Applied Catalysis A: General 358 (2009) 42–48

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Applied Catalysis A: General

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Insight to sulfur species in the hydrodeoxygenation of aliphatic esters oversulfided NiMo/g-Al2O3 catalyst

Eeva-Maija Ryymin *, Maija L. Honkela, Tuula-Riitta Viljava, A. Outi I. Krause

Department of Biotechnology and Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FI-02015 TKK, Finland

A R T I C L E I N F O

Article history:

Received 2 December 2008

Received in revised form 21 January 2009

Accepted 28 January 2009

Available online 6 February 2009

Keywords:

Hydrodeoxygenation (HDO)

Methyl heptanoate

Sulfided NiMo

Reduced NiMo

Surface sulfur

A B S T R A C T

The hydrodeoxygenation (HDO) network of aliphatic esters was investigated over reduced and sulfided

NiMo/g-Al2O3 catalysts in a batch reactor at 250 8C and 7.5 MPa. Methyl and ethyl heptanoate and their

main HDO reaction intermediates were used as reactants. The participation of surface OH� and SH�

groups in the various reaction steps and the stability of the surface sulfur are discussed. Although the

surfaces of the reduced and sulfided catalysts under H2 have identical properties, the reactivity of the

oxygen-containing compounds is greater over the sulfided catalyst. This is due to the stronger

nucleophilic strength of the SH� groups compared to the OH� groups. Methyl and ethyl heptanoate react

via various reaction pathways, of which alkaline hydrolysis is demonstrated to be of special importance.

Sulfur-containing intermediates are formed in the HDO reactions with surface SH� groups. Accordingly,

sulfur level on the catalyst surface is diminished.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Diminishing fossil fuel reserves and environmental challengesassociated with the greenhouse effect are driving an intensivesearch for new biomass-based energy sources. Biomass can eitherbe combusted as such or converted to energy-rich gaseous andliquid products [1–3]. Thermochemical conversion of wood bypyrolysis yields a complex mixture of aromatic and aliphaticcompounds. The liquid part of the product is known as bio-oil.Mechanical extraction is typically used to produce vegetable oilsfrom the seeds of various biomass crops. Vegetable oils of rapeseedcan also be upgraded to obtain rapeseed methyl ester (RME),known as biodiesel. Unfortunately, wood-based bio-oils, vegetableoils, and RME contain even as much as 35–40 wt% oxygen-containing compounds, which are responsible for severalunwanted properties, e.g. low energy density, immiscibility withhydrocarbon fuels, and chemical instability. Upgrading by hydro-deoxygenation is essential to remove at least part of the oxygenatoms. As positive effects, hydrotreated oils do not produceharmful SOx emissions, and NOx emissions are lower than those offossil fuels. Hydrotreated oils are also accepted as CO2 neutral. Thework reported in this paper is a contribution to resolving thecomplex chemistry of hydrodeoxygenation of bio-oils.

* Corresponding author. Tel.: +358 9 451 2625; fax: +358 9 451 2622.

E-mail address: eeva-maija.ryymin@tkk.fi (E.-M. Ryymin).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.01.035

Hydrodeoxygenation (HDO) reactions are most commonlycarried out over sulfided NiMo/Al2O3 or CoMo/Al2O3 catalysts. Itis generally assumed that the active sites of the catalysts arecoordinatively unsaturated sites (CUS) such as sulfur anionvacancies. The probable location of the active sites is at the edgesof the MoS2 or Ni–Mo–S or Co–Mo–S structures [4]. In addition,protonic sulfhydryl groups (SH�) are proposed to be involved in thesupply of hydrogen and in providing Brønsted acidity [5].

Delmon [6] has categorized the active sites for hydrogenationand hydrogenolysis as follows: hydrogenation sites consist ofthreefold coordinatively unsaturated Mo atoms and hydrogeno-lysis sites of coordinatively unsaturated Mo atoms with neighbor-ing SH� or H+ groups. In addition, acid sites have a function basedon Lewis acidity (sites on the support) and Brønsted acidity (thesulfided phase, as SH� groups). Bunch and Ozkan [7] have modifiedthis model by proposing that hydrogenolysis sites are formed bythe adsorption and dissociation of H2S molecules. Further, theyproposed that the hydrogenolysis sites are Brønsted acid centersassociated with Mo atoms. The hydrogenation sites, in turn, couldbe sulfur vacancies associated with Ni or Mo atoms. The theorydescribing the differences between hydrogenation and hydro-genolysis sites is under debate, however [4].

