Unravelling the catalytic influence of naturally occurring salts on biomass pyrolysis chemistry using glucose as a model compound: A combined experimental and DFT study

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Ansari, J. W. Chew, P. J. Dauenhauer and S. H. H. Mushrif, Catal. Sci. Technol., 2019, DOI:

10.1039/C9CY00005D.

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Catalysis Science & Technology

Unravelling the Catalytic Influence of Naturally Occurring Salts on Biomass P_Dy_Or_Io_:l₁y_0.s₁₀is₃^V₉ⁱ_/^e_C^w₉^A_C^rt_Y^ic₀^le₀^O₀₀^nl₅ⁱⁿ_D^e

Chemistry Using Glucose as a Model Compound: A Combined Experimental and DFT Study

Jyotsna S. Arora^†, Khursheed B. Ansari^†1, Jia Wei Chew^†, Paul J. Dauenhauer^‡, Samir H. Mushrif ^§*

^†School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang

Drive, Singapore 637459, Singapore

^‡Department of Chemical Engineering and Materials Science, University of Minnesota, 421

Washington Ave. SE, Minneapolis, Minnesota 55455, United States

^§Department of Chemical and Materials Engineering, University of Alberta, 9211-116 St NW,

Edmonton, Alberta T6G1H9, Canada.

^*Corresponding author, email: mushrif@ualberta.ca

Phone: 780-492-4872

Fax: 780-492-2881

¹Current Affiliation: Department of Chemical Engineering, Aligarh Muslim University, AMU

Campus, Aligarh 201002 Uttar Pradesh, India

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DOI: 10.1039/C9CY00005D

Abstract

Fast pyrolysis is an efficient thermochemical decomposition process to produce bio-oil and renewable

chemicals from lignocellulosic biomass. It has been suggested that alkali- and alkaline-earth metal

(AAEM) ions in biomass alter the yield and composition of bio-oil, but little is known about the intrinsic

chemistry of metal-catalyzed biomass pyrolysis. In this study, we combined thin-film pyrolysis

experiments and density functional theory (DFT) calculations to obtain insights into AAEM-catalyzed

glucose decomposition reactions, especially forming major bio-oil components and char. Experiments

reveal the difference in the yield and composition of bio-oil of metal-free and AAEM complexed

glucose. Metal-free glucose produced 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-Pyran-4-one

(DHMDHP) as the predominant compound in bio-oil, while 1,6-anhydroglucofuranose (AGF)

maximized in Na(I)/glucose, levoglucosan (LGA) in K(I)/glucose, levoglucosenone (LGO) in

Ca(II)/glucose and furfural in Mg(II)/glucose. To evaluate the stereoelectronic basis of metal ions in

altering pyrolysis reaction kinetics, the reaction mechanisms of AGF, LGA, 5-hydroxymethylfurfural

(5-HMF), furfural, 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose (ADGH), LGO, and char

formation were investigated using DFT calculations. DFT results showed that the presence of Ca(II)

and Mg(II) ions catalyzed furfural and LGO formation, while alkali ions decatalyzed the formation of

on these products. Conversely, Na(I) and K(I) ions catalyzed the concerted dehydrative ring closure of

glucofuranose during AGF formation. For ADGH, AAEMs showed anti-catalytic effect. We also

described a novel route for char formation via coupling between 1,2-anhydroglucopyranose and a

carbonyl compound. The presence of alkali ions catalyzed char formation. Thus, the atomistic insights

obtained from DFT calculations assist in understanding the observed change in experimental yields of

individual bio-oil compounds governing its composition.

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Catalysis Science & Technology

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DOI: 10.1039/C9CY00005D

1. Introduction

Vast amounts of lignocellulosic biomass produced in nature can be potentially converted into renewable

fuels without affecting the nutritional requirements of the world.¹Utilization of the second generation

lignocellulosic biomass can help in replacing fossil fuels, which, in turn, can reduce the emission of

carbon dioxide and other greenhouse gases. Fast pyrolysis is a sustainable technology for generating

renewable fuels/blends from biomass, compared to biochemical (i.e., fermentation) and gasification

processes.^{2, 3}In fast pyrolysis, biomass decomposes at high temperature (400-600 ^oC) in the absence of

oxygen and generates bio-oil, along with non-condensable gases and char. Thermal deconstruction of

polymeric biomass comprises of multiple reactions i.e., depolymerization, dehydration, ring

fragmentation, and repolymerization, which generate a number of bio-oil components and solid char.^4,

⁵The storage, handling, and direct utilization of bio-oil is hindered by its high oxygen and water content

and therefore bio-oil requires further processing through deoxygenation/hydrodeoxygenation in the

presence of a heterogeneous catalyst before it can be used as a fuel.⁶

Obtaining insights into the intrinsic reaction chemistry of biomass pyrolysis is crucial for developing

the technology, because the yield and composition of bio-oil are governed by the reactions that take

place in the condensed-phase. However, reaction chemistry, kinetics and transport effects are inter-

related⁷, which makes it difficult to study the intrinsic reactions of biomass pyrolysis under the given

operating conditions. To deconvolute the effect of transport on pyrolysis reactions, advanced

experimental methods like the thin-film technique have been developed.^{7, 8}In thin-film experiments,

biomass decomposition occurs in an isothermal, reaction-controlled regime, and insights into the

fundamental chemistry of pyrolysis can be obtained by minimizing the secondary decomposition

reactions.⁸

1.1 Understanding biomass pyrolysis chemistry: State of the art experiments

The underlying chemistry of biomass pyrolysis has been investigated for decades. The knowledge of

pyrolysis reactions started with the global pathway of biomass/cellulose decomposition into gas, tar,

and char, which was further improved as direct and indirect lumped pathways/mechanisms, and finally

a component specific reaction mechanism was proposed. However, the prediction capability of these

models remains limited considering the conventional trial and error approach taken in these studies.

Further, the conventional pyrolysis system with allied analytical instruments has limitations in capturing

the pyrolysis chemistry and kinetics due to the multiphase decomposition process with convoluted, fast

chemistry and transport effects. However, recently advanced pyrolysis reactor systems such as CDS

Pyroprobe,⁹micropyrolyzer,^{7, 10, 11}wire-mesh reactor,¹²and Pulsed-Heated Analysis of Solid Reactions

(PHASR)^{13, 14}enhance the heating rate of thin-film biomass samples (heating rate ~ 200 – 12,000 ^oC.s^-

¹) compared to the conventional method (TGA, heating rate ~ 3 – 4 ^oC.s^-1).¹⁵The kinetics of cellulose

conversion, measured using the PHASR reactor, deconvoluted the complex chemistry of cellulose

decomposition and showed that intra-chain scission of cellulose, resulting in the formation of low-

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molecular-weight volatiles, was favored above 467 ^oC, termed as the reactive melting poin_Dt_O(T_{I: 1}_R₀_M_.1)₀,₃w_9/h_C9il_Ce_Y00005D

below T_RM, chain-end mechanisms exhibited primary cellulose initiation reactions.¹⁶The effect of chain

end type, i.e., presence of a reducing end vs. levoglucosan-end (LG-end) in the oligosaccharide, on the

product distribution was investigated by Leng et al.¹⁷Their results suggested that the presence of a

reducing-end in the sugar molecule favored ring opening reactions generating higher yield of furans,

while the LG-end promoted dehydration reactions generating pyrans and secondary reaction products.¹⁷

We recently discussed the decomposition chemistry of glucose under isothermal conditions at different

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o

operating temperatures (300-500 C) and proposed a detailed reaction network map of glucose

breakdown based on the change in the product yields with increasing temperature.²

Although, there are multiple experimental studies on cellulose decomposition, the experimental results

can only provide macroscopic information about the decomposition process and insights into the

formation of short-lived intermediates; reaction mechanisms are limited. Attempts to understand the

reaction chemistry involved the use of isotopic labelling studies^{18, 19}as well as first principles

calculations^20-22. Molecular modelling is an effective tool to understand the reaction mechanisms and

kinetics.³Selected examples from the literature are as follows: Seshadri and Westmoreland investigated

the pyrolysis of β-D-glucose and suggested concerted reaction pathways for several elementary reaction

steps including dehydration, retro-aldol condensation and keto-enol tautomerization²³. Hu et al.

performed Density Functional theory (DFT) calculations to evaluate the lowest-energy pathways for

the formation of 1,6-anhydroglucofuranose (AGF), 1,4,3,6-dianhydroglucopyranose (DAGP) and 1,5-

anhydro-4-deoxy-D-glycerohex-1-en-3-ulose (ADGH).²⁰Mayes et al. combined quantum mechanical

calculations with experiments to evaluate the pathways for 5-hydroxymethylfurfural (5-HMF)

formation and unimolecular 1,2-dehydration reactions of anomers α- and β-D-glucopyranose.²⁴In

summary, molecular simulations can provide a comprehensive understanding of the elementary steps

that take place in the complex reaction network of cellulose pyrolysis.

