ENVIRONMENTALLY BENIGN AND CATALYTIC PROCESSES...

1 ENVIRONMENTALLY BENIGN AND CATALYTIC PROCESSES LABORATORY

PRACTICE

ENVIRONMENTALLY BENIGN AND

CATALYTIC PROCESSES LABORATORY

PRACTICE

Production of Raw Materials for the Chemical

Industry by Homogeneous Catalytic Hydrogenation

and Catalytic Transfer Hydrogenation Reactions

BME, Department of Chemical and Environmental Process

Engineering

Budapest

2018


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Introduction

Our excessively wasteful and environmentally damaging lifestyle is highly depended on

products of chemical industry. One of the main aims of chemistry is to create such new and

effective technologies, which have less negative impact on the environment. Gamma-

valerolactone (GVL) was considered as a promising renewable platform chemical, which can

easily be produced from avaiable biomass-based feedstocks including waste stream reducing

the excluding edible carbohydrates- without unethically producing chemical raw materials from

foodstuff. Due to its advantageous physical and chemical properties, it can be equally applied

as an organic platform molecule, solvent, octane booster, fuel additive etc..

Primarily solar-, wind-, hydro- and geothermal energy are used as renewable energy

sources. We need to pay extra attention to the biomass based energy production,i because

carbohydrates, the most common organic compounds on Earth, are regenerated from carbon

dioxide and water by solar energy, can be utilized as a renewable raw material even in large

scale.ii There is approximately 170 billion tons of biomass generated on the planet annually, of

which 75% is carbohydrate, still humanity utilizes only about 3-4% of this significant amount.iii

The composition of the biomass strongly depends on the source. In general, biomass consists

of 38 – 50% of cellulose, 23 – 32% of hemicellulose and 15 – 25% of lignin. (Fig 1.). The

cellulose is an unbranched-chained, water-insoluble polysaccharides, that can consists of

hundreds or even tens of thousands of glucose units. The cellulose is the most common

naturally occurring biopolymer, that is assumed to be generated around 2 x 109 tons annually.

The hemicellulose is a polymer that has amorphous structure, has lower molceular weight, than

cellulose, its monomers are hexoses (glucose, mannose and galactose) and pentoses (mainly

arabinose and xilose). The third constituent is lignin, which is an intensely cross-linked

polymer, it consists of three types of phenylpropene components, and functions as some kind

of glue, connecting the cellulose and hemicellulose threads.


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Figure 1. Average composition of the plant origin biomass

The most common carbohydrates are the pentose and hexose containing five and six

carbon atoms, respectively. Nowadays, the most sought for and most produced monosaccharide

is glucose obtained from corn, rice or potato extracted starch, its amount is approximately 5

million tons per year.iv The diversity of carbohydrates offers much more possibilities for us

(Fig 2.).

Figure 2. Industrial processes for converting carbohydrates

Conversion of the biomass into carbonaceous compounds is therefore becoming more

and more importance. István Horváth and his colleagues demonstrated first that the dehydration

of carbohydrates combined with hydrogenation involves the preparation of various oxygen-

containing compounds, furfuryl alcohol, levulinic acid (LA), gamma valerolactone (GVL), 2-


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methyltetrahydrofuran (2-Me-THF), through further hydrogenation processes, even alkanes can

be produced (Fig 3).v This proposed cycle involves sustainability (cyclicality), free from ethical

issues (as it does not start from edible carbohydrates), its versatility is demonstrated by the

variance of products.

Figure 3. The proposed conversion of carbohydrates

Laboratory practice focuses on the use of GVL as a solvent in transfer hydrogenation

reactions, as well as reduction of furfural to furfuryl alcohol, which is the major step in the

production of GVL from hemicellulose. Since most chemical reactions take place in the

presence of a catalyst, therefore the appropriate selection of the catalyst, separation and

recovery of the catalyst from the reaction mixture are crucial points for all catalytic processes.

In the case of homogeneous catalysis, the separation of the product and the catalyst, which are

in the same phase, can be carried out in several ways. One option is to change the polarity of

the reaction medium. An example: conversion of levulinic acid to γ-valerolactone in the

presence of water-soluble ruthenium catalyst. After removing water and GVL (e.g. distillation),

the precipitated catalyst complex can be re-dissolved and reused for the production further γ-

valerolactone molecules. Another possibility is the membrane separation process based on the

difference in size of the molecules.

-valerolactone


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Homogeneous catalytic reduction of 2-furfural to furfuryl alcohol in various solvent is

known in the literature, but the reduction using Ru(acac)3/BDPP catalyst system was very

recently demonstrated by Tukacs et al. It was shown that the activity of the Ru-based catalyst

systems can be increased by the application of bidentate phosphine ligands (Ph2P(CH2)nPPh2,

n = 1 – 3) keeping the environmentally benign benefits of a solvent-free system (Fig 4.).vi,vii

Furthermore, the activity was also influenced by the number of the methylene spacers between

the phosphorus atoms of the bidentate ligands e.g. by the size of the chelate ring of the active

form of the catalyst. Main advantages of this method are the good solubility and stability of the

catalyst (not susceptible to oxidation).