In earlier studies [8,9] exploring the reaction pathways of HDOof methyl heptanoate over sulfided catalysts, we observed theformation of alkanethiols even in the absence of H2S addition.Alkanethiols were concluded to form from primary alcohol by theSN2 nucleophilic substitution mechanism. Other research groups[10,11] have also reported the formation of sulfur-containing

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–48 43

compound, but their studies have mainly been performed underpartial pressure of H2S. In work exploring the simultaneoushydrodenitrogenation (HDN) of alkylamines and hydrodesulfur-isation (HDS) of alkanethiols over sulfided Mo-based catalysts,Zhao and Prins [10] deduced that, in the HDN of alkylamines,alkenes and alkanes are formed indirectly by elimination andhydrogenolysis of alkanethiol intermediates. As a new insight, theypointed out that thiols react (hydrogenolysis) on vacancies on thecatalyst surface that can be regarded as Lewis acid sites. HDO of n-propanol and n-butanol with H2S was investigated by Mashkinaand Khairulina [11] over catalysts with Lewis acid sites such asAl2O3 or W/Al2O3. The catalysts were sulfided before the reactiontests. The selectivity of alkanethiol formation was 20–50%. Theyconcluded that alcohols react to alkanethiols and alkenes bydistinct routes. In addition, catalysts that contain both Lewis acidsites and basic sites were noted to be active for the formation ofalkanethiols in the presence of H2S.

NiMo/Al2O3 and CoMo/Al2O3 catalysts are typically reducedand sulfided before use in the HDO process. The surface structureof the catalyst is strongly influenced by the catalyst pretreatmentconditions [12,13]. Early on it was suggested [14] that both OH�

and SH� groups on the catalyst surface could provide activehydrogen atoms for heteroatom removal. More recently, participa-tion of OH� groups in the HDO reaction of benzofuran was reportedby Bunch and Ozkan [7]. In their work, a reduced catalyst exhibiteda similar hydrogenolysis function to a sulfided catalyst. As aconsequence, they proposed that the action of the SH� groupscould be supported by the OH� groups, and that the two speciescould be simultaneously active on the surface of the sulfidedcatalyst.

In the present study, we investigated further the reactionpathways involved in the HDO of aliphatic esters in order tounderstand the role of sulfur on the catalyst surface. The stability ofthe surface species and the role of surface OH� groups in thereactions were of interest, too. For these purposes, experimentswith methyl and ethyl heptanoate and with their main inter-mediates were carried out over reduced and sulfided NiMo/g-Al2O3 catalysts. Supplementary experiments were carried out withthe addition of sulfiding agent, H2S, to the gas feed.

2. Experimental

2.1. Reactor

The HDO experiments were performed in a 50-ml batch reactor(Autoclave Engineers) equipped with a fixed catalyst basket and amagnetic stirrer. The catalyst loading was 0.5 g. The stirring ratewas kept at 1000 rpm.

2.2. Catalyst and chemicals

A commercial NiMo/g-Al2O3 catalyst was crushed and sieved toa fraction of 0.59–0.75 mm, dried at 100 8C for 5 h, and packed intothe catalyst basket. The catalyst was pretreated in situ before thereaction studies by reduction or sulfidation. Reduction consisted ofdrying the catalyst for 3 h at 350 8C under N2 flow and 2 hreduction at 350 8C under H2 flow. In sulfidation the drying periodof 3 h at 350 8C under N2 flow was followed by 2 h sulfidation at350 8C under H2S/H2 (5 mol%) flow. All the gases were obtainedwith 99.999% purity from AGA.

The main reactants were methyl heptanoate (Merck, >98%), 1-heptanoic acid (Merck, �99%), and 1-heptanol (Merck, �99%), andthese reactants were diluted with n-dodecane (Merck, �99%) to5 wt%, 2 wt%, and 2 wt% mixtures, respectively. n-Decane (Merck,>98%) was used as an internal standard during the reaction tests.Supplementary experiments were carried out using 1-heptene

(Merck, �99%), 1-heptanal (Fluka, �95%), or ethyl heptanoate(Aldrich, 99%) as a reactant in 2 wt% mixtures diluted with n-dodecane.