1.2 Influence of inorganic metal salts on biomass pyrolysis

Except for few studies,^25-29a majority of the experiments and multiscale theoretical calculations have

been performed for individual biomass components with emphasis on glucose/cellulose decomposition

^{7, 8, 30, 31}because cellulose is the most abundant component of plant biomass comprising of an ordered

polymeric structure of β-D-glucose units. However, biomass contains about 0.5–5 wt% alkali- and

alkaline-earth metal (AAEM) salts.^{26, 27}These metal salts are predominantly concentrated in the cells and

partitioned into almost all subcellular compartments of leaves and roots of plant biomass. The presence of these

inherent metal salts alter the yield and composition of bio-oil and char.^{26, 27}Thermogravimetric analysis

(TGA) and differential thermal analysis (DTA) of AAEM salts impregnated cellulose pyrolysis showed

that the presence of alkali salts did not promote weight loss of the impregnated samples, which, in turn,

was related to the dehydration and carbonization reactions during cellulose pyrolysis.²⁸On the other

hand, maximum yields of water from Mg(II) and Ca(II)-impregnated cellulose were obtained at lower

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Catalysis Science & Technology

temperatures i.e., 200 ^oC for Mg(II), while 250 ^oC for Ca(II)-chloride compared to 350 ^oC for untr^Veⁱa^ewte^Ad^{rticle Online}

DOI: 10.1039/C9CY00005D

cellulose.²⁸The effect of Na(I), K(I), Ca(II) and Mg(II) salts on primary pyrolysis product distribution

of cellulose showed that chloride salts of Na(I) and K(I) result in a product distribution similar to that

of untreated cellulose, while Mg(II) and Ca(II) chloride salts showed enhanced formation of furans and

levoglucosenone (LGO).²⁷Zhu et al. evaluated the overall yield and composition of volatile organic

vapours and char by comparing the results of Ca(II) and Mg(II) loaded cellulose powder (diffusion-

limited) and thin-film (isothermal reaction-controlled regime) pyrolysis experiments. Their results

suggested that Ca(II) ions favored primary catalytic reactions of cellulose to char, while Mg(II) ions

affected secondary char formation.²⁶However, the reaction mechanisms and pathways proposed were

based on the final product distribution.²⁶Moreover, due to extremely fast timescales, the presence of

multiple competing reactions and difficulty in measuring pyrolysis kinetics experimentally, insights

into the intermediates and kinetics of reaction mechanisms in the presence of AAEMs cannot be

obtained based on the final product distribution. Therefore, computational studies play a key role in

understanding the reaction chemistry and also for developing the reaction map of metal-assisted

biomass pyrolysis, which will be beneficial to comprehend the catalytic (or anti-catalytic) effect of

metal salts on primary/secondary reactions of pyrolysis.

DFT calculations combined with experiments to study glucose decomposition in the presence and

absence of Na(I) salt suggested that the presence of Na(I) ion stabilized the charged atoms of the

transition state, which, lowered the activation barriers for levoglucosan (LGA) and a few dehydration

reactions of glucose/ its intermediates. However, this study also hypothesized that other indigenous

metal ions i.e., K(I), Ca(II) and Mg(II) have an effect similar to that of Na(I) ion on the reaction kinetics

and product distribution.²⁵Recently, we used DFT simulations to investigate the effect of Na(I), K(I),

Ca(II) and Mg(II) ions on the glycosidic bond cleavage via six different mechanisms, categorized as

direct glycosidic bond breaking pathways and two-step pathways to produce (precursors to) LGA and

furans as products. The results highlighted that all four metal ions have different effects on the activation

barriers of glycosidic bond breaking reactions.³²

Although metal-assisted biomass pyrolysis has been studied in the past, few questions remain

unanswered:

1. Do metal cations catalyze primary or secondary reactions of biomass decomposition?

2. Do all AAEMs exhibit similar (catalytic or anti-catalytic) effect on biomass pyrolysis

chemistry?

To answer these questions, it is imperative to perform a comprehensive analysis of the chemistry for

the formation of all major bio-oil components and char in the presence of all four AAEM ions.

Therefore, the present study is an attempt to bridge the aforementioned knowledge gaps and obtain

molecular level information of the effect of AAEM cations on pyrolysis reactions and associated

kinetics. In this work, we present an integrated experimental and theoretical approach to investigate

AAEM catalyzed pyrolysis chemistry. Thin-film experiments of glucose pyrolysis are performed to

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investigate the influence of alkali (Na(I) and K(I)) and alkaline-earth (Ca(II) and Mg(II)) metal^Vii^eo^wn^Ars^{ticle Online}

DOI: 10.1039/C9CY00005D

(primarily found in biomass) on the yield and composition of bio-oil and char. β-D-glucose is used as

model biomass compound, because glucose is the monomer of cellulose and also one of the

intermediates formed during cellulose pyrolysis. Next, atomistic insights into the reaction chemistry of

AAEM-catalyzed glucose decomposition generating major bio-oil components and char are obtained

using DFT calculations.

The methodology for thin-film sample preparation and details of pyrolysis experiments are presented

in Section 2. Section 3 describes the methods used for molecular simulations to investigate metal ion

catalyzed-glucose decomposition reactions forming bio-oil components and char. In Section 4.1, the

product yields of bio-oil and char obtained from glucose thin-film pyrolysis experiments in the presence

and absence of AAEM salts at 200 ºC are discussed. In the latter part of section 4.1, we briefly discuss

the integrated approach taken in the current study i.e., we correlate the experimentally observed yield

of individual bio-oil components and char with DFT-calculated activation barriers along the reaction

pathways. In section 4.2, we describe the binding sites of AAEM ions with glucose to obtain the most

stable configurations of metal-glucose complexes. The reaction map of glucose decomposition is

discussed in section 4.3, and section 4.4 gives the results and discussion of the DFT calculations for the

formation of bio-oil components, namely, 1,6-anhydroglucofuranose (AGF), 1,5-anhydro-4-deoxy-D-

glycerohex-1-en-3-ulose (ADGH), levoglucosan (LGA) and levoglucosenone (LGO); 5-

hydroxymethylfurfural (5-HMF), furfural and char. Finally, we summarize the findings of this work in

section 5.

2. Materials and Methods

2.1. Materials used and Sample Preparation

Glucose (Lot No: G8270) and metal salts i.e., sodium chloride (Lot No: 746398), potassium chloride

(Lot No: 746436), magnesium chloride (Lot No: M2670) and calcium chloride (Lot No: 746495) were

purchased from Sigma Aldrich, Singapore. Initially, glucose thin-films were prepared by dissolving

1.0% (weight basis) of glucose in deionized (DI) water. Then, 25 μL of 1.0 wt % clear glucose solution

was transferred into a cylindrical pyrolysis crucible; 4 mm × 8 mm (diameter × height) in size. The

water was then removed via room temperature evacuation, leaving behind a micrometer-sized film of

glucose on the inner wall of the crucible.^{2, 7, 11}

To study the effect of AAEMs, glucose was mixed with metal salts individually i.e., NaCl, KCl, MgCl₂

and CaCl₂. The required amount of NaCl, KCl, MgCl₂and CaCl₂(0.35 mmol metal salt/g of glucose)²⁷

was measured and then was added to 1.0 g of glucose; followed by deionized water addition which

yielded a ~1.0 wt% clear solution of glucose homogeneously mixed with metal salts. A similar

procedure of water evaporation, as mentioned above, was followed which generated a micrometer-sized

film of glucose/metal salts. The thickness of the films was measured using a digital microscope (Leica

DVM6) and are shown in Fig. 1[A] and 1[B]. The image analysis showed that glucose films in the

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presence and absence of AAEMs were 10–12 μm thick, which exist inside the reacti_Do_On_I-_:c₁₀o_.1n₀t₃r₉o_/l_Cl₉e_Cd_Y00005D

regime of experimental pyrolysis.¹¹The method of glucose thin-film preparation and its thickness

measurement are also reported elsewhere.²

[A]

[B]

Carbon tape

Glucose thin-film

0.01 cm

Figure 1: Thin-film images acquired from Leica DVM6 digital microscope [A] Glucose thin-film

without metal [B] Glucose thin-film with 0.35 mmol magnesium chloride/g of glucose

2.2. Pyrolysis Experiments

Thin films of metal-free glucose and homogeneously mixed glucose with AAEMs (NaCl, KCl, MgCl₂

and CaCl₂) were pyrolyzed using a micropyrolyzer (PY-3030S single shot pyrolyzer, Frontier

Laboratories Ltd., Japan) at 200 ^oC. The operating temperature for pyrolysis experiments was selected

as 200 ºC as the melting point of glucose is 146-150 ^oC, therefore at 200 ^oC, only primary reactions of

glucose decomposition in the presence and absence of the metal ion will be predominant. This in-turn

generated few (~ 9–11) volatile compounds along with char, thus minimizing secondary reactions of

the intermediates and/or primary products, which assisted in evaluating the direct influence of metal

ion(s) on pyrolysis reactions. Moreover, the temperature of glucose thin-film samples were raised by

∼1,000,000 °C/min, in the micropyrolyzer, which is three-to-five orders of magnitude faster than

traditional heating rates in pyrolysis techniques.^{2, 11, 33}This allowed glucose thin-film samples to

decompose under an isothermal and reaction-controlled regime. Thus, the pyrolysis product distribution

obtained is a representation of the intrinsic chemistry of glucose decomposition (free of transport-

limitations).

Pyrolysis volatile products were immediately transferred (using a continuous flow of helium gas) to a

gas chromatograph (Agilent, Model 7890B) coupled with a mass spectrometer (Model 5977B MSD)

for identification and quantification. Post reaction, the solid residue of pyrolysis (char) was quantified

via combustion by injecting the oxygen pulse into the micropyrolyzer.^{2, 7, 11}The volatile products were

identified using both mass spectrometry and by comparing retention times (RT) of these compounds to

that of pure standards. The compounds, such as 1,4,3,6-dianhydroglucopyranose (DAGP); 1,6-

anhydroglucofuranose (AGF); 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose (ADGH); 2,3-

dihydro-3,5-dihydroxy-6-methyl-4HPyran-4-one (DHMDHP); and 1,2-cyclopentanedione (CPD) were

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2

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DOI: 10.1039/C9CY00005D

quantified using previously reported method. Pyrolysis experiments of glucose thin-films in the

presence and absence of metal salts were conducted in triplicate and the average values (product yields,

% C basis) are shown in the results and discussion section 4.1 with the experimental error. The standard

deviation in individual product yield ranged between 1–2 %. The overall carbon balance for thin-film

pyrolysis of glucose/metal salts, including volatile products and char, was 82-90%.