Figure 4. Two-way phosphine ligands

Purpose of the practice

Production of chemical raw materials (1-phenyl-ethanol, i-propyl-alcohol, gamma-

valerolactone, cinnamonalcohol, furfuryl alcohol, etc.) by homogeneous catalytic reduction

using molecular hydrogen and catalytic transfer hydrogenation reaction. During the reactions,

the Ru-based catalyst system is used, while as a solvent – in case of the catalytic transfer

hydrogenation reactions (if necessary) – gamma-valerolactone (sustainable liquid) is applied,

which can also be obtained from natural biomass.

Scheme of the chemical reaction

Figure 5. Homogeneous catalytic reduction of Furfural


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Structure of the applied Ru-based catalyst

Applied green chemistry principles:

Use of renewable raw materials

Application of catalyst

Development of less dangerous syntheses

Design of safer products

Required chemicals

Name Formula M (g mol-1)

2-furfural C5H4O2 96.1

1,4-bis(diphenyl-

phosphino)butane

DPPB

C28H28P2 426.5

Ruthenium(III)-

acetylacetonate

Ru(acac)3

(C5H7O2)3Ru 398.4

Ru(acac)3

Ruthenium(III) acetylacetonate


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Equipment

Measuring cylinder (30 mL), 120 mL Hastelloy-C Parr reactor (Fig 6.), analytical scale

for accurate weighing in, sample containers (2 – 4 mL).

Figure 6. High pressure reactor system

Experimental work

1. During the experiment, pour 30 ml of 2-furfural into the Hastelloy-C Parr reactor, then

add 0.038 g of Ru(acac)3 and 0.416 g of DPPB.

2. Place the clamps on the reactor, connect it to the H2 cylinder, then flush the reactor

several times, then set the desired pressure.

3. After installing the heating block, set the appropriate temperature (80 – 160 °C) and the

mixing speed (500 – 600 rpm) on the user interface.

4. During the 2 hours reaction the decrease in pressure has to be corrected for.

5. After the desired reaction time, the sample prepared directly from the single-phase

mixture is analysed by gas chromatography and the conversion is determined. During

the analysis, 10 µL of toluene (internal standard) is added to 10 µL of the reaction

mixture, 1 mL of dichloro-methane is used as the solvent. Additionally, 1H- and 13C-

NMR spectroscopic analysis of the final product of the reaction, spectral evaluation.

Toluene internal standard is used to determine the sample purity.


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Transfer hydrogenation

Transfer hydrogenation is an alternative process where hydrogenation is carried out with

a non-gaseous hydrogen donor, therefore these reactions are executed under mild conditions at

atmospheric pressure. Since alternative hydrogenation molecules tend to be more soluble in

substrates, the reaction time is shortened. Most commonly, mono- or bivalent alcohols, formic

acid, formates, or cyclic ethers, amines, aldehydes or water are used as hydrogen sources.viii

The general equation for transfer hydrogenation is shown in Fig 7., where S is the substrate to

be reduced, DH2 indicates the alternative hydrogen donor.

Figure 7. The general equation of transfer hydrogenation

Catalytic transfer hydrogenation of levulinic acid to GVL has not been previously

described in the literature. Viktória Fábos presented in her doctoral dissertation in 2009 that

using a formic acid (HCOOH) as a hydrogen donor, in the presence of a ruthenium-based

catalyst the reaction can be successfully implemented.ix The formic acid that is a co-product of

the production of levulinic acid from biomass, is capable of the reduction of the levulinic acid

during transfer-hydrogenation. During the process carbon-dioxide (CO2) and 4-hydroxyvaleric

acid (4-HVA) are produced, the cyclisation of the latter, together with water loss, result the

final product, gamma-valerolactone (Fig 8.).

Figure 8. Transfer hydrogenation of levulinic acid in the presence of formic acid and catalyst

Transfer hydrogenation using diruthenium-complex {[2,5-Ph2-3,4-(p-MePh)2(η5-

C4CO)]2H}Ru2(CO)4(μ-H) as catalyst proposed by Shvo and latter Casey led to a yield of GVL

nearly 100%. The method was patented.x The reaction showed an excellent selectivity, also the


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overhydrogenated byproducts were missed from product mixtures. The highest conversions

were achieved at 2-fold formic acid / levulinic acid molar ratio with a levulinic acid / catalyst

molar ratio of 1200 and 2400 – 8 hours reactions led to the formation of GVL with a yield of

>99.9% at 100 °C (Fig 9.).

Figure 9. Transfer hydrogenation of levulinic acid with Shvo-catalyst

General reaction equation

Figure 10. General reaction equation

Application of Ruthenium-based catalyst (Shvo-catalyst)

The preparation of the catalyst is a three-step process (Fig 11.). Providing the

cyclopentadienyl ring of the catalyst firstly the 2,5-diphenyl-3,4-bis(4'-methylphenyl)

cyclopentadienone (4) and the ruthenium-containing triruthenium dodecacarbonyl (3) reagents

must be prepared. The physical and physico-chemical properties of the various types of Shvo

catalysts can be easily modify by fine tuning of the substituents on the phenyl rings of the (4)

ligand.