2.3. Experiments

After catalyst reduction or sulfidation the reactor was cooleddown to the temperature of the HDO reaction (250 8C) under N2

flow to ensure that the gas phase was free of the pretreatmentgases. Liquid feed was introduced to the preheated reactor(250 8C), and the pressure was set to 7.5 MPa with H2 or N2. Theliquid feed occupied one-third of the total reactor volume. Thereduced catalyst was always studied under H2 atmosphere andthe sulfided one under N2 or H2 atmosphere. The change in theliquid-to-gas ratio was minimized by limiting the number ofliquid samples to five. The length of the experiments wasbetween 5 min and 120 min. A series of experiments with H2S/H2

mixture (200 ppm or 400 ppm) as gas reactant were performedto study the formation and reactivity of sulfur-containingintermediates.

2.4. Analytical methods

Liquid samples were analyzed with an Agilent Technologies7890A gas chromatograph equipped with a capillary column (HP-1, 60 m � 0.25 mm � 1 mm) and a flame ionization detector. Theproducts were quantified by the internal standard method.Supplementary qualitative analysis was performed with an AgilentTechnologies 5975C mass spectrometer connected to the gaschromatograph. In some of the experiments, the gas phase wasquantitatively probed for H2S with Drager tubes. The catalystsamples were dried in a separate oven for 24 h at 100 8C beforeanalysis of the sulfur and carbon contents of the catalysts with aLECO SC-444 series analyzer. The reactor system did not allow inerttransfers.

Carbon balance was calculated on the basis of the acid partof methyl heptanoate as alcohol part forms gaseous compoundsthat were not analyzed. The amount of CO was estimated on thebasis of C6 formation. At low conversion levels the carbon balancewas 90 � 2% and decreased in the course of the experiment to78 � 2%. The loss was considered to be mainly due to vapor–liquidequilibrium of the compounds. In addition, coke deposition wasobserved.

2.5. Definitions

In this paper, totally deoxygenated molecules consisting only ofcarbon and hydrogen atoms are referred to as hydrocarbonproducts. Thus, alkenes and alkanes are summed together. Themolar concentration of the product is the number of moles ofproduct divided by the total number of moles of reactant andproducts multiplied by 100%. These values are calculated from theanalyzed liquid samples.

3. Results

3.1. Reactivity of methyl heptanoate

The products of deoxygenation of methyl heptanoate over thereduced and sulfided NiMo are listed in Table 1. The main productsover the sulfided NiMo under H2 atmosphere are the same asreported earlier [8]. In addition, the improved qualitative analysisallowed the detection of 2-heptanone and 2-heptanol. The HDO ofethyl heptanoate over the sulfided catalyst produced identicalproducts but with ethanol and diethyl ether in place of methanoland dimethyl ether. Under H2 atmosphere the reduced and the

Table 1Reaction products of methyl heptanoate over reduced and sulfided NiMo/g-Al2O3 catalyst in H2 and inert (N2) atmosphere.

Oxygen-containing products Sulfur-containing products Hydrocarbon products

Reduced NiMo under H2a 1-Heptanoic acid, 1-heptanal,

1-heptanol, methanol, dimethyl

ether, and heptyl heptanoate

Not detected n-Hexane and n-heptane

Sulfided NiMo under N2b 1-Heptanoic acid and dimethyl ether Not detected Hexenesd

Sulfided NiMo under H2c 1-Heptanoic acid, 1-heptanal,

1-heptanol, methanol, dimethyl

ether, heptyl heptanoate [8].

2-heptanone and 2-heptanol

1-Methanethiol, 1-hexanethiol,

1-heptanethiol [8]

Hexenesd, heptenese, n-hexane,

n-heptane [8]

a Conversion after 90 min 33%, total hydrocarbon amount 1 mol%.b Conversion after 90 min 24%, total hydrocarbon amount 1 mol%.c Conversion after 90 min 100%, total hydrocarbon amount 95 mol%.d Isomers of hexenes: 1-hexene, cis/trans-2-hexene, and cis/trans-3-hexene.e Isomers of heptenes: 1-heptene, cis/trans-2-heptene, and cis/trans-3-heptene.

Fig. 1. Formation of 1-heptanoic acid (~) and 1-heptanol (*) in HDO of methyl

heptanoate over reduced NiMo/g-Al2O3 in H2 (– – –), sulfided NiMo/g-Al2O3 in N2

(� � �), and sulfided NiMo/g-Al2O3 in H2 (—).