3. Computational Methodology

All-electron DFT calculations were performed using Gaussian 09 code.³⁴The 6-311++G(2d,p) basis set

with the hybrid M06-2X functional was used in the present study as it provided accurate estimates of

energy barriers for carbohydrate chemistry.^{20, 24, 25}No constraints were implemented on the atoms during

geometry optimizations and transition state (TS) search. The Berny algorithm, as implemented in the

Gaussian code, was used for the TS search. Geometry optimizations were followed by frequency

calculations to differentiate between the saddle points and local minima on the potential energy surface

i.e., the TSs and ground state structures, by the presence and absence of an imaginary frequency,

respectively, which relates to the reaction coordinate. Next, intrinsic reaction coordinate (IRC)

calculations were performed to verify that the molecular structure of the obtained TS linked with the

anticipated reactant and product on the potential energy surface.

Glucose is chosen as the model compound in the present study as it is the monomer of cellulose and an

important intermediate that is formed during cellulose pyrolysis.³⁵The Polarizable Continuum Model

(PCM) using the integral equation formalism variant (IEFPCM) was taken into account for all the

geometry optimization and TS search calculations intended for emulating the condensed phase of

cellulose pyrolysis. Ethanol was used as the implicit solvent in the continuum model. This is because

the dielectric constant (ε) of EtOH (24.85) is comparable to that of glucose (21.0) at 200 ^oC.³⁶

The electronic properties and the bonding character of AAEM ions with glucose/intermediates/the TS(s)

were evaluated using the Natural Bond Orbital (NBO) analysis. For evaluating metal-glucopyranose

and metal-intermediate complexation, the metal ion was complexed with various oxygen atoms in

glucopyranose and the intermediate (formed in a multi-step reaction). Enthalpy of complexation

(∆퐻_{푐표푚푝푙푥푛}) is calculated using eqn (1).

∆퐻_{푐표푚푝푙푥푛}= 퐻_{푐표푚푝푙푥}− (퐻_{푔푙푢푐표푝푦푟푎푛}+ 퐻_{푚푒푡푎푙−푖표푛}

)

(1)

The metal coordinated structure which results in the most favourable ∆퐻_{푐표푚푝푙푥푛}was taken as the

ground state reactant/intermediate complex and was used for activation barrier calculations. Figure 2

shows the atom numbering in β-D-glucopyranose (1). The oxygen atom bonded to the carbon have that

carbon’s number. Similarly, the hydrogen attached to the oxygen has that oxygen’s number, while the

hydrogen atoms attached to carbon are numbered sequentially from H₇to H₁₃, commencing from C₁.

Throughout the discussion, the atoms will be referred to by this nomenclature. In addition to this, the

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reactant, intermediate(s) and products from each reaction pathway are assigned a specific nu^Vm^iewbe^Arr^{ticle Online}

DOI: 10.1039/C9CY00005D

(written in parenthesis), while each TS is given a label, e.g. AG-ts1, where the first two letters ‘AG’

indicate the chemical species i.e., anhydroglucofuranose, while ‘ts1’ indicates the transition state of

step-number 1 in a multi-step reaction. This label is written on the reaction arrow of each reaction

pathway. The enthalpic activation barriers, reported in kcal.mol^-1, were computed at 1.0 atm and 200

°C.

Levoglucosan

(LGA)

1,2-cyclopentanedione

Levoglucosenone

(CPD)

(LGO)

Furfural

1,6-anhydroglucofuranose

β-D-glucopyranose

(AGF)

5-hydroxymethylfurfural

(5-HMF)

1,5-anhydro-4-deoxy-D-glycerol

2,3-dihydro-3,5-dihydroxy-

6-methyl-4HPyran-4-one

(DHMDHP)

hex-1-en-3-ulose (ADGH)

Figure 2: Major compounds (bio-oil components) obtained from the pyrolysis of β-D-glucopyranose in

metal-free conditions. The atom numbering scheme in β-D-glucopyranose (1) is also shown.

4. Results and Discussion

4.1 Glucose thin-film pyrolysis with and without metal salts: Product distribution and bio-oil

composition

Figure 3[A] shows bio-oil and char yields in the presence and absence of AAEM ions. Thin-film

pyrolysis of glucose without metal salts at 200 ºC produced 20.93 ± 0.18 % of bio-oil and 61.71 ± 1.05

% char. Pyrolysis of Mg(II) and Ca(II) /glucose thin-film produced similar yields of bio-oil (~ 20.5 %)

and char (~ 62.05 %), while Na(I)/glucose and K(I)/glucose thin-films decreased bio-oil yield by 2-6%,

which, in turn, increased char yield by 9-10%.

The bio-oil so formed is comprised of anhydrosugars, namely levoglucosan (LGA) and 1,6-

anhydroglucofuranose (AGF); pyrans: 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose (ADGH) and

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2,3-dihydro-3,5-dihydroxy-6-methyl-4HPyran-4-one (DHMDHP); furans i.e., furfural,^{View Article Online}

5-

DOI: 10.1039/C9CY00005D

hydroxymethylfurfural (5-HMF), furan and 5-methylfurfural, as well as light oxygenates like 1,2-

cyclopentanedione (CPD). In the absence of the metal ions, the highest yields from glucose pyrolysis

were of DHMDHP (~ 8 %), followed by LGA (~ 7%), furfural (~ 2.6 %) and then ADGH and AGF (~

1.2 %). 5-HMF, furan, 5-methylfurfural and CPD were obtained in lower yields (< 1 %) and remained

as minor products. Figure 3[B] shows major products of glucose thin-film pyrolysis in the presence and

o

absence of AAEMs at 200 C. Table S1 of the supporting information lists the complete product

distribution of glucose pyrolysis.

In the bio-oil obtained from Mg(II)/glucose thin-film pyrolysis, anhydrosugars comprised of

levoglucosenone (LGO) and 1,4,3,6-dianhydroglucopyranose (DAGP) were additional products along

with LGA and AGF. Presence of Mg(II) does not exhibit a significant change in AGF yield, while LGA

yield increased by 2 %, as compared to metal-free glucose. Furans such as furfural also increased by

4.5 % for Mg(II)/glucose thin-film, while DHMDHP and ADGH decreased by 7 % and 1 %,

respectively, compared to the yields of these components obtained from pure glucose. Among minor

products for Mg(II)/glucose, furan and 5-methylfurfural increased, while 5-HMF and CPD decreased.

Further, the comparison of bio-oil components from Ca(II)/glucose with metal-free glucose pyrolysis

showed that the yields of AGF in the presence of Ca(II) increased by 2.8 %, while LGA decreased by

1.2%. Similar to Mg(II)/glucose thin-film pyrolysis, LGO (~ 3.8 %) and DAGP (~ 0.5 %) were obtained

as additional anhydrosugar compounds in the bio-oil of Ca(II)/glucose. Furfural in the bio-oil also

increased by 2.8 % for Ca(II)/glucose, while no significant change in 5-HMF yield was observed for

Ca(II)/glucose when compared to product distribution from pure glucose pyrolysis. Pyrans viz.

DHMDHP and ADGH decreased by 6.7 % and ~ 1 %, respectively, in Ca(II)/glucose.

In the bio-oil from Na(I)/glucose and K(I)/glucose pyrolysis, anhydrosugars included LGA and AGF

only; no LGO and DAGP were observed. This trend is similar to that shown by metal-free glucose.

Further, LGA yield decreased by 2 % and 1.2 %, respectively, in the presence of Na(I) and K(I) ions.

The yield of AGF was the highest (~ 3.8 %) in the presence of Na(I) ions, while AGF yield slightly

decreased to ~ 3.4 % in the presence of K(I) ions. Furfural increased marginally (~ 0.5 and 1.5 % for

Na(I) and K(I) ions, respectively), while HMF and 5-methylfurfural, remained unchanged for Na(I) and

K(I)/glucose pyrolysis compared to that of metal-free glucose. The yield of pyrans, viz., DHMDHP and

ADGH from Na(I) and K(I) assisted glucose pyrolysis also decreased by 5-6 % and 0.6 %, respectively,

compared to their yields in the absence of metal ion.

Similar to Na(I)/glucose pyrolysis, anhydrosugars generated from K(I)/glucose pyrolysis comprised of

LGA and AGF only. Further, LGA yield decreased by about 1.2 %, while furfural yield increased by

1.5 % compared to that of metal-free glucose. The yield of ADGH and DHMDHP also decreased by

about 0.6 % and 5.5 %, respectively, during K(I)/glucose pyrolysis.

10

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DOI: 10.1039/C9CY00005D

25

20

15

10

5

75

70

65

60

55

Bio-oil

62.1

char

60.5

71.1

70.1

61.7

0

Glucose Gluc+Mg Gluc+Ca Gluc+Na Gluc+K

(Gluc)

[A]

10

Glucose (Gluc)

Gluc+Mg(II)

Gluc+Ca(II)

Gluc+Na(I)

Gluc+K(I)

8

6

4

2

0

DHMDHP

LGA

Furfural

AGF

ADGH

LGO

[B]

Figure 3: [A] shows bio-oil and char yields and [B] shows major bio-oil components from glucose thin-

film pyrolysis, in the presence and absence of alkali and alkaline-earth metal salts at 200 ^oC. The metal

salt loading is 0.35 mmole /g of glucose at 200 ^oC. [Nomenclature for pyrolysis products: DHMDHP:

2,3-dihydro-3,5-dihydroxy-6-methyl-4H-Pyran-4-one;

LGA:

levoglucosan;

AGF:

Anhydroglucofuranose; ADGH: 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose and LGO:

Levoglucosenone]

In summary, our experimental results show that Mg(II)/glucose pyrolysis resulted in the highest yield

of furfural, while, Ca(II)/glucose produced more LGO than metal-free glucose. Further, Na(I)/ glucose

resulted in a higher yield of AGF, while K(I)/glucose thin-film resulted in the highest yield of LGA.

Interestingly, DHMDHP, the major product from metal-free glucose decomposition, decreased

significantly in the presence of AAEM ions. Thus, it is evident from experiments that the presence of

metal ions influences glucose pyrolysis product distribution differently, especially the yield and

composition of bio-oil. Therefore, to evaluate the change in the reaction chemistry and kinetics brought

11

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Page 12 of 40

about by the complexation of metal ions with the reactant/intermediates/the TS, DFT calculations^Vw^iewer^Ae^{rticle Online}

DOI: 10.1039/C9CY00005D

performed.

