both in bulk-liquid and on the solid surface (mol) nLA,solid : Moles of LA adsorbed on the solid surface (mol) ntotal : Total moles in the standard solutions (mol) t : Time (min) XGLC : Conversion of glucose (mol %) XHMF : Conversion of HMF (mol %) yFA : Partitioning coefficient of FA on the solid surface of ZSM–5 zeolites (−) yLA : Partitioning coefficient of LA on the solid surface of ZSM–5 zeolites (−)

39

Chapter 2

YFA/GLC : Yield of FA from glucose (mol %) YFA/HMF : Yield of FA from HMF (mol %) YHMF : Yield of HMF from glucose (mol %) YLA/GLC : Yield of LA from glucose (mol %) YLA/HMF : Yield of LA from HMF (mol %)

2.6 References

[1] Klass, D. L., Biomass for Renewable Energy, Fuels, and Chemicals. Academic Press: New York, 1998.

[2] Kamm, B.; Kamm, M.; Gruber, P. R.; Kromus, S., Biorefinery Systems - An Overview. In Biorefineries - Industrial Processes and Products: Status Quo and Future Directions Volume 1, Kamm, B.; Gruber, P. R.; Kamm, M., Eds. Wiley-VCH: Weinheim, 2006; pp 3-40.

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