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–4844

sulfided catalysts yielded almost the same products. Naturally, nosulfur-containing compounds were formed over the reducedcatalyst. The experiment over the sulfided catalyst in N2 atmo-sphere revealed only two oxygen-containing products (heptanoicacid and dimethyl ether) with the isomers of hexenes ashydrocarbon products. Sulfur-containing compounds were formedover the sulfided catalyst under H2 but were not detected under N2

atmosphere.Conversions of methyl heptanoate after 90 min were 33% with

the reduced NiMo under H2 and 24% with the sulfided NiMo underN2 atmosphere. At this point the amount of hydrocarbons was low.The total hydrocarbon amount was only 1 mol% in both cases. Theconversion of methyl heptanoate over the sulfided NiMo under H2

atmosphere was complete in 90 min and nearly all the inter-mediates were consumed, too.

Fig. 1 shows the formation of the two main oxygen-containing products of methyl heptanoate, i.e., heptanoic acidand heptanol, over the reduced and sulfided catalysts as afunction of the reaction time. The formation of heptanoic acidand heptanol was the fastest over the sulfided catalyst underH2 atmosphere. Furthermore, the concentration profile wastypical for a reaction intermediate, and heptanoic acid wasnoticeably consumed during the 60-min period. The reducedNiMo under H2 and the sulfided NiMo under N2 producedheptanoic acid, but the further reactivity of the acid was weak.In our earlier work [8] we concluded that heptanoic acid andheptanol are intermediates over the sulfided catalyst under H2

atmosphere.

Table 2Reaction products of 1-heptanoic acid over reduced and sulfided NiMo/g-Al2O3 catalys

Oxygen-containing products

Reduced NiMo under H2 1-Heptanal, 1-heptanol,

and heptyl heptanoate

Sulfided NiMo under N2 Not detected

Sulfided NiMo under H2 1-Heptanal, 1-heptanol,

and heptyl heptanoate [8]

a Isomers of hexenes: 1-hexene, cis/trans-2-hexene, and cis/trans-3-hexene.b Isomers of heptenes: 1-heptene, cis/trans-2-heptene, and cis/trans-3-heptene.

Table 3Reaction products of 1-heptanol over reduced and sulfided NiMo/g-Al2O3 catalyst in H

Oxygen-containing products

Reduced NiMo under H2 Diheptyl ether

Sulfided NiMo under N2 Diheptyl ether

Sulfided NiMo under H2 Diheptyl ether [8]

a Isomers of heptenes: 1-heptene, cis/trans-2-heptene, and cis/trans-3-heptene.

3.2. Reactivity of main intermediates: 1-heptanoic acid and 1-

heptanol

To gain more insight into the reaction steps involved, weseparately tested the reactivity of the intermediates, i.e., heptanoicacid and heptanol, under the same three reaction conditions:reduced NiMo under H2, sulfided NiMo under N2, and sulfided

t in H2 and inert (N2) atmosphere.

Sulfur-containing products Hydrocarbon products

Not detected n-Hexane and n-heptane

Not detected Hexenesa, heptenesb, n-hexane,

and n-heptane

1-Hexanethiol and

1-heptanethiol [8]

Hexenesa, heptenesb, n-hexane,

and n-heptane [8]

2 and inert (N2) atmosphere.

Sulfur-containing products Hydrocarbon products

Not detected Heptenesa and n-heptane

1-Heptanethiol Heptenesa and n-heptane

1-Heptanethiol [8] Heptenesa and n-heptane [8]

Table 4Reactivity of 1-heptanoic acid and 1-heptanol over reduced and sulfided NiMo/g-Al2O3 catalyst in H2 and inert (N2) atmosphere.

1-Heptanoic acid 1-Heptanol

Conversion (%) Hydrocarbon products (mol%) Conversion (%) Hydrocarbon products (mol%)

Reduced NiMo under H2a 56 3 94 74

Sulfided NiMo under N2a 42 14 100 92

Sulfided NiMo under H2b 100 97 100 98

a At 90 min (heptanoic acid) or 60 min (heptanol).b At 30 min (both reactants).

Fig. 2. Formation of H2S during H2 flushing of the sulfided NiMo/g-Al2O3 catalyst at

250 8C.