Thermal decomposition reactions of metal/glucose complexes were computed using DFT to understand

the stereoelectronic basis of the AAEM ions on the intrinsic chemistry of glucose pyrolysis. Numerous

elementary reaction mechanisms have been proposed in the literature for the formation of 5-HMF, LGA

and AGF^{20, 21, 23, 24}. To investigate the catalytic (or anti-catalytic) effect of metal ions on the reaction

pathways of LGA, AGF, ADGH, furfural and 5-HMF formation, only those reaction pathway(s) that

are kinetically favored over other reported mechanisms described in the literature were computed in the

present study. The activation barriers from these calculations were then correlated with the experimental

yields of bio-oil components and char so as to obtain atomistic insights into the reaction chemistry of

AAEM-catalyzed glucose decomposition.

4.2 Coordination complexes of alkali- and alkaline-earth metal ions with β-D-glucopyranose

The positively charged AAEMs Na(I), K(I), Ca(II) and Mg(II) ions coordinate with the electron-rich

hydroxyl and pyranose oxygen atoms of β-D-glucopyranose (1). The stability of the metal/sugar

complex is governed by the coordination site of the sugar molecule with the metal ion.³⁷Na(I) ion was

chosen as the model ion to investigate the most suitable coordination site for the complexation of metal

ions with glucose. The stable metal-glucose complexes will be used as a reference point or starting

structures to evaluate the activation barriers of glucose decomposition reactions in the presence of

AAEMs. NBO charge distribution of β-D-glucopyranose (1) shows that hydroxyl groups possess

slightly higher electronic charge (i.e., -0.75-0.77 au) compared to the ether oxygen atom having atomic

charge of -0.65 au. Hence, there are four possible sites for the Na(I) ion to coordinate with β-D-

glucopyranose (1), which are O₁, O₂categorized as site1; O₃, O₄: site2; O₄, O₆: site3 and O₅, O₆: site4.

Figure 4[A] shows the NBO charge distribution of (1). Table 1 lists the complexation enthalpies of

Na(I) ion coordinated at all the four sites.

The sites 2 and 4 result in similar activation enthalpies of -5.11 and -5.13 kcal.mol^-1, respectively, for

Na(I) ion coordination with β-D-glucopyranose. On the other hand, Na(I)/glucose complexes at sites 1

and 3 have complexation enthalpies of -4.55 and -3.26 kcal.mol^-1, respectively. This is because site1

Na(I)/glucose complex exhibits lesser charge transfer between Na(I) ion and O₁, O₂donor atoms, while

Na(I)/glucose site3 complex is devoid of an intramolecular H-bond between H₆and O₅atoms compared

to the coordination structures of Na(I)/glucose at site2 and site4. Literature studies discussed that metal

(aquo) chloride/hydroxide complexes preferred to complex with C₁and C₂oxygen atoms of glucose.³⁸

However, the reported complexation studies were evaluated in the aqueous phase/in the presence of a

solvent as opposed to the condensed-phase environment observed during pyrolysis, which in turn,

resulted in different binding sites of the metal ion with β-D-glucopyranose. Figures 4[B] to 4[E] show

Na(I)/β-D-glucopyranose-coordination complexes at sites 1 to 4.

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DOI: 10.1039/C9CY00005D

Table 1: Complexation enthalpies (ΔH_complx) of Na(I) ion with β-D-glucopyranose

Molecular structure of glucose-

Na(I) complex*

Oxygen atoms

Site

ΔH_complx, kcal.mol^-1

O₁, O₂

O₃, O₄

O₄, O₆

O₅, O₆

1

2

3

4

-4.55

-5.11

-3.26

-5.13

Na

*Representation of Na(I)-glucose coordination complex

Following the Na(I)/β-D-glucopyranose complexation studies, the coordination of K(I), Ca(II) and

Mg(II) ions with β-D-glucopyranose at sites 2 and 4 was evaluated. It was observed that the divalent

Mg(II) and Ca(II) ions preferred to coordinate at site4 (O₅, O₆) over site2 (O₃, O₄), while Na(I) and

K(I) ions exhibited similar complexation enthalpies at both sites 2 and 4. Mg(II) and Ca(II) ions

coordinated at site4, had complexation enthalpies of -23.2 and -11.7 kcal.mol^-1, respectively; while

Na(I) and K(I) ions showed complexation enthalpies of -5.13 and -5.88 kcal.mol^-1, respectively. Higher

relative stability of divalent ions glucose complexes over alkali-ion complexes was because of divalent

metal ions’ higher charge density, which enabled stronger coordination of divalent metal ions with

oxygen atoms. Table 2 summarizes the complexation enthalpies of metal ions with sites 2 and 4 oxygen

atoms and also lists the changes in geometrical parameters of β-D-glucopyranose on metal coordination

and change in NBO charges of metal ion after complexation with glucose.

The C(1)-O(5) and C(5)-O(5) bonds elongate by 0.015–0.04 Å in Mg(II)/glucose-site4 complex

(1+Mg(II)) compared to the C-O bond lengths of β-D-glucopyranose. This implies the prospect of π-

back donation from the metal ion to β-D-glucopyranose, thus stabilizing the Mg(II)/glucose-site4

complex. In contrast, the remaining metal complexes showed that the C-O bond lengths get minimally

altered (~0.02 Å) which indicated lower stability.

In summary, the stable metal/glucopyranose configurations serve as ground-state reactant complexes

for evaluating the activation enthalpies of glucose decomposition. Further, insights into AAEM-

glucopyranose coordination complexes suggest that binding of AAEMs with oxygen atoms of

glucopyranose stabilize the metal/glucopyranose coordinate complexes, which in turn, can provide

stability to the energy intensive transition states of the reaction. However, the binding site of AAEMs

in the TSs may not be the same as its binding in the ground-state complexes. This is because the TS

involves a change in the electronic/bonding properties of the atoms involved in the reaction, which, in

turn, governs the binding of the metal ion in the TS. The details of metal binding in the TS and the

intermediate species generated during the reaction pathway are discussed in section 4.4.

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DOI: 10.1039/C9CY00005D

-0.762

-0.655

-0.765

-0.756

Na

-0.770

-0.767

[A]

[B]

[C]

[F]

[I]

Na

K

Na

[D]

[E]

K

[G]

[H]

2.01

Mg

[J]

[K]

Figure 4: [A] Charge distribution in β-D-glucopyranose (au); [B] Na(I) ion coordination with O₁, O₂

atoms: site1; [C] Na(I) ion coordination with O₃, O₄atoms: site2; [D] Na(I) ion coordination with O₄,

O₆atoms: site3 [E] Na(I) ion coordination with O₅, O₆atoms: site4; [F] K(I) ion coordination at site2;

[G] K(I) ion coordination at site4; [H] Ca(II) ion coordination at site2; [I] Ca(II) ion coordination at

site4; [J] Mg(II) ion coordination at site2; [K] Mg(II) ion coordination at site4. The units of the

highlighted distances are Å. Grey, red and white balls indicate C, O, and H atoms, respectively.

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Table 2: Enthalpies of complexation of alkali- and alkaline-earth metal ions with β-D-glucopyranose

DOI: 10.1039/C9CY00005D

(1) at sites 2 and 4. Changes in the geometrical parameters, Natural Bond Order (NBO) charges of metal

ion, and difference in the formal and NBO charge of the metal after complexation with β-D-

glucopyranose at site4 are listed.

(1)

(1)+Mg(II)

(1)+Ca(II) (1)+Na(I) (1)+K(I)

Atoms of β-D-glucopyranose

involved in metal ion coordination

Site2: O(3) & O(4)

Enthalpy of complexation (kcal.mol^-1)

--

-21.9

-23.2

-11.1

-11.7

-5.11

-5.13

-5.31

-5.88

Site4: O(5) & O(6)

^aGeometrical parameters of site4 complexes

C(1)-O(1) [Å]

C(1)-O(5) [Å]

C(5)-O(5) [Å]

C(6)-O(6) [Å]

q_metal[au]

1.387

1.414

1.423

1.146

1.372

1.454

1.442

1.957

0.043

1.382

1.435

1.434

1.971

0.029

1.385

1.388

1.421

1.426

1.425

0.986

0.014

1.418

1.424

1.423

0.988

0.012

^b(Mⁿ⁺) - (q_metal

)

^aThe units of geometrical parameters and NBO charge are written in square brackets; ^bformal charge

on the metal ion before complexation with β-D-glucopyranose.

4.3: Reaction Map of Glucose Decomposition

Figure 5 shows the reaction network of glucose pyrolysis, resulting in the formation of experimentally

observed products i.e., AGF, ADGH, LGA, LGO, 5-HMF, furfural, and char.

The formation of AGF from β-D-glucopyranose (1) involves cleavage of the 1,5-acetal bond to form

acyclic D-glucose (2). 1,4-acetal bond formation in D-glucose (2) generates β-D-glucofuranose (3). The

glucofuranose moiety (3) forms 1,6-acetal bond with loss of water in a concerted fashion resulting in

AGF (4).^{23, 39, 40}

ADGH formation from β-D-glucopyranose (1) takes place by the loss of a water molecule i.e., O₁H and

H₂atoms generating 1,2-anhydroglucopyranose (5). 1,2-anhydroglucopyranose (5) can further undergo

dehydration i.e., loss of H₃and O₄H generating intermediate (6), which subsequently undergoes

tautomerization of enol group at C₃forming ADGH (7). LGA (8) formation from β-D-glucopyranose

(1) can follow two pathways. The first reaction pathway involves loss of water i.e., -O₁H and H from

O₆followed by ring closure of O₆at C₁in a concerted manner.²³The second reaction pathway for LGA

formation consists of isomerization of 1,2-anhydrolucopyranose (5). Under pyrolysis conditions, LGA

is unstable and reactive.⁴¹LGO, a secondary reaction product of LGA decomposition⁴¹, is formed by

loss of water molecule i.e., H₂and O₃H atoms from LGA (8) resulting in the formation of an enol

intermediate (9), which generates a ketone intermediate (10) and finally another loss of water molecule

H₈and O₄H groups forming LGO (11).