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Figure 11. Preparation of Shvo-catalyst

Figure 12. Precursor of Shvo catalyst (1) and the active form (2)

The Shvo-catalyst precursor (Fig 12.) is a symmetric ruthenium complex {2,3,4,5-

Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H) bounded by a hydride bridge, originally used to reduction of

aldehydes, ketones, alkenes and alkines. Shvo and colleagues have shown that in the presence

of formic acid and hydrogen {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H)-diruthenium complex

and various phenyl-substituted derivatives are capable for the reduction of aliphatic, cyclic or

aromatic ketones and aldehydes forming the corresponding alcohol product.

While the reduction of ketones were highly selective, in case of aldehydes aldol

condensation products were also observed in the reaction mixture. The experimental results

clearly show that the excess of formic acid accelerates the reaction, but at the same time the it

promotes the formation of formate esters. To eliminate this issue, sodium formate and a small

amount of water were added to the reaction mixture. Reduction experiments of unsaturated

aldehydes and ketones has shown that when not conjugated double bonds are existing in the


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substrate, hydrogenation is selective for the carbonyl group, resulting alcohols, whereas double

bonds are reduced when conjugated double bonds are in the substrate. They also confirmed if

the reaction is carried out in a solvent it has an effect on the rate of reaction. The mechanism of

reduction was also studied, in which it was found that the active intermediate involved in the

reaction is formed from the diruthenium complex and contains a relatively acidic hydroxyl

group on the cyclopentadienyl ring as well as a hydride directly connected to ruthenium.

Reaction analysis

Atom-efficiency (e.g. transfer hydrogenation of levulinic acid)

C5H8O3 + H2 → C5H8O2 + H2O

M = 116.11 M = 2 M = 100.16 M = 18

Hatom = 100 * Mproduct / Mstarting materials = 100*(100.16/116.11+2) = 84,8 %

Required chemicals

Name Formula M (g mol-1)

Levulinic acid C5H8O3 116.12

Acetone C3H6O 58.08

Acetophenone C8H8O 120.15

Cinnamaldehyde C9H8O 132.16

Equipments

Measuring cylinder (10 mL), 8 mL glassreactor, stirring bar, analytical scale for accurate

weighing in, sample containers (2 – 4 mL).

Safety instructions

During the experimental work, protective goggles and lab coat must be worn. Due to

the high temperature of the reactor, care must be taken while working with it.

Experimental work

6. During the experiments acetofenone, levulinic acid, acetone, cinnamaldehyde are used

as model substrate. Since acetofenone containes two types of functional groups


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(carbonyl- and phenyl group), its reduction is suitable to follow-up the selectivity of the

catalyst. It should be noted that the electron withdrawing properties of the phenyl group

promotes the reduction of the carbonyl group

7. Measure into the reactor tube (Hach tube) 0.5 mL (5.22 mmol) of GVL, 0.0021 g (0.002

mmol) of catalyst, 0.2 mL (5.22 mmol) of HCOOH and 0.3 mL of substrate (2.61 mmol).

Close up the reactor tube and place it into the thermostat bath preheated to 95°C

previously. Set the magnetic mixer to 375 rpm. The reaction time is 2 hours. After the

desired reaction time sample taken from the reaction mixture analyzed by

gaschromatographic method using toluene as internal standard.

To be submitted

A brief summary of the experimental work

Writing up the chemical equations for the reactions

Determination and calculation of the conversion and selectivity of the products

produced with different hydrogenation reactions

Attach chromatograms and spectra

Test questions (example)

Write up the equation for the GVL production!

What is atomic efficiency and what is the atomic efficiency of the examined test reaction like?

What kind of safety instructions must be taken during practice?

Which metal atom does the catalyst contain used during practice?

Draw the Shvo-catalyst precursor and its active form!


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Literature

i Tuck, C. O.; Pérez, E.; Horváth, I. T.; Sheldon R. A.; Poliakoff, M., Science, 2012, 695.

ii Fábos V., PhD Thesis, ELTE, Budapest, 2009.

iii Corma, A.; Iborra, S.; Velty, A., Chem. Rev., 2007, 107, 2411.

iv Lichtenthaler, F. W., Acc.Chem. Res., 2002, 35, 728.

v Mehdi, H.; Tuba, R.; Mika, L. T.; Bodor, A.; Torkos, K.; Horváth, I. T., „Renevable Resources

and Renevable Energy, Taylor and Francis, 2006, Boca Raton, 2007, 55.

vi O. Kröcher; R.A. Köppel and A. Baiker; Chem. Commun. 1997, 453.

vii J. M. Tukacs; M. Novák; G. Dibó; L. T. Mika; Catal. Sci. Technol. 2014, 4, 2908.

viii Joó, F., Aqueus Organometallic Catalysis, Kluwer Academic Publisher, 2001.

ix Fábos, V.; Koczó G.; Mehdi, H.; Boda, L.; Horváth, I. T., Energy & Environ. Sci., 2009, 2,

767.

x Horváth I.T; Mehdi, H.; Fábos V.; Kaposy, N., 2008: Szabadalom HU 08 00662

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