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–48 45

NiMo under H2. The products are reported in Tables 2 and 3.Heptanoic acid as reactant did not produce sulfur-containingcompounds either over the reduced catalyst under H2 or over thesulfided catalyst under N2. Surprisingly, no oxygen-containingintermediates were formed under N2 atmosphere. The products ofdeoxygenation of heptanol over the sulfided NiMo were the sameunder N2 and H2 but no sulfur-containing compounds were formedover the reduced catalyst.

Table 4 presents the conversions of the reactants and the molarconcentrations of the hydrocarbon products in the deoxygenationof heptanoic acid and heptanol. After 90 min, the conversion ofheptanoic acid was 56% over the reduced NiMo under H2 and 42%over the sulfided NiMo under N2. The total amounts of hydro-carbons were 3 mol% and 14 mol% at these points. Heptanolreacted, however, fast. After only 60 min the conversion was 94%for the reduced NiMo and 100% for the sulfided NiMo under N2

atmosphere. Moreover, the reactivity of heptanol to hydrocarbonswas high. The total hydrocarbon amount was 74 mol% for thereduced NiMo and 92 mol% for the sulfided NiMo under N2. Overthe sulfided catalyst under H2, the conversions of both reactantswere 100% after 30 min and the yield of hydrocarbons was close tothat indicating increased reactivity.

3.3. Effect of H2S addition on reactions of heptanol

In our earlier work [8,9], we detected sulfur-containingproducts such as 1-heptanethiol and 1-hexanethiol in the HDOof methyl heptanoate over the sulfided catalyst in the absence ofsulfiding agent. The reactivity of alcohol with and without H2Saddition was now investigated in more detail in order to followquantitatively the formation and reactivity of sulfur-containingproducts. Primary alcohol, 1-heptanol, was chosen as modelreactant because the reaction steps of heptanol HDO are simplerthan those of methyl heptanoate. Heptanethiol was formed overthe sulfided catalyst under H2, and the maximum molarconcentrations of heptanethiol in the liquid phase ranged from0.1 mol% with no H2S addition to 0.4 mol% with 400 ppm of H2S.

Both heptanol and heptanethiol were highly reactive both inthe absence and in the presence of H2S. In the absence of H2S, at2 min, as much as 51% of heptanol had converted to deoxygenatedproducts over the sulfided catalyst. The maximum molarconcentration of heptanethiol as reported above was detected atthat conversion level. At 90% conversion of heptanol, no discernibleamount of heptanethiol was left in the liquid phase indicating thatheptanethiol is an intermediate, but also indicating high HDSreactivity. In the presence of H2S, heptanol conversion was notsignificantly changed as a function of time compared to the HDO ofheptanol without H2S. We have previously reported the formationof ethers and alkenes as intermediates in the HDO of heptanol oversulfided catalysts [8,9]. Here, the concentration of diheptyl etherwas unaffected by the presence of H2S. However, the formation ofheptanethiol and alkenes increased and the formation of alkanedecreased with the addition of sulfiding agent.

In order to identify all the conceivable routes to alkanethiols, wealso studied the formation of heptanethiol under HDO conditions

over the sulfided catalyst using 1-heptene as reactant. Theseexperiments revealed that heptanethiol was indeed formed in theabsence of H2S. However, the yield was lower than in the HDO ofheptanol even though the initial amount of reactant was the same.Two types of reactant, alcohol and alkene, produced alkanethiolsunder H2 atmosphere without the addition of H2S to the gas phase.

3.4. Sulfur-containing compounds in the reaction mixture

Detachment of sulfur species from the catalyst surface wasstudied by measuring the H2S concentration in the gas phase of thebatch reactor. The measurements were done after sulfidation of thecatalyst and after the HDO of methyl heptanoate. H2S was chosenas probe molecule because over the sulfided catalyst sulfur-containing intermediates, such as thiols, would react to thecorresponding alkane and H2S. After sulfidation, followed by30 min flushing under N2 flow, no H2S was detected in the gasphase. Fig. 2 illustrates the H2S concentration in the gas phase at250 8C after change of the flushing gas from N2 to H2. MaximumH2S concentration was 230 ppm. After a sharp initial rise, theconcentration dropped to 30 ppm. In a separate run, methylheptanoate was introduced to the reactor immediately after30 min flushing with N2. After 60 min reaction the H2S concentra-tion was 250 ppm. As reported above, alkanethiols were detectedin the liquid phase. Thus, both liquid and gas phase containedsulfur-containing compounds. Since the gas phase was probed onlyfor H2S, the presence of other sulfur-containing compounds in thegas phase is not ruled out.