The reaction pathway for 5-HMF formation from β-D-glucopyranose (1) involves a series of ring

opening and dehydration reactions. Two routes for keto-intermediate (15) formation are possible²⁴.

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Route one involves D-fructose (12) as an intermediate (path shown in black (2)-(15)), while route two

DOI: 10.1039/C9CY00005D

involves cyclization D-glucose (2) with water loss (pathway shown in brown (2) to (15)). Furthermore,

the ketone intermediate (15) also exhibits two competing routes for 5-HMF (18) formation i.e., loss of

the first water molecule can occur from C_βof aldehydic group (path in green (15) to (16)) or from C_β

of alcoholic group (path in black (15) to (17)). Further, the most favourable path for furfural (19)

formation is by deformylation of 5-HMF.²⁵

Char formation is a convoluted process and predominantly involves dehydration and bimolecular

condensation reactions of C5 molecules and dehydrated volatile products/intermediates.²²The primary

OH group in 1,2-anhydroglucopyranose (5) can attack electron deficient C of C5 species (e.g. furfural

or 5-HMF) by nucleophilic addition reaction to form an α,β-diol intermediate Ch-int1 (20). The Ch-int1

intermediate then undergoes dehydration, forming two enol intermediates, namely Ch-int2G (21) and

Ch-int2F (22). The next step involves tautomerization of the enol generating ketone intermediates i.e.,

Ch-int3G (23) and Ch-int3F (24). The ketone intermediates are highly reactive due to the presence of

α-H atoms and hence can further react with a carbonyl intermediate or can undergo nucleophilic

addition reaction with the primary hydroxyl group of an intermediate/product. A series of reactions of

ketone intermediates can take place in the pyrolysis condensed-phase thus contributing to char.

After mapping out the plausible route of glucose decomposition chemistry, molecular insights into the

catalytic/ anti-catalytic effect of AAEM ions for the formation of AGF (4), ADGH (7), LGA (8) LGO

(11), 5-HMF (18), furfural (19) and char were investigated.

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Catalysis Science & Technology

(16)

(18)

(19)

(14)

(23)

(24)

(21)

(22)

(17)

(15)

(13)

(20)

(12)

(1)

(5)

(6)

(3)

(7)

(2)

(4)

(10)

(11)

(8)

(9)

Figure 5: Reaction network of β-D-glucopyranose decomposition to form bio-oil components anhydroglucofuranose (4), 1,5-anhydro-4-deoxy-D-glycerohex-

1-en-3-ulose (7), levoglucosan (8), levoglucosenone (11), 5-hydromethylfurfural (18), furfural (19) and char. The numbers in parenthesis indicate the

intermediate/product components formed during the course of the reaction. β-D-glucopyranose is shown in blue. The product components observed

experimentally are shown inside the dashed box.

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4.4: Mechanistic pathways of metal/glucose decomposition reaction chemistry

DFT calculations were performed to gain insights into the reaction kinetics and understand the

stereoelectronic basis of the metal cation when coordinated to the energy-intensive transition state of

the reaction pathway.

4.4.1: Formation of Anhydroglucofuranose (AGF)

The formation of AGF from β-D-glucopyranose (1) involves ring opening to form acyclic D-glucose

(2), followed by 1,4-acetal bond formation in (2) generating β-D-glucofuranose (3), which, in turn,

undergoes 1,6-acetal bond with loss of water in a concerted fashion, resulting in AGF (4).^{23, 39, 40}

Formation of β-D-glucofuranose (3) is the most crucial step responsible for AGF formation.²⁰DFT

computed activation enthalpies of the three steps involved in AGF formation i.e., AG-ts1, AG-ts2 and

AG-ts3 are 45.4, 38.3 and 54.7 kcal.mol^-1, respectively, in the absence of metal ions. The activation

enthalpy values indicate that 1,6-acetal bond formation accompanied by the loss of a water molecule,

i.e., AG-ts3, is the rate-determining-step during AGF formation from glucopyranose. Therefore, the

relative change in activation enthalpies of the rate-limiting step in the presence and absence of AAEMs

is compared with the relative change in AGF experimental yields. Figure 6 shows the reaction scheme

for the conversion of β-D-glucopyranose to AGF and the correlation plot of AGF experimental yields

with the computed activation barriers of the rate-limiting step AG-ts3, in the presence and absence of

AAEMs, respectively. The activation enthalpies of all the steps during the formation of AGF i.e., AG-

ts1, AG-ts2 and AG-ts3; in the presence and absence of metal ions are listed in Table S2 of the

supporting information.

( ) ( )

The complexation of Mg(II) ions lowers the activation barriers of the first two steps i.e., ퟏ → ퟐ and

-1

( ) ( )

ퟐ → ퟑ to 40.0 and 34.8 kcal.mol , respectively for AG-ts1 and AG-ts2. Similarly, the complexation

of Ca(II) ions also decreases the activation barriers of AG-ts1 and AG-ts2 to 40.7 and 34.3 kcal.mol^-1,

respectively. On the other hand, Mg(II) increases the activation enthalpy of the rate-limiting step i.e.,

-1

( ) ( )

ퟑ → ퟒ to 61.9 kcal.mol , while complexation with Ca(II) ion results in an activation barrier of 56.7

kcal.mol^-1. Furthermore, Na(I) and K(I) ions complex with AG-ts1 and AG-ts2 and result in barriers of

43 - 44 and 35 - 36 kcal.mol^-1, respectively, while the activation barrier of AG-ts3 decreases to 45.4 and

44.7 kcal.mol^-1, respectively, compared to 54.7 kcal.mol^-1for metal-free glucose. In summary, the trend

in activation enthalpies of the rate-limiting step for AGF formation is as follows: Mg(II) > Ca(II) >

K(I) > Na(I)/glucose, which, in turn, corresponds to the variation in AGF yield i.e., highest for

Na(I)/glucose and the least is for Mg(II)/glucose.

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Catalysis Science & Technology

AG-ts3

AG-ts2

AG-ts1

β-D-glucopyranose

(1)

Anhydroglucofuranose

(AGF) (4)

β-D-glucofuranose

(3)

D-glucose

(2)

% yield

5

80

60

40

20

activation enthalpy: 1,6-acetal bond formn

4

3

2

1

0

61.9

54.7

56.7

45.4

44.7

No metal

Mg

Ca

Na

K

Figure 6: Correlation plot of the change in activation enthalpies of the rate-limiting step i.e., 1,6-acetal bond formation (shown in red in the reaction mechanism),

in the presence and absence of alkali- and alkaline-earth metals with the change in 1,6-anhydroglucofuranose (AGF) experimental yields. The yield of AGF is

expressed as %C basis. The blue arrows in the molecular structures show the electron flow during the reaction. Transition states of the reactions are illustrated

above the arrow (in brackets), where the atoms not taking part in the reaction are faded.

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4.4.2: Formation of ADGH

ADGH is categorized as a pyran. The experimental results of glucose pyrolysis show that in the absence

of metal ions, ADGH yield is higher than that of metal-complexed glucose thin-film samples. ADGH

formation from β-D-glucopyranose (1) takes place by the loss of a water molecule, i.e., O₁H and H₂

atoms generating 1,2-anhydroglucopyranose (5), which further undergoes dehydration and

tautomerization forming ADGH (7). The activation barriers of the aforementioned three steps are

labelled as AD-ts1, AD-ts2 and AD-ts3. In AD-ts1, the leaving group O₁H is stabilized by H from O₆H

group. This results in elongation of O₆H bond distance to 1.06 Å. On the other hand, no intramolecular

H-bonding is present with O₄H group and H₉atoms of AD-ts2. In AD-ts3, tautomerization of the enol

group at C₃forms a four-membered cyclic transition state generating ADGH. In the absence of the metal

ion, the activation enthalpies of AD-ts1, AD-ts2 and AD-ts3 are 60.2, 69.9 and 50.3 kcal.mol^-1,

respectively. These activation barrier values are in agreement with the barriers reported by Hu et al. for

20

ADGH formation

.

The intermediate 1,2-anhydroglucopyranose can also undergo isomerization generating levoglucosan

(LGA) ⁴¹. Additionally, 1,2-anhydroglucopyranose can polymerize^{22, 41}or react with other components,

and can initialize char formation. The TS for 1,2-anhydroglucopyranose (5) to LGA (8) isomerisation

i.e., Ch-tsR has an activation enthalpy of 53.3 kcal.mol^-1, while the reverse reaction i.e., LGA to 1,2-

anhyglucopyranose, Ch-tsF, has an activation enthalpy of 61.0 kcal.mol^-1. Additionally, the

condensation reaction of 1,2-anhydroglucopyranose with a reaction intermediate/ product of pyrolysis,

in this case furfural, involves nucleophilic addition of 1,2-anhydroglucopyranose to carbonyl ‘C’ of

furfural. The TS, Ch-ts1, has an activation barrier of 79.6 kcal.mol^-1in the absence of the metal ion.

Thus, in the absence of the metal ion, the formation of ADGH is kinetically favorable as the activation

enthalpies i.e., Ch-tsF and AD-ts2, for the generation and conversion of 1,2-anhydroglucopyranose (5),

an important intermediate during ADGH formation, are lower than the other two competing reactions.

Figure 7 shows the reaction scheme and the correlation plot of ADGH yields in the presence and absence

of metal ions with the activation enthalpies of 1,2-anhydroglucopyranose to LGA isomerization, rate-

limiting step i.e., AD-ts2 in ADGH formation and the reaction of 1,2-anhydroglucopyranose, i.e., Ch-

ts1, to form char.

