Hydrodeoxygenation of aldehydes catalyzed by supported palladium catalysts

Applied Catalysis A: General 332 (2007) 56–64 www.elsevier.com/locate/apcata

Hydrodeoxygenation of aldehydes catalyzed by supported palladium catalysts Dana Procha´zkova´ a, Petr Za´mostny´ a, Martina Bejblova´ b, Libor Cˇerveny´ a, Jirˇ´ı Cˇejka b,* b

a Institute of Chemical Technology, Prague, Technicka´ 5, CZ-166 28 Prague 6, Czech Republic J. Heyrovsky´ Institute of Physical Chemistry of AS CR, v.v.i., Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic

Received 19 June 2007; received in revised form 3 August 2007; accepted 3 August 2007 Available online 10 August 2007

Abstract This contribution focuses on the hydrogenation and hydrodeoxygenation of aldehydes (benzaldehyde, 2-phenylpropionaldehyde, 3-phenylpropionaldehyde, cinnamaldehyde, 4-isopropylbenzaldehyde, heptanal) on supported palladium catalysts, namely zeolites and active carbon. The effects of the substrate structure, solvent, type of support and its acidity, reaction temperature and pressure on activity and selectivity of Pd catalysts were investigated in detail. Catalytic hydrodeoxygenation of aldehydes was carried out in a liquid phase in an autoclave in the temperature range from 30 to 130 8C and at pressure ranging from 1 to 6 MPa. Hydrogenation reactions performed in non-polar hexane proceeded with a significantly higher rate than in a polar methanol. The presence of isopropyl substituent in 4-isopropylbenzaldehyde increased both the reaction rate as well as the selectivity to hydrodeoxygenation. Hydrodeoxygenation of 2-phenylpropionaldehyde, 3-phenylpropionaldehyde, cinnamaldehyde, and heptanal did not proceed, the main products were dialkylethers, which evidences the importance of the direct attachment of C O group to the benzene ring. The presence of hydrogenation—hydrogenolytic mechanism as well as the direct hydrogenolysis of C O bond in the transformation of benzaldehyde was verified. # 2007 Elsevier B.V. All rights reserved. Keywords: Hydrogenation; Hydrodeoxygenation; Benzaldehyde; Cinnamaldehyde; Phenylpropionaldehyde; Pd supported catalysts; Zeolites; Pd carbon catalyst

1. Introduction Aldehydes can be easily reduced to alcohols. In some cases, it is desirable to perform the reduction even towards corresponding hydrocarbons. Clemmens (acidic) and WolffKizhner (basic) reductions are the oldest procedures enabling the transformation of a carbonyl group of aldehydes to a methyl group [1]. Metal hydrides, e.g. LiAlH4–AlCl3 [2], NaBH4– CF3COOH [1], NaCNBH3–BF3OEt2 [3], Et3SiH–BF3 [4], monomeric or dimeric [(C2H5)2Ti] [5], are capable to deoxygenate carbonylic compounds. Two-step synthesis over intermediates enoltriphlates, tosylhydrazones, selenides or selenoacetals [4,6–9] can be used to transform carbonyl group to hydrocarbonylic one. In addition, phase transfer catalysis [4], hydrogenation by hydrogen transfer [1,4] and homogeneous catalysis [10] can be employed for these hydrogenation reactions. At present, heterogeneous catalysis is frequently

* Corresponding author. ˇ ejka). E-mail address: [email protected] (J. C 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.08.009

used to hydrodeoxygenation. Hydrogenation reactions can be accompanied by hydrogenolytic splitting. Hydrodeoxygenation is often carried out in an acidic environment [11,12]. Pd and Ni are the most frequently used metals for this purpose [13,14]. It is generally known that Pd catalysts are highly active in hydrogenations of double and triple carbon-carbon bonds while they are much less active in hydrogenation of aromatic hydrocarbons and C O bonds. This is particularly true for aliphatic aldehydes and ketones. In such cases, under severe reaction conditions elimination of hydrogen occurs [14]. Different catalytic behavior of Pd compared with other platinum metals is usually explained by the formation of pallyl complex of carbonyl compounds with Pd [15]. However, the number of papers dealing with hydrogenation of aldehydes or ketones over Pd catalysts is still rather limited. Generally, the transformation of carbonyl group to methyl one can theoretically proceed via three mechanisms: i) hydrogenation–dehydration mechanism consisting of transformation of a carbonyl compound to alcohol, followed by a dehydration producing olefin and hydrogenation of the