The sulfur content of the catalyst was measured at three stages:immediately after sulfidation and N2 flush, after 180 min H2

flushing, and after 60 min HDO experiment over the sulfidedcatalyst under H2. The measured values are listed in Table 5. Theloss of sulfur was significant during the H2 flow: the sulfur contentdropped from 5.7 wt% to 4.8 wt% in 180 min, a loss of 15%. Thesulfur content of the catalyst after the HDO experiment waspractically the same as that after sulfidation (margin of error

Table 5Sulfur and carbon contents of the NiMo/g-Al2O3 catalyst after sulfidation, H2

flushing, and HDO experiment.

After

sulfidation

After 180 min

H2 flushing

After HDO experiment

under H2

Sulfur (wt%)a,b 5.7 4.8 5.6

Carbon (wt%) 0.3 0.4 5.7

a Sulfur content of the catalyst on carbon-free basis.b Margin of error �0.3 wt%.

Scheme 2. Proposed reaction mechanism for alkaline hydrolysis of aliphatic ester

with SH� nucleophile.

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–4846

�0.3 wt%). During the H2 flow the reactor was continuously flushedwhereas the HDO experiment was carried out batch-wise as all theHDO experiments in this study.

4. Discussion

4.1. Reaction mechanisms for methyl heptanoate

Sulfidation of the catalyst clearly enhanced the HDO of methylester in experiments comparing the reduced and sulfided NiMocatalysts under H2. Although the activity of the catalysts wasdifferent, intermediates and end products were nearly the same(Tables 1–3). In the case of the reduced catalyst, hexenes, 2-heptanone, and 2-heptanol would probably have been present inthe reaction mixture, too, if the reaction temperature or theconcentration of the reactants had been higher. Furimsky [14] inhis study of HDO of furan and Bunch and Ozkan [7] in their study ofHDO of benzofuran have previously suggested that reducedcatalysts could provide similar types of active sites for hydro-genolysis and hydrogenation reactions to those offered by sulfidedcatalysts.

Reasons for the high reactivity of aliphatic ester in the HDO overthe sulfided catalysts were sought by considering the conceivablereactions of esters, i.e., acid hydrolysis, alkaline hydrolysis,transesterification and reduction. In this context the acid–baseproperties pertain to the surface of the catalyst. Previously, wehave considered only acid-catalyzed hydrolysis for the productionof heptanoic acid and methanol and reduction for the production ofheptanol and methanol [8,9]. The proposed [8] reaction scheme forthe HDO of methyl heptanoate is presented in Scheme 1. Theformation of acid has been recognized to be fast at the beginning ofthe reaction, although acid-catalyzed hydrolysis is expected toproceed with the help of water molecules. Since the water contentis low at the beginning of the reaction, the reaction mechanism ofthe ester is in need of further clarification.

In alkaline hydrolysis, the carbonyl carbon of an ester can beattacked by a good nucleophile without prior protonation of the

Scheme 1. Reaction pathways of HDO reactions of

ester [15]. Alkaline hydrolysis of ester is an irreversible reaction,thus giving better yields of carboxylate ions and alcohol than acid-catalyzed hydrolysis. When protons are available, the carboxylateion reacts further to acid. It is well known that the OH� group canact as a nucleophile in the reaction of ester [15]. Also SH� groupsare good nucleophiles, and we postulate that they could have asimilar effect to OH� groups. The proposed reaction mechanism ispresented in Scheme 2. Attack of the SH� group on methylheptanoate could produce thioheptanoic acid and methanol. Theformation of thioheptanoic acid could be followed immediately bythe simultaneous addition of H2 and cleavage of H2S and leading tothe formation of heptanal. Thioheptanoic acid was not detected inour experiments. However, on the basis of the strong attraction ofSH� nucleophiles to carbonyl carbon of ester, it seems probablethat thioheptanoic acid was formed. When heptanal was used as areagent over the sulfided NiMo under N2 atmosphere, heptanoicacid was formed. Thus, the formation of thioheptanoic acid andheptanal could increase heptanoic acid concentration. Correspond-ing products were formed in supplementary experiments carriedout with ethyl heptanoate as reactant. It follows that the reactionmechanism we propose would apply for other esters, too. Vogelaaret al. [16] studied the HDS of thiophene and suggested that therate-determining step of this reaction is catalyzed by structuralsulfur in the form of SH� groups. To conclude this, probably bothOH� and SH� groups are acting as nucleophiles and initiating thealkaline hydrolysis reaction of aliphatic esters. In addition, wepropose that for aliphatic esters under HDO conditions the alkalineand acid hydrolysis proceed simultaneously.