In Mg(II)/glucose, unlike H₆bonding to the leaving group O₁H, intramolecular H-bonding between H₄

and O₆exists due to the coordination of Mg(II) ions with O₃and O₄atoms in AD-ts1, which increases

the activation enthalpy of AD-ts1 to 70.9 kcal.mol^-1. In the TS of the second step, AD-ts2, Mg(II) ions

bind to the leaving O₄H group, facilitating the loss of water and lowers the activation enthalpy to 56.9

kcal.mol^-1. On the other hand in AD-ts3, Mg(II) ions coordinate with oxygen atom of the enol group at

C₃and increases the activation barrier to 56.1 kcal.mol^-1. Moreover, the coordination of Mg(II) ion with

O₆atom of 1,2-anhydroglucopyranose decreases the activation enthalpy to 33.0 kcal.mol^-1for the

isomerization of 1,2-anhydroglucopyranose to LGA, while the activation barrier for the reverse

reaction, Ch-tsF, increases to 76.9 kcal.mol^-1due to the coordination of Mg(II) ions with O₃and O₄in

20

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Catalysis Science & Technology

Ch-tsF. The subsequent reaction of 1,2-anhydroglucopyranose (5) with furfural (19) res_Du_Olt_Ii_:n₁₀g_.10in₃^V₉ⁱc_/^e_C^wh₉a^A_C^rr^t_Y^ic₀^le₀^O₀₀^nl₅ⁱⁿ_D^e

formation is not catalyzed by Mg(II) ions, thus resulting in an activation barrier of 80.9 kcal.mol^-1. Thus,

the effect of Mg(II) ions on ADGH yield is anti-catalytic, as complexation of Mg(II) ion with 1,2-

anhydroglucopyranose intermediate (5) favors isomerization to LGA over the other three competing

reactions.

The coordination of Ca(II) ion with electron rich O-atoms in AD-ts1, AD-ts2 and AD-ts3 is similar to

that exhibited by Mg(II) ion. The activation enthalpies of Ca(II) coordinated AD-ts1, AD-ts2 and AD-

ts3 are 72.3, 69.4 and 53.5 kcal.mol^-1, respectively. During 1,2-anhydroglucopyranose to LGA

isomerization, Ca(II) binds to O₃and O₆atoms in Ch-tsR and results in an activation enthalpy of 33.5

kcal.mol^-1, whereas in the TS of the reverse reaction i.e., Ch-tsF, Ca(II) ion coordinates with O₃and O₄

atoms and results in an activation enthalpy of 69.4 kcal.mol^-1. In addition to this, during the reaction of

1,2-anhydroglucopyranose (5) with furfural (19), Ca(II) ion binds to the aldehydic oxygen of the

furfural moiety and destabilizes Ch-ts1, resulting in the activation enthalpy of 83.8 kcal.mol^-1.

Therefore, on comparing the activation enthalpies of the four parallel reactions taking place during

ADGH formation, presence of Ca(II) ion kinetically favors conversion of intermediate 1,2-

anhydroglucopyranose to LGA and hence the yield of ADGH is low.

However, in the presence of Na(I) and K(I) ions, the activation enthalpies of Na(I)/AD-ts1, Na(I)/AD-

ts2 and Na(I)/AD-ts3 are 57.1, 73.0 and 51.9 kcal.mol^-1, respectively, while 57.5, 73.5 and 53.5

kcal.mol^-1are shown by K(I) assisted TSs. The enthalpic barrier of AD-ts1 corroborates with the

reported activation energy value of 59.0 kcal.mol^-1for 1,2-dehydration in Na(I)/glucopyranose²⁵. In AD-

ts1, AD-ts2 and AD-ts3 TSs, the O-atoms coordinating with Na(I) and K(I) ions are similar. In AD-ts1,

the alkali ion coordinates with O₁H and O₆H atoms, which slightly increases intramolecular H₆--O₁

bond distance to 1.45 Å. In alkali ion complexed AD-ts2, the univalent ions coordinate with O₄and O₆

atoms, which, in turn, lowers the affinity of the leaving group O₄H to abstract the neighbouring H-atom

and increases the activation enthalpy compared to metal-free AD-ts2. In AD-ts3, the alkali ion

coordinates to the enolic O-atom of the 4-membered cyclic TS. The intermediate 1,2-

( )

anhydroglucopyranose (5) can isomerize to LGA (8) and vice versa. The enthalpic barrier of ퟓ →

-1

( )

ퟖ isomerization, Ch-tsR, has activation enthalpies of 48.2 and 50.2 kcal.mol , while the TS of the

-1

( ) ( )

reverse reaction ퟖ → ퟓ , Ch-tsF, has enthalpic barrier of 60.0 and 62.1 kcal.mol , for Na(I) and K(I)

ions, respectively. Further, complexation of Na(I) and K(I) ions with Ch-ts1assists in the coupling

reaction of 1,2-anhydroglucopyranose (5) with furfural (19) initializing char formation and results in

activation barrier values of 76.3 and 72.8 kcal.mol^-1, respectively.

Thus, by comparing the activation enthalpy values of AD-ts2, Ch-tsR, Ch-tsF and Ch-ts1, it can be seen

that there are significant differences in the enthalpic barriers with and without AAEM ions. While the

difference in the activation energies of these four parallel reactions is only about 7-10 kcal.mol^-1in the

presence and absence of Na(I) and K(I) ions, about 25-30 kcal.mol^-1difference exists when Mg(II) and

21

Catalysis Science & Technology

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View Article Online

DOI: 10.1039/C9CY00005D

(20)

Ch-ts1

AD-ts1

AD-ts3

AD-ts2

(5)

(1)

(6)

ADGH (7)

Ch-tsF

Ch-tsR

(8)

% yield

F: AD-ts2

C: Ch-ts1

C: Ch-tsR

76.3

1.5

1.2

0.9

0.6

0.3

0

100

F: Ch-tsF

83.8

72.1

80.9

79.6

80

60

40

20

73.5

72.8

76.9

33

69.9

61

73

60

69.4

62.1

56.9

50.2

48.2

53.3

33.5

No metal

Mg

Ca

Na

K

Figure 7: Reaction scheme and correlation plot of 1,5-anhydro-4-deoxy-D-glycerohex-1-en-3-ulose

(ADGH) yield with the activation enthalpies of four competing reactions affecting ADGH formation in

the presence and absence of alkali- and alkaline-earth metal ions. The yield of ADGH in the correlation

plot is expressed as %C basis. The dotted arrow in the reaction scheme indicates secondary reactions.

The labels of the transition states e.g. AD-ts1, are written on the reaction arrow. The labels F:AD-ts2

and F:Ch-tsF indicate the reactions that form ADGH/ its intermediate 1,2-anhydroglucopyranose, while

C:Ch-ts1 and C:Ch-tsR indicate the reactions that convert the reaction intermediate of ADGH. The blue

arrows in the molecular structures show the electron flow during the reaction. Transition states of the

reactions are illustrated above the arrow (in brackets), where the atoms not taking part in the reaction

are faded.

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Catalysis Science & Technology

View Article Online

Ca(II) ions are present. These results explain the experimentally observed trend in ADGH yield, where

DOI: 10.1039/C9CY00005D

the presence of metal ions effectively does not kinetically favour ADGH formation.

4.4.3: Formation of Levoglucosan (LGA) and the subsequent conversion of LGA to Levoglucosenone

(LGO)

LGA is one of the predominant anhydrosugars formed during glucose pyrolysis.³⁵The most common

reaction pathway for LGA (8) formation is from β-D-glucopyranose (1), which involves loss of water

i.e., -O₁H and H from O₆followed by ring closure of O₆at C₁in a concerted manner.²³The second

reaction pathway for LGA formation involves isomerization of 1,2-anhydroglucopyranose (5). In

addition to this, LGA, being unstable, can convert back to 1,2-anhydroglucopyranose (5) or even

undergo two successive dehydrations and tautomerization generating LGO (11). Figures 8 [A] and 8

[B] show the reaction schemes and the correlation plot of LGA yield with the activation enthalpies of

all four competing reactions affecting LGA formation in the presence and absence of AAEM ions.

In the absence of the metal ion, the TS for LGA formation from β-D-glucopyranose (1), LG-ts, shows

that O₆is stabilized by H-bonding with H of HO₃group²³, as evident by the elongation of O₃-H₃bond

distance from 0.98 Å in the ground state of β-D-glucopyranose to 1.02 Å in LG-ts. The activation

barrier for LGA formation from β-D-glucopyranose, LG-ts, is 47.2 kcal.mol^-1. 1,2-

anhydroglucopyranose (5), generated as an intermediate during ADGH formation, can isomerize to

LGA. The activation barrier of Ch-tsR is 53.3 kcal.mol^-1. For the conversion of LGA to LGO, the

activation enthalpies of the three reaction steps i.e., LO-ts1, LO-ts2 and LO-ts3 are 71.5, 57.3 and 51.6

kcal.mol^-1, respectively. Of the three reaction steps in LGO formation, the first step of water loss i.e.,

LO-ts1, has the highest activation barrier and is the rate-limiting step in LGO formation. Furthermore,

the TS for LGA to 1,2-anhydroglucopyranose conversion, Ch-tsF, results in an activation barrier of 61.0

kcal.mol^-1. Thus, in the absence of the metal ion, LGA is obtained in relatively high yield, because the

activation enthalpies of the reactions forming LGA with TSs Ch-tsR and LG-ts are 5-15 kcal.mol^-1

lower than the activation enthalpies of the reactions that convert LGA to other products, i.e., LO-ts1

and Ch-tsF.