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double bond occurring in the last step (C–C O ! C–C– OH ! C C ! C–C), ii) in the case of hydrogenation-hydrogenolytic mechanism, hydrogenation of carbonyl compound to alcohol occurs, followed by the hydrogenolytic splitting of the bond C–O producing hydrocarbon (C–C O ! C–C–OH ! C–C), iii) the third mechanism is a direct hydrogenolysis of C O bond (C–C O ! C–C). The metal hydrogenation component of the bi-functional catalyst initiates the hydrogenation steps while the acid component is responsible for dehydration of the alcohol produced. The objective of this contribution is to investigate catalytic hydrodeoxygenation of aldehydes differing in the shape and size of molecules over Pd-based heterogeneous catalysts. The effects of the substrate, type of the support and its acidity, solvent and reaction conditions on the mechanism of the hydrodeoxygenation reaction were studied. 2. Experimental 2.1. Catalysts Palladium supported catalysts with 5 wt.% of palladium were used to investigate the mechanism of hydrodeoxygenation reactions. Active carbon (Doduco GmbH) and zeolites beta and ZSM-5 (Zeolyst), with different Si/Al ratio, were used as supports. The catalysts Pd/zeolite were prepared by the impregnation of zeolite with a water solution of PdCl2. The suspension of zeolite, distilled water and 40% solution of PdCl2 in HCl was stirred for 9 h. It was followed by evaporation using a vacuum rotary pump, the samples were dried at 120 8C in nitrogen atmosphere for 2 h and thermally treated in a stream of nitrogen at the temperature of 400 8C for 1 h. After that stream of nitrogen was switched to hydrogen and PdCl2 was decomposed and reduced to Pd. Hydrochloric acid formed during decomposition of PdCl2 was absorbed in NaOH solution and analyzed. It was confirmed that all chlorine was removed from the catalyst, which evidences the complete reduction of palladium. The list of used catalyst supports with their structural properties is given in Table 1. X-ray powder diffraction was used to characterize parent and modified zeolites as for the crystallinity and phase purity using Bruker D8 Advance operating in Bragg-Brentano mode with Vantec-1 detector.

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The content of palladium was determined by XRF ARL 9400 (ARL, Switzerland). The total surface area was obtained from nitrogen adsorption isotherms at 196 8C by the BET method using a Micromeritics ASAP 2020 (USA). Surface areas of parent supports are in Table 1, the decrease in the surface after modification was less then 10% for each sample. Scanning electron micrograph (SEM) images were collected on a Hitachi S-4700. The nature and the concentration of acid sites in parent and Pd-modified zeolites was determined by Fourier transform infrared spectroscopy (FTIR) using d3-acetonitrile as probe molecule and a Nicolet FTIR Prote´ge´ 460 spectrometer. For details on the determination of the concentrations of acid sites, samples activation and measurement conditions you can see ref. [16]. 2.2. Kinetic tests Catalytic tests were carried out in a liquid phase in a 300 ml stainless-steel autoclave (Parr 4842) with a magnetic stirrer. The autoclave was filled with the reaction mixture consisting of a substrate (4.5 g), solvent (150 ml, hexane or methanol) and catalyst (50–100 mg). After that the autoclave was closed and heated to the required reaction temperature (30–130 8C) and pressurized with hydrogen (1–6 MPa). Subsequently, the stirring was started. The reaction time was measured from the start of the stirring. 2.3. Analytical methods Samples of the reaction mixture were analyzed on a gas chromatograph GC Varian CP-3800 with the flame-ionization detector VA FID 11, the injector VA 1177, the non-polar column VA-1 (length 60 m, diameter 0.25 mm, stationary phase thickness 0.25 mm). The reaction intermediates were determined by GC–MS (Varian Saturn 2000). The reproducibility of kinetic data based on GC analysis was better than 2%. 3. Results and discussion 3.1. Characterization of catalysts under study Zeolite catalysts before and after modification with Pd were characterized using X-ray powder diffraction, sorption of nitrogen, scanning electron microscopy, chemical analysis