Methyl part of methyl heptanoate is assumed to react tomethanol by alkaline or acid-catalyzed hydrolysis or reduction.Our detection of methanethiol in the liquid phase suggests thatmethanol can further react with SH� groups to form methanethiol.Similarly, ethyl part of ethyl heptanoate reacted to ethanol andethanol would convert to ethanethiol. The mechanism is the same

methyl hepatanoate over sulfided catalyst [8].

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–48 47

as that presented earlier [8]: heptanol protonation is followed bySH� addition and formation of heptanethiol (SN2 mechanism).When dimethyl ether or diethyl ether is formed from alcohol,water is formed as a side product, and water either catalyzes theacid hydrolysis of aliphatic esters or reacts with aldehyde to formthe acid.

4.2. Effect of surface species on reaction steps

We have earlier [8] suggested that over the sulfided catalystunder H2, heptanoic acid might produce saturated and unsaturatedC6 hydrocarbons directly. The direct production of C6 alkenes wasnow supported by the present experimental data since bothmethyl ester and heptanoic acid reacted under N2 to hexenes whileno heptanal was detected (Tables 1 and 2). The decarboxylationroute from acid to alkane has also been reported [8]. In fact, wedetected a very low concentration of hexane when heptanoic acidwas used as a reactant under N2 atmosphere. The surface of thesulfided catalyst includes groups such as S2�, SH� and H+ [5]. This iswhy surface species might participate in the hydrogenation ofalkenes to alkane, i.e., hexenes to hexane. Hydrogenation was alsoevident in our experiment with 1-heptanol over the sulfided NiMounder N2 (Table 3): alkenes and alkane, i.e., heptenes and heptane,were observed. Thus, there is no indication that the decarboxyla-tion of acid proceeded at a discernible rate.

Tøpsoe and Tøpsoe [17] have suggested that the protoniccharacter of the SH� groups give rise to Brønsted acidity and supplyhydrogen. We observed hydrogenation under N2 atmosphere overthe sulfided catalyst as discussed above. This occurs most likelydue to the protonic SH� species. However, although protonicspecies are proposed to participate in the hydrogenation reactions,protonic SH� groups did not enable oxygen removal reactions inwhich H2 is needed (Scheme 1). Under N2 atmosphere, methylheptanoate did not react to heptanol or heptanal, and heptanoicacid did not form heptanal (Tables 1 and 2).

The formation of heptanoic acid from methyl heptanoate byacid-catalyzed hydrolysis is not a deoxygenation reaction sinceoxygen atoms are not removed from the methyl ester molecule.However, the nucleophilic SH� attack on methyl heptanoate(Scheme 2) is partial deoxygenation since thioheptanoic acid iscomposed of fewer oxygen atoms than methyl ester. Thus, underH2 atmosphere surface SH� group enhances oxygen removal, andalkaline hydrolysis reaction should be favoured in processes wherea high deoxygenation rate of aliphatic ester is needed.

4.3. Formation of sulfur-containing intermediates

Alcohol yields unsaturated hydrocarbons by acid-catalyzeddehydration [15,18]. The NiMo catalyst, regardless of the pretreat-ment conditions and the atmosphere used in the deoxygenationexperiments, catalyzed the deoxygenation of alcohol (Table 3). Thereactivity was, however, clearly enhanced over the sulfided catalyst,especially under H2 atmosphere (Table 4). In our earlier discussionof correlation between OH� and SH� groups, we proposed thatthese nucleophiles are present over the sulfided catalyst. Thesimilarity of the reaction products of alcohol on differentlypretreated catalysts (Table 3) indicated that also the acid propertiesof the catalysts are of the same kind.