In LG-ts, Mg(II) ions coordinate with O₁and O₂atoms. However, no H-bonding between H₃and O₆is

observed in Mg(II)/LG-ts, unlike the TS in the absence of the metal. The activation barrier for LG-ts in

the presence of Mg(II) ions is 48.2 kcal.mol^-1, which is comparable to the activation enthalpy of metal-

free TS i.e., 47.2 kcal.mol^-1. On the other hand, the activation barrier of 1,2-anhydroglucopyranose (5)

conversion to LGA, Ch-tsR, decreases to 33.0 kcal.mol^-1, as compared to 53.3 kcal.mol^-1in the metal-

free reaction. In Ch-tsR, Mg(II) ions bind to O₆atom facilitating ring closure and shortens the O₆-C₁

bond distance to 2.88 Å compared to 2.92 Å in metal-free TS. Further, Mg(II) ion coordinates with O₃

and O₄atoms during LGA to 1,2-anhydroglucopyranose reaction and results in an activation enthalpy

of 76.9 kcal.mol^-1for Ch-tsF. Next, binding of Mg(II) ions with O₂, O₄and O₅atoms in LO-ts1 results

in activation enthalpy of 70.6 kcal.mol^-1which is similar to that of metal-free TS i.e., 71.5 kcal.mol^-1.

23

Catalysis Science & Technology

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In LO-ts2, Mg(II) ions complex with O-atom of the enol group and increases the activation barrier to

DOI: 10.1039/C9CY00005D

66.3 kcal.mol^-1. Mg(II) ions coordinate with the leaving OH group i.e., O₄H in LO-ts3, which, in turn,

increases the activation barrier to 55.2 kcal.mol^-1.

In the presence of Ca(II) ions, the metal ion binds to O₂and O₄atoms, with H₃atom weakly bonded to

O₆in LG-ts. The activation barrier for LGA formation from (1) in the presence of Ca(II) ion is 52.0

kcal.mol^-1. The competing reaction of LGA (8) formation from 1,2-anhydroglucopyranose (5) is

kinetically favorable as the activation barrier of Ch-tsR is 33.5 kcal.mol^-1. In Ch-tsR, Ca(II) ion binds

to O₆atom of (5). Further, the conversion of LGA (8) to 1,2-anhydroglucopyranose (5) in the presence

of Ca(II) ions results in an activation barrier of 72.1 kcal.mol^-1, where Ca(II) ions complex with O₃and

O₄atoms in Ch-tsF. In addition to this, the activation barrier of LO-ts1, which is also the rate-

determining step in LGA to LGO reaction pathway, has an activation barrier of 72.1 kcal.mol^-1, while

the subsequent steps i.e., LO-ts2 and LO-ts3 have activation enthalpies of 63.5 and 49.9 kcal.mol^-1. The

oxygen binding sites for Ca(II) ions in LO-ts1, LO-ts2 and LO-ts3 are similar to that preferred by Mg(II)

ions.

The coordination of Na(I) and K(I) ions in LG-ts is preferred with O₃atom. Coordination of alkali ions

at O₃result in slight elongation of the interatomic distances between O₆, C₁and O₁atoms i.e., O₆-C₁=

2.57-2.59 Å; O₁-C₁= 2.80-2.82 Å; O₆-H₆= 1.65-1.67 Å in Na(I) and K(I) complexed LG-ts compared

to 2.62, 2.77 and 1.60 Å in the absence of the metal ion. However, the activation barrier values i.e., 47.0

and 48.2 kcal.mol^-1respectively, for Na(I) and K(I) assisted LG-ts, are similar to metal-free

glucopyranose. Na(I) and K(I) binding to O₆atom of 1,2-anhydroglucopyranose (5) in Ch-tsR decreases

the activation enthalpies to 48.2 and 50.2 kcal.mol^-1, respectively, compared to 53.3 kcal.mol^-1in metal-

free TS. On the other hand, the activation enthalpies for the ring opening of LGA to 1,2-

anhydroglucopyranose in the presence of Na(I) and K(I) ions have activation enthalpies of 60.0 and

62.1 kcal.mol^-1, respectively, which are similar to metal-free Ch-tsF of 61.0 kcal.mol^-1. In Ch-tsF, Na(I)

and K(I) ions bind to O₆atom of LGA, interatomic distance is metal(I)-O₆= 2.17 and 2.47 Å,

respectively. The metal ion and O₆bond distance in Ch-tsF is 0.03-0.05 Å higher than Ch-tsR, while

the interatomic distances between H₂, O₆and C₁are similar in Ch-tsF and Ch-tsR. Thus, the difference

in the activation barriers of Ch-tsF and Ch-tsR is attributed to the higher stability of the metal(I)-LGA

complex compared to metal(I)-1,2-anhydroglucopyranose complex. In addition to this, the activation

barriers of LO-ts1, LO-ts2 and LO-ts3 during LGO formation are 75.4, 61.0 and 49.7 kcal.mol^-1in

presence of Na(I) ions, while 74.4, 60.2 and 50.9 kcal.mol^-1, respectively for K(I) assisted. In LO-ts1,

Na(I) and K(I) ions bind to O₂, O₄and O₅atoms, while, the metal ions coordinate with O-atom of the

enol group in LO-ts2 and with O₄atom in LO-ts3.

In summary, the presence of Mg(II) and Ca(II) ions catalyzes the reaction steps for the formation of

LGA, i.e., Ch-tsR and LG-ts, owing to activation enthalpies of 33.0, 48.2 and 33.5, 52.0 kcal.mol^-1for

Mg(II) and Ca(II) catalyzed TSs, respectively. However, the activation barriers for Ch-tsR in the

presence of Na(I) and K(I) increase by 15-17 kcal.mol^-1, compared to the activation enthalpies for

24

Page 25 of 40

Catalysis Science & Technology

[A]

AD-ts1

(1)

(5)

Ch-tsR

Ch-tsF

LO-ts1

LO-ts3

LO-ts2

LG-ts

(10)

(11)

(9)

(8)

LGA yield

C:LO-ts1

LGO yield

C: Ch-tsF

F:LG-ts

F:Ch-tsR

75.5

[B]

[C]

Na(I) & K(I)

10

8

90

70

50

30

10

76.9

74.4

72.1

71.4

70.6

48.2

33

62.1

50.2

61

53.3

6

60

48.2

52

48.2

47.1

4

47.2

Ca(II) &

Mg(II)

33.5

2

0

No metal

Mg

Ca

Na

K

Figure 8: [A] and [B] Reaction scheme and correlation plot of levoglucosan (LGA) yield with the activation enthalpies of four competing reactions affecting

LGA formation in the presence and absence of alkali and alkaline-earth metal ions. [C] Fate of LGA in the presence of alkali and alkaline-earth metal ions. The

yield of LGA in the correlation plot is expressed as %C basis. The dotted arrows in [C] indicate the secondary reactions of the molecules. The labels of the

transition states e.g. LG-ts, are written on the reaction arrow. The labels F:LG-ts and F:Ch-tsR indicate the reactions that form LGA, while C:Ch-tsF and C:LO-

ts1 indicate the reactions that convert LGA. The blue arrows in the molecular structures show the electron flow during the reaction. Transition states of the

reactions are illustrated above the arrow (in brackets), where the atoms not taking part in the reaction are faded.

25

Catalysis Science & Technology

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Mg(II) and Ca(II) assisted reactions. On the other hand, Mg(II) and Ca(II) ions also_DOc_Ia_:t₁a₀l_.1y₀z₃e_9/Ct₉h_Ce_Y00005D

subsequent breakdown of LGA (8) to LGO (11) as these ions lower the activation barrier of LO-ts1,

which is the rate-limiting step, but have an anti-catalytic effect on LGA conversion to 1,2-

anhydroglucopyranose. Moreover, Na(I) and K(I) exhibit a reverse trend for LGA conversion reactions

compared to Ca(II) and Mg(II) ions, i.e., Na(I) and K(I) ions have a neutral effect on LGA conversion

to 1,2-anhydroglucopyranose, while the activation enthalpies to convert LGA to LGO increases by 4-5

kcal.mol^-1. Therefore, in the absence of metal ions and in the presence of alkali ions LGA yield is

lowered as 1,2-anhydroglucopyranose formation is favoured, which further reacts to give char.⁴¹

Conversely, the presence of Ca(II) and Mg(II) ions kinetically favors conversion of 1,2-

anhydroglucopyranose to LGA, which, in turn, reacts to form LGO. Thus, the fate of LGA is decided

by its chemical surroundings and the relative reaction rates of all these reaction pathways influence the

experimental yield of LGA. Figure 8 [C] summarizes the fate of LGA in the presence of AAEM ions.

4.4.4: 5-hydroxymethylfurfural (5-HMF) formation and its conversion to furfural

5-HMF is an important product obtained during biomass pyrolysis.²⁴However, experimental results

show that lower yield (<1%) of 5-HMF is obtained in metal-free and AAEM-complexed glucose

pyrolysis. Figure 9 shows the reaction pathway for 5-HMF formation from β-D-glucopyranose (1) as

well as the correlation of the experimental yield of 5-HMF with the activation barriers for the formation

and conversion of 5-HMF. 5-HMF formation involves a series of ring opening and dehydration

reactions of β-D-glucopyranose (1). Two routes for keto-intermediate (15) formation are possible.²⁴

Route one involves D-fructose (12) as an intermediate (reaction arrows in black), while route two

involves cyclization of D-glucose (2) with water loss (reaction arrows shown in pink). Furthermore, the

ketone intermediate (15) also exhibits two competing routes for 5-HMF formation depending upon the

loss of first water molecule from ‘C’ alpha to the aldehydic side group (intermediate shown in red in

the reaction scheme) or primary hydroxyl group.