Table 1 Utilized catalysts Support C Beta (12.5) Beta (35) Beta (70) ZSM-5 (15) ZSM-5 (140)

Si/Al

Dimensions of structure

Channel type

Size of pores (nm)

– 12.5 35 70 15 140

SBET (m2/g) 976

3D

12

0.76  0.64

493 510 530

3D

10

0.53  0.55

308 325

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and adsorption of d3-acetonitrile followed by FTIR spectroscopy to determine the type and concentration of acid sites. X-ray powder patterns (not given here) provided clear evidence on the good crystallinity of investigated samples and did not show any changes after the modification with Pd. This indicates that the modification with Pd did not cause any structural collapse of the crystalline zeolite framework. Scanning electron micrograph images (Fig. 1) depicts some catalysts with Pd and show typical sizes and shapes of zeolite used with agglomeration of small particles to large objects. From the chemical analysis it was observed that the amount of Pd for modification is very close to the preset value being 5 wt.%. SEM image of Pd/ZSM-5 may indicate a presence of very small particles of Pd on the external surface of zeolite crystallites. Concentration of Broensted and Lewis acid sites is summarized in Table 2. The concentration of aluminum in zeolites obtained, expressed in Si/Al, from the chemical analysis corresponds well to the aluminum concentration calculated from integral intensities of absorption bands [17]. Both ZSM-5 and beta zeolites exhibit some amount of aluminum in Lewis type of acid sites. The concentrations of Broensted as well as Lewis acid sites after modification with Pd was decreased of about 20% not depending on the type of zeolite and Si/Al ratio, see Table 2. No specific preference for interaction of Pd particles with Broensted or Lewis acid sites was observed. Based on these results and SEM images, Pd/ZSM-5, it can be assumed that Pd particles are mainly located on the external surface or in surface layers of zeolites.

3.2. Hydrodeoxygenation of benzaldehyde Benzaldehyde, as the first investigated substrate, was utilized as the model substance for studying hydrogenation of aldehydes having an aromatic ring in the molecule. This is due to the possibility to hydrogenate both the benzene ring as well as a carbonyl group. Moreover, hydrodeoxygenation of benzaldehyde may also occur [18–21]. The proposed reaction scheme is given in Fig. 2. Benzaldehyde (A) forms toluene (C) either through benzylalcohol (B) as intermediate or by direct cleavage of C O carbonyl bond in hexane environment. Benzaldehyde (A) can form toluene through intermediates dimethylacetal benzaldehyde (D), benzylmethylether (E) and benzylalcohol (B) in methanol environment. Dimethylacetal benzaldehyde is formed by acid catalyzed reaction. It is formed due to the presence of acid sites on zeolite support and it may be speculated that Pd might also contribute to this reaction. The selectivity to toluene production is defined as the ratio of the concentration of the produced toluene to the sum of concentrations of all products. The selectivity values were determined at 99% conversion of benzaldehyde. 3.3. Effect of support type Palladium supported catalysts with 5 wt.% of palladium were used to investigate the reaction mechanism of benzaldehyde hydrodeoxygenation. Large pore zeolite beta and medium pore zeolite ZSM-5 were employed to investigate the effect of zeolite structure and their acidity on the reaction mechanism. It was observed that the most suitable catalysts for hydrodeox-

Fig. 1. Scanning electron images of catalysts Pd/ZSM-5(140), Pd/ZSM-5(15) and Pd/beta(35).

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Table 2 Concentration of acid sites in parent and modified zeolites Zeolite

Beta (12.5) Beta (70) ZSM-5 (15) ZSM-5 (140)

Concentration of acid sites

Pd/zeolite

Clc (mmol/g)

Cbc (mmol/g)

0.21 0.08 0.29 0.01

0.69 0.07 0.40 0.01

Pd/Beta (12.5) Pd/Beta (70) Pd/ZSM-5 (15) Pd/ZSM-5 (140)

Concentration of acid sites Clc (mmol/g)

Cbc (mmol/g)

0.14 0.05 0.25 0.01

0.58 0.03 0.33 <0.01

Fig. 2. Reaction scheme of benzaldehyde hydrodeoxygenation.

ygenation of benzaldehyde were the catalysts with the large pore zeolite beta as support. The selectivity to toluene was 93.6% after 15 min of the time-on-stream (T-O-S) over catalyst Pd/beta (35) and 94% over the catalyst Pd/beta (70). The selectivity values indicate that the difference in acidity had only a marginal effect on the selectivity in this reaction. Hydrodeoxygenation of benzaldehyde is influenced by acidity of the support over medium pore zeolites ZSM-5. The selectivity to toluene was 56.5% over the catalyst Pd/ZSM-5 (15) and only 22.2% on Pd/ZSM-5 (140). Substantially lower selectivity in hydrodeoxygenation of benzaldehyde over Pd/ZSM-5 compared with Pd/beta could be also connected with a lower diffusivity of reactant and products in 10-membered ring channels of ZSM-5 zeolites. Even lower selectivity to toluene was 29.1% over the catalyst Pd/C, which indicates the importance of acidity in this reaction.

ygenation when compared with other tested catalysts apparently owing to its low acidity and narrow pores. The values of selectivity of the toluene production were higher in hexane than in methanol on all the tested catalysts. 3.5. Mechanism and kinetics The concentrations of reactant and products in time were processed by regression analysis using the ERA 3.0 program [22] to determine the contribution of individual reactions to the overall mechanism (A ! B ! C or A ! C) and to compare the performance of catalysts given in Table 1 as well. Langmuir– Hinshelwood model was proposed as mathematical model formally corresponding to the reaction scheme depicted in Fig. 2 to describe the reaction system kinetics. The products of all reaction steps were observed in the hydrodeoxygenation of benzaldehyde in methanol. Only reactions r1, r2 and r3 occurred

3.4. Effect of solvent The investigation of hydrodeoxygenation of benzaldehyde was carried out with two solvents (non-polar hexane or polar methanol). Hydrodeoxygenation of benzaldehyde in methanol was accompanied by the production of some intermediates, namely dimethylacetal benzaldehyde and benzylmethylether. Fig. 3 depicts the selectivity to toluene at 99% conversion of the benzaldehyde. The selectivity to toluene was the lowest for hexane as well as methanol on the catalyst Pd/ZSM-5 (140) 22.2% for hexane and 3.4% for methanol. Therefore, it could be stated that the catalyst Pd/ZSM-5 (140) was not very active for hydrodeox-

Fig. 3. Selectivity to toluene production, 100 mg cat., 130 8C, 6 MPa.

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when hexane was used as a solvent. The transient kinetics model was also tested, but it was found too complex for the proposed reaction scheme and available data even at maximum simplification. Unfortunately in transition kinetics model, it was impossible to obtain reliable parameter estimates similarly as it was observed in earlier paper [23]. Also in the Langmuir– Hinshelwood model, it was necessary to proceed to certain simplifying presumptions, like the quasi-equilibrium approximation and the most abundant reaction intermediates assumption [24]. Langmuir–Hinshelwood model of the reaction system is based on the assumption that the reaction rate of each reaction is controlled by a single reaction step, in this case by the surface reaction. Reaction rate equations used in the model were in form ri ¼ w

k i K X cX 1 þ K A cA þ K B cB þ K C cC þ K D cD þ K E cE

where ri is the rate of the i-th reaction in accordance with the Fig. 1, respectively, ki its rate constant, w catalyst amount and K the formal adsorption coefficient. The index X indicates the substance, which is educt of the i-th reaction step. The measured data (e.g. Fig. 4) indicate that there is a loss of the catalytic activity as the reaction proceeds. There is an inability of transformation of benzylalcohol to toluene in hydrodeoxygenation of benzaldehyde over 50 mg of catalyst Pd/beta (35) in comparison with experiment over 100 mg of catalyst. In order to estimate kinetic parameters of the reaction system, the model of the catalyst ‘‘deactivation’’ was established. The model describes the deactivation by introducing the catalyst activity a as a dimensionless quantity, the value of which is equal to 100% at the beginning of the reaction and diminishes with the time of the reaction. Owing to the concept of single active site type taking part in all reactions, used for the mathematical modelling, the rates of all reactions should decrease proportionally to the decreasing activity. ri ¼

ki ða=100ÞK A cA 1 þ K A cA þ K B cB þ K C cC þ K D cD þ K E cE

Such an equation also corresponds to the idea that the catalyst activity loss is caused by building up the non-specific hindrance

to reaction species reaching the active site (as is discussed later). Various semi-empirical models representing different mechanisms of catalyst activity loss were compared. The best results were obtained using the model established upon assumption that the deactivation is proportional to the amount of toluene formed and indirectly proportional to the amount of remaining benzaldehyde. da kd ða=100ÞK C cC  kd =K DR ða=100Þ P ¼ rd ¼ dt 1 þ j K jc j The rate of deactivation is characterized by the rate constant of deactivation kd and the equilibrium constant of deactivation KDR. The mechanistic interpretation of the equation given above takes two possible deactivation mechanisms into account. First possibility consists in the loss of the catalyst activity due to toluene strongly adsorbed to the catalyst surface [25]. Second one involves enveloping the catalyst surface by water formed by the reaction and, thus, forming a mass-transfer hindrance to the reaction. Both the mechanisms above can be modelled by the single equation (since they are both related to the formation of reaction products), which is entirely sufficient as the detailed deactivation study is beyond the scope of this work. Thus, the deactivation was modelled as a equilibrium process, which is in agreement with the experimental data showing that the catalyst activity drops to relatively low equilibrium value that ceases to diminish further since then. It is demonstrated in Figs. 4 and 5 that benzylalcohol transformation to toluene proceeded even after the period of steep catalytic activity loss, albeit at much lower rate. The actual computed activity profiles can be seen from these Figures as well for different catalysts. It is important to note that the deactivation rate is proportional to toluene concentration on the catalyst surface (computed by the model) not that in the solvent phase. Therefore, the deactivation proceeds significantly only after the benzaldehyde is consumed by the reaction. Until then, the competitive adsorption of benzaldehyde decreases the amount of adsorbed toluene and inhibits the deactivation. Fig. 4 shows the model ability to fit the experimental data for different catalyst amounts used for experiments. The catalyst deactivation was much more important in experiments carried out in

Fig. 4. Typical profiles of reaction components experimental concentrations by benzaldehyde hydrodeoxygenation (*: benzaldehyde, & : benzylalcohol, ~: toluene) fitted by the mathematical model (solid lines) and the computed profile of catalyst activity loss (dashed line) on catalyst Pd/beta (35) for different catalyst amounts (50 mg left, 100 mg right) (c (%): molar concentration).

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Fig. 5. Typical profiles of the reaction components experimental concentrations by benzaldehyde hydrogenation (*: benzaldehyde, & : benzylalcohol, ~: toluene) fitted by the mathematical model (solid lines) and the profile of computed catalyst activity loss (dashed line) on different catalysts: Pd/C (a), Pd/beta (35) (b), Pd/beta (70) (c), Pd/ZSM-5 (140) (d) and Pd/ZSM-5 (15) (e) in hexane (c (%): molar concentration).

hexane than in those carried out in methanol (Figs. 5 and 6). The deactivation rate constants estimated differed insignificantly among different experiments in hexane and, therefore, they were fixed at the average value obtained from all experiments to achieve direct comparability of parameter estimates from different experiments. We may suspect the high value of deactivation rate constant in non-polar environment is caused mainly by the catalyst interaction with water formed by the reaction. This conclusion is also supported by additional measurement with added water to reaction mixture (4.5 g benzaldehyde, 100 ml hexane, 50 ml water, 100 mg cat.). The catalyst deactivation in methanol is much slower due to miscibility of water and methanol. Therefore, it can be ascribed to the sticking toluene to the catalyst surface. In methanol, the deactivation is also dependent on the catalyst support. It does not proceed when the support is active carbon and it is pronounced by increased Si/Al ratio of the zeolite catalyst.

Rate constants are highly correlated with adsorption coefficients multiplicating them in fraction numerators. Global kinetic terms representing products of these parameters were estimated by regression analysis. The amount of catalyst was also included into the kinetic term. Because it was necessary to reduce the number of model parameters as much as possible, the significance of individual elements Kjcj of adsorption term was tested individually for different catalysts. Elements which were found as non-significant with respect to available experimental data on the significancy level of 95%, were eliminated from the model according the most abundant reaction intermediates assumption. As a matter of form, the insignificancy of the member may be approached by satisfying the following premise: X 1þ K jc j  K jc j j

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Fig. 6. Typical profiles of the reaction components experimental concentrations by benzaldehyde hydrogenation (*: benzaldehyde, & : benzylalcohol, ~: toluene, *: acetal, &: benzylmethylether) fitted by the mathematical model (solid lines) and the profile of computed catalyst activity loss (dashed line) on different catalysts: Pd/C (a), Pd/beta (35) (b), Pd/beta (70) (c), Pd/ZSM-5 (140) (d) and Pd/ZSM-5 (15) (e) in methanol (c (%): molar concentration).

Optimum values of kinetic parameters estimated by regression analysis are for individual catalysts and solvents mentioned in Tables 3 and 4. The values are always mentioned only for those parameters, which were proved as statistically significant on the confidence level of 95%. The missing parameters (not mentioned in these Tables) were deemed insignificant at given confidence level and neglected. The graphical representation to fitted data is shown in Figs. 5 and 6. When the kinetic terms of reactions r1–r3 (those are common for experiments carried out in both solvent environments) are compared the reaction r1 can be clearly deemed being the fastest one. Both reaction pathways leading to toluene, i.e. the reaction benzaldehyde ! benzylalcohol ! toluene, and the direct hydrogenolysis to toluene take part in toluene formation and they are about equally important if the palladium catalyst with non-acidic support (Pd/C) is used. Different behavior was observed, when palladium was supported on zeolites. In this case benzylalcohol

was very active on zeolite beta catalyst and the importance of direct hydrogenolysis on such catalyst was decreased. On the other hand, ZSM-5 zeolite supported catalysts have shown low or negligible ability to further hydrogenation of benzylalcohol and the majority of toluene was formed by the direct hydrogenolysis of benzaldehyde. While the observations above are valid not only when hexane was as a solvent but also in the methanol, the experiments carried out in methanol showed very significant formation of dimethylacetalbenzaldehyde and benzylmethylether. The formation of acetal is actually the fastest reaction occurring in the reaction system, but the equilibrium of the reaction is not entirely shifted to the reaction product under the experimental conditions used. Thus, the reversible reaction plays an important role as well. The formation of the ether by the reaction of benzylalcohol with methanol does not occur at a large extent and all ether can be expected to be the intermediate

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Table 3 Estimates of kinetic parameters of benzaldehyde hydrogenation on different catalysts in hexane (kxKXw: kinetic terms’ dimension is min1, KX are dimensionless) and their 95% confidence intervals Hexane

Pd/C

Pd/beta(35)

Pd/beta(70)

 k1KAw k2KBw k3KAw KA KB KC kd KDR

38.2 1.8 2.7 1.2

9.7 0.2 1.6 0.3

3.7 30.0 210.0

0.8

 136.2 56.9

21.1 7.8

0.8 2.3

0.3 0.5

Pd/ZSM-5(140) 

30.0 44.4

146.4 68.2 34.2

20.7 7.2 10.2

3.4

0.6

30.0 34.0

Pd/ZSM-5(15)

 6.1

2.8

1.6 3.1

0.8 1.7

30.0 27.0

 18.9 2.8 6.0 3.1

8.6 1.4 4.3 1.6

30.0 4.2

Table 4 Estimates of kinetic parameters of benzaldehyde hydrogenation on different catalysts in methanol (kxKXw: kinetic terms’ dimension is min1, KX are dimensionless) and their 95% confidence intervals MeOH

Pd/C

Pd/beta(35) 

k1KAw k2KBw k3KAw k4KAw k5KDw k7KEw k8KDw KD kd KDR

4.8 0.016 1.2 31.3 0.22 0.002 1.5

4.9 0.001 1.3 34.2 0.01 0.000 0.7

Pd/beta(70) 

2.3 0.158

1.6 0.018

8.9 0.26 0.043 0.9

8.8 0.03 0.012 0.6

Pd/ZSM-5(140) 

29.7 0.032 2.2 106.2 0.22 0.010 1.2

0.2 0.3

of acetal hydrogenation to toluene reaction pathway. The acetal hydrogenation to ether is fast reaction on all tested catalysts. The subsequent hydrogenolysis of ether to toluene is much slower and it is also slower than the hydrogenation of benzylalcohol to toluene. 3.6. Hydrodeoxygenation of 4-isopropylbenzaldehyde Hydrodeoxygenation of 4-isopropylbenzaldehyde proceeded with 100% conversion and 100% selectivity over the catalysts Pd/beta (35), Pd/beta (12.5), Pd/C and Pd/ZSM-5 (15). In the case of Pd/ZSM-5 (140), selectivity to toluene was 57% at 100% conversion of 4-isopropylbenzaldehydes, which could be associated with low concentration of acid sites on the zeolite support. These results indicate that an electron-donor substituent in para-position has a positive effect on hydrodeoxygenation of an aldehyde group. 3.7. Effect of distance of aromatic ring from carbonyl group

60.2 0.002 4.5 216.7 0.01 0.001 0.2

0.2 0.5

Pd/ZSM-5(15)

 1.5 0.014 0.4 14.8 0.07 0.032 1.1 0.03 0.6 1.0

0.9 0.009 0.2 8.8 0.04 0.022 0.7 0.03

 9.6 0.062 1.2 39.3 0.58 0.012 3.0 0.24 2.1 1.2

8.3 0.059 1.1 33.2 0.49 0.011 2.4 0.23

product of 2-phenylpropionaldehyde, cumene, was observed in the reaction mixture in a negligible quantity and only over Pd/beta (35). The main reaction was the production of etherbis(2-phenylpropyl)ether. 3-Phenylpropionaldehyde had not undergone any hydrodeoxygenation reactions over any of the catalysts tested. The product of the reaction was bis(3phenylpropyl)ether. Cinnamaldehyde contains in its structure aromatic ring, double bond and aldehydic group. Under given reaction conditions, at first, hydrogenation of C C double bond of cinnamaldehyde occurred producing 3-phenylpropionaldehyde. The main product of the reaction was bis(3-phenylpropyl)ether. The production of ether could be explained by two ways: either by the reaction of two molecules of alcohol or by the transformation of hemiacetal, which could have been produced from aldehyde and alcohol [26,27]. It was concluded that the distance of the aromatic ring from the aldehyde group has a significant impact on hydrodeoxygenation yield. With the increasing distance, the reactivity of aldehydes decreased very substantially. 3.8. Heptanal

Benzaldehyde, 2-phenylpropionaldehyde, 3-phenylpropionaldehyde differ in the distance of their aldehydic group from the aromatic ring. In the case of benzaldehyde, a significant presence of hydrodeoxygenation reactions was proved on all the tested catalysts. Hydrodeoxygenation

Heptanal, as representative of linear aldehyde, was another substrate investigated. The main product of the reaction carried out in methanol was methylheptylether. When hexane was used as the solvent, the main product of the reaction was bis(2-

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heptyl)ether. Heptanal had not undergone any hydrodeoxygenation tranformations.

mostly produced over the Pd catalysts with large pore zeolites beta as supports.

4. Conclusions

Acknowledgement

The substrate structure had the determining effect on the course of hydrodeoxygenation reaction. In contrast, benzaldehyde had readily undergone hydrodeoxygenation transformations. The selection of the solvent and the catalyst had a significant effect on the rate of these reactions. In hexane, all reactions proceeded more rapidly and selectively on all tested catalysts. This is probably connected with smaller interaction energy of hexane molecules with acid sites of zeolites as compared with more hydrophilic methanol. Pd/ZSM-5 (140) catalyst was the least active for hydrodeoxygenation. The reactions proceeding in methanol were complicated by the production of side-products—acetal and methyl (alkylphenyl)ethers. The evaluation of the experimental data in the program ERA 3.0 proved that the transformation of benzaldehyde to toluene proceeded by the hydrogenation-hydrogenolytic mechanism as well as by the direct hydrogenolysis of C O bond. Simultaneously, it was confirmed that a catalyst deactivation occurred during the benzaldehyde reaction. Isopropyl substituent of 4-isopropylbenzaldehyde in paraposition to carbonyl group had a positive effect on the rate and selectivity of hydrodeoxygenation. It was demonstrated that with increasing distance of the aromatic ring from the carbonyl group, the substrate reactivity in hydrodeoxygenation reactions decreases. When 2-phenylpropionaldehyde was used, a trace quantity of cumene was produced only on the catalyst Pd/beta (35). The main reaction of 2-phenylpropionaldehyde was its transformation to bis(2-phenylpropyl)ether. 3-Phenylpropionaldehyde had not undergone any hydrodeoxygenation transformations. The main product of the reaction was bis(3phenylpropyl)ether. During reactions of cinnamaldehyde, very rapid hydrogenation of double bond occurred producing 3phenylpropionaldehyde, which further transformed as far as to bis(3-phenylpropyl)ether. Heptanal was transformed to heptylmethylether in the reactions proceeding in methanol, diheptylether in hexane. The quantity of the produced ether was affected by the used catalyst support. Ethers were the

The authors acknowledge the Grant Agency of the Czech Republic (Grant No. 104/06/0684) for the financial support. References [1] R.H. Mitchel, Y. Lai, Tetrahedron Lett. (1980) 2637. [2] R.F. Nystrom, C. Rainer, A. Berger, J. Am. Chem. Soc. 80 (1958) 2896. [3] A. Srikrishna, R. Viswajanani, J.A. Sattigeri, Ch.V. Yelamaggad, Tetrahedron Lett. 36 (1995) 2347. [4] H.A. Zahalka, H. Alper, Organometallics 5 (1986) 1909. [5] E.E. Tamelen, J.A. Gladysz, J. Am. Chem. Soc. 96 (1974) 5290. [6] Y. Nishiyama, S. Hamanaka, J. Org. Chem. 53 (1988) 1326. [7] S. Kim, Ch.H. Oh, J.S. Ko, K.H. Ahn, Z.J. Kim, J. Org. Chem. 50 (1985) 1927. [8] G.W. Kabalka, S.T. Summers, J. Org. Chem. 46 (1981) 1217. [9] R.O. Hutchins, B.E. Maryanoff, C.A. Milewski, J. Am. Chem. Soc. (1971) 1793. [10] G. M. Intille, US 4067900, C07C067/08 (1975). ˇ erveny´, Chem. Listy 98 (2004) 981. [11] M. Bejblova´, L. C [12] H. Lansink-Rotgering, M. Scholz, A. Freund, G. Kunz, US 5773677, C07C1/20. [13] R.A. Sheldon, H.V. Bekkum, Fine Chemicals Through Heterogeneous Catalysis, Wiley/VCH, Weinheim, 2001, pp. 363. [14] H.U. Blaser, A. Indolese, A. Schnyder, H. Steiner, M. Studer, J. Mol. Catal. A 173 (2001) 3. ˇ erveny´, Elsevier, Amster[15] K. Tanaka, In