Our present study supports the assumption that increasedBrønsted acidity in the presence of H2S promotes acid-catalyzed E2

elimination and SN2 nucleophilic substitution to form alkenes andalkanethiols. HDO of heptanol over the reduced and sulfided NiMoyielded the same reaction products except for heptanethiol. Thisindicates that alkanethiols are not obligatory intermediatesbetween alcohol and alkenes. Evidently, both NiMo surfacescatalyzed the formation of alkenes directly from alcohol. Our

observations with n-heptanol are also in good agreement with theobservation of Mashkina and Khairulina [11]. They observed thatalkenes and thiols are formed directly from shorter alcohols, n-butanol and n-propanol. By contrast, Zhao and Prins [10] havepostulated, that in HDN of alkylamine, alkanethiol is anintermediate, i.e., between the alkylamine and hydrocarbons.More experiments will need to be done to resolve the differencesbetween the reactions of oxygen-containing and nitrogen-contain-ing compounds.

Mashkina [19] has published an extensive report on the synthesisof alkanethiols from alcohol and H2S. It was reported that a catalyst isa prerequisite for this reaction to occur. Our tests with sulfidedcatalyst in the absence of sulfiding agent proved that the SH� groupson the catalyst surface take part in the alkanethiol formation, too.In the presence of sulfiding agent the SH� groups on the catalystsurface and the gaseous H2S may both have enhancing effect onthe alkanethiol formation. The formation of alkanethiols is alsooccurring in the reaction between SH� nucleophile and a double-bond of alkene, as 1-heptanethiol was formed in our experimentswith 1-heptene over the sulfided catalyst. 1-Heptene is known to bethe most reactive of the isomers of heptenes [15]. Hence, only 1-heptanethiol was formed. Mashkina [19] has earlier suggested thereaction of alkene and H2S for production of alkanethiol.

4.4. Release of sulfur from the catalyst surface

H2S concentration in the gas phase was studied to obtaindetailed information about the stability of the sulfur species on thecatalyst. A clear jump in H2S concentration was observed at thebeginning of H2 flushing (Fig. 2). This observation was confirmedby measurement of the sulfur content of the sulfided NiMo catalyst(Table 5). Mild heating of the catalyst before determination of thesulfur content of the catalyst can be presumed to have removed thefree water, residual solvent, and other molecules from the pores ofthe catalyst. Thus, it would seem that H2 reacted with surfacesulfur groups and the sulfidation state of the NiMo catalystdecreased. Change in the sulfidation stage would be a disadvantagein continuous systems. Kogan et al. [20] have reported similarfindings over the presulfided CoMo catalyst: the sulfur content ofthe catalyst did not change during treatment with an inert He flow,but H2 flow caused the formation of H2S and the catalysts thencontained less sulfur than before the treatment. They noted thatthe maximum amount of mobile sulfur on the catalyst was about40% of the stoichiometric amount of sulfur. In our experiments 15%decrease was observed, which was smaller than in the tests ofKogan et al. This was probably due to the different flushingtemperatures and different promoter metal.

5. Conclusions

HDO of methyl heptanoate was studied over reduced andsulfided NiMo/g-Al2O3 catalyst. The sulfided catalyst was used inboth N2 and H2 atmospheres. The aim was to determine the stabilityof sulfur on the catalyst surface and to discuss the types of surfacespecies over the sulfided catalyst. Except for the sulfur-containingcompounds, which were formed over the sulfided catalyst, theproducts were the same over the reduced and the sulfided catalystsunder H2. The reactivity was notably enhanced over the sulfidedcatalyst, however. Thus, we concluded that the high nucleophilicstrength of SH� groups is important in the hydrodeoxygenationreactions of aliphatic ester. The action of OH� groups is weaker thanthat of SH� groups but the two species most likely act similarly.Accordingly, with the help of surface species aliphatic esters react byalkaline hydrolysis as well as by acid hydrolysis.

A few sulfur-containing compounds are formed in the HDO ofmethyl heptanoate over the sulfided catalyst. The source of the

E.-M. Ryymin et al. / Applied Catalysis A: General 358 (2009) 42–4848

sulfur is the sulfur attached to the catalyst surface during thepretreatment, i.e., sulfidation. Two independent routes for theformation of heptanethiol were presented: either a correspondingalcohol or alkene reacts with the surface SH� groups. It was alsodemonstrated that H2, as reactant gas, reacts with sulfur on thecatalyst surface and forms H2S. Sulfur from the surface of thecatalyst is transferred to the liquid or gas phase and alters the HDOconditions in the course of the experiment.

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