The activation barriers for glucopyranose ring opening (AG-ts1), isomerization of glucose to fructose

(HM-ts1), fructofuranose formation (HM-ts2) and dehydration (HM-ts3) followed by tautomerization

(HM-ts4) are 45.4, 39.3, 29.5, 66.5, 58.9 kcal.mol^-1, respectively, while dehydrative cyclization of D-

glucose (2) to ketone-intermediate (15) (HM-ts5) exhibits an activation barrier of 78.1 kcal.mol^-1. These

activation enthalpies suggest that the formation of ketone intermediate via the fructose intermediate is

kinetically favorable. Once the ketone intermediate is formed, loss of a water molecule can occur from

‘C’ alpha to alcoholic side group, HM-ts6, or from ‘C’ alpha to aldehydic side group, HM-ts7, resulting

in enthalpic barriers of 73.7 and 56.7 kcal.mol^-1, respectively. The enthalpic barrier of HM-ts7 is about

18 kcal.mol^-1lower than that of HM-ts6, because HM-ts7 is stabilized by conjugation with the

neighbouring aldehydic group. Loss of another water molecule from intermediates (16) and (17) results

in the formation of 5-HMF. The activation enthalpies of TSs, HM-ts8 and HM-ts9, are 51.1 and 55.2

kcal.mol^-1, respectively. The above results are in corroboration with the findings reported by Mayes et

26

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Catalysis Science & Technology

al.²⁴for 5-HMF formation. 5-HMF once formed, can undergo subsequent deformylation generating

View Article Online

DOI: 10.1039/C9CY00005D

furfural (FU-ts), thus resulting in an activation barrier of 78.4 kcal.mol^-1. Analysis of the activation

barriers of metal-free glucose shows that 5-HMF formation via the fructose intermediate is the

kinetically preferred route and the dehydration of β-D-fructofuranose to enol i.e., HM-ts3 is the rate-

determining step.

The activation enthalpies of AG-ts1 and HM-ts1 to HM-ts4 for Mg(II)/glucose complex are 39.3, 25.9,

24.3, 38.3 and 57.1 kcal.mol^-1, while in the presence of Ca(II) ions the activation enthalpies are 39.5,

29.2, 36.7, 47.9 and 66.0 kcal.mol^-1, respectively. The complexation of divalent Mg(II) and Ca(II) ions

in the TSs, HM-ts1 to HM-ts4, lower the activation enthalpies by 5-15 kcal.mol^-1when compared to

metal-free TSs. Furthermore, the activation enthalpies for the conversion of Mg(II) complexed ketone

intermediate (15) to HMF via loss of water from the hydroxyl side group, HM-ts6 and HM-ts8 are 63.1

and 42.9 kcal.mol^-1, while the loss of water from aldehydic side group in (15), HM-ts7 and HM-ts9, are

44.9, 64.5 kcal.mol^-1, respectively. For Ca(II) assisted HM-ts6 and HM-ts8, the activation barriers are

64.5 and 44.3 kcal.mol^-1, while HM-ts7 and HM-ts9 have activation enthalpy values of 73.7, 63.1

kcal.mol^-1, respectively. It can be seen that two successive dehydrations of the ketone intermediate (15)

are preferred from the hydroxyl side chain for Ca(II) coordinated intermediate (15), while Mg(II) ions

kinetically favor loss of first water molecule from the aldehydic side group. This can be attributed to

the fact that H of CHO side group forms a weak H-bond with the hydroxyl group of αC (bond distance

2.68 Å) in Ca(II) complexed HM-ts7, while this bond distance is >3.5 Å for Mg(II) complexed TS. The

presence of H-bonding in Ca(II)/HM-ts7 lowers the affinity of the hydroxyl group to abstract the H

atom from the neighbouring C-atom in case of Ca(II), thus increasing the activation barrier to 73.7

kcal.mol^-1as compared to 44.9 kcal.mol^-1for Mg(II). Furthermore, the activation barriers for the

conversion of HMF to furfural, FU-ts, are 60.1 and 65.6 kcal.mol^-1, respectively for Mg(II) and Ca(II)

assisted TSs. On comparing the activation barrier of the rate-determining step for 5-HMF formation

i.e., HM-ts3 with that of its conversion reaction, FU-ts, it can be seen that the delta between HM-ts3

and FU-ts activation barriers is about 6 kcal.mol^-1for metal-free conditions, while presence of Mg(II)

slightly lowers the delta of activation barriers and Ca(II) ions have a neutral effect. Further, the slight

difference in the activation enthalpies for the formation and conversion reaction, with the conversion

reaction being kinetically favorable for Mg(II) and Ca(II), explains the lower yield of HMF in the

presence of divalent ions.

In the presence of Na(I) and K(I) ions, the activation enthalpy values of AG-ts1 and HM-ts1 to HM-ts4

are 42.5, 31.9, 31.5, 62.9 and 62.8 kcal.mol^-1for Na(I)/glucose complex, while in the presence of K(I)

ions the activation enthalpies are 41.8, 38.2, 30.7, 64.9 and 63.1 kcal.mol^-1, respectively. The

complexation of Na(I) and K(I) ions in the TSs AG-ts, HM-ts1 to ts4 slightly lower the activation

enthalpies by 3-5 kcal.mol^-1when compared to metal-free TSs. Similar to the case Mg(II) ions, the

complexation of Na(I) and K(I) ions with the ketone intermediate (15) also kinetically favors loss of

water from the aldehydic side. The activation enthalpies of Na(I) complexed HM-ts6 and HMts8 are

27

Catalysis Science & Technology

Page 28 of 40

-1

View Article Online

DOI: 10.1039/C9CY00005D

68.7 and 47.5 kcal.mol , while 51.7 and 62.5 kcal.mol , respectively for HM-ts7 and HM-ts9. On the

other hand, the activation enthalpies of K(I) complexed HM-ts6 and HMts8 are 68.0 and 46.9 kcal.mol^-

¹, while 51.4 and 58.0 kcal.mol^-1, respectively for HM-ts7 and HM-ts9. Moreover, the activation barriers

for the conversion of HMF to furfural, FU-ts, are 74.5 and 76.4 kcal.mol^-1for Na(I) and K(I) assisted

TSs, which are 2-4 kcal.mol^-1lower than metal-free FU-ts. In summary, the presence of Na(I) and K(I)

ions have a neutral effect on conversion of HMF and thus the yield of HMF in presence of alkali ions

is similar to that of metal-free glucose.

4.4.5: Formation of furfural and the condensation reaction of furfural with 1,2-anhydroglucopyranose

initiating char formation

Figure 10 shows the correlation plot and the reaction scheme for furfural formation and its subsequent

reaction with 1,2-anhydroglucopyranose which initiates char formation and correlates furfural yield

with the activation barriers associated with TSs FU-ts and Ch-ts1.The experimental results show that

furfural yield is maximum from Mg(II)/glucose thin-film samples, followed by Ca(II) ions and least is

in metal-free glucose. The computed activation barrier for deformylation of 5-HMF²⁵, FU-ts, in the

absence of metal ion is 78.4 kcal.mol^-1. As discussed in section 4.4.2, furfural can also undergo a

coupling reaction with 1,2-anhydroglucopyranose intermediate resulting in the formation of an α,β-

dihydroxy adduct (20). The activation barrier for the above mentioned coupling reaction is 79.5

kcal.mol^-1, which is similar to that of FU-ts.

Complexation of Mg(II) ions with oxygen of the hydroxyl side group in FU-ts assists in C-C bond

breaking and results in the activation enthalpy of 60.1 kcal.mol^-1, while the coupling reaction of furfural

(19) with 1,2-anhydroglucopyranose (5) is decatalyzed by complexation of Mg(II) ions with O₃and O₄

-1

(

)

( )

(

)

atoms of (5). The reaction ퟏퟗ + ퟓ → ퟐퟎ has an activation barrier of 80.9 kcal.mol . Similarly,

Ca(II) ion complexation also assists in furfural formation resulting in 65.6 kcal.mol^-1for Fu-ts, while

the activation barrier of Ch-ts1 increases to 83.8 kcal.mol^-1when Ca(II) ions coordinate with O₆and

aldehydic oxygen of furfural.

Conversely, binding of Na(I) and K(I) ions with oxygen of the hydroxyl side group in HMF slightly

lowers the activation enthalpies to 74.5 and 76.4 kcal.mol^-1, respectively compared to metal-free FU-ts,

whereas the enthalpic barriers for Ch-ts1 are lowered by 3-6 kcal.mol^-1. To summarize, the presence of

Na(I) and K(I) ions as well as in metal-free conditions, the formation of furfural and its subsequent

conversion reaction compete with each other owing to similar activation enthalpies. Further, the

activation enthalpies for FU-ts and Ch-ts1 are higher by 6-8 kcal.mol^-1compared to the enthalpic

barriers of Na(I) and K(I) ions, which explains the lower yield of furfural in metal-free conditions

compared to Na(I) and K(I) ions. In contrast, Mg(II) and Ca(II) complexed FU-ts result in activation

barriers which are about 18-20 kcal.mol^-1lower than Ch-ts1 and hence the yield of furfural is high as

compared to metal-free and alkali ion assisted furfural yields.

28

Page 29 of 40

Catalysis Science & Technology

AG-ts1

HM-ts1

HM-ts2

HM-ts3

HM-ts4

(15)

(14)

(12)

(2)

(13)

(1)

HM-ts5

HM-ts8

HM-ts6

FU-ts

(16)

(18)

(19)

HM-ts7

(15)

HM-ts9

(17)

HMF

F: HM-ts3

C:FU-ts

0.4

0.3

0.2

0.1

0

78.4

73.7

80

70

60

50

76.4

74.5

68.7

68

65.6

63.1

60.1

64.5

No metal

Mg

Ca

Na

K

Figure 9: Reaction scheme and correlation plot of 5-hydroxymethylfurfural (5-HMF) yield with the activation enthalpies of two competing reactions affecting

HMF formation in the presence and absence of alkali and alkaline-earth metal ions. The yield of HMF in the correlation plot is expressed as %C basis. The

labels of the transition states e.g. AG-ts1, are written on the reaction arrow. The labels F:HM-ts3 and C:FU-ts indicate the reactions that form and convert HMF.

The blue arrows in the molecular structures show the electron flow during the reaction. Transition states of the reactions are illustrated above the arrow (in

brackets), where the atoms not taking part in the reaction are faded.

29

Article Doi

Unravelling the catalytic influence of naturally occurring salts on biomass pyrolysis chemistry using glucose as a model compound: A combined experimental and DFT study

DOI: 10.1039/c9cy00005d

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c9cy00005d

Full text of DOI:10.1039/c9cy00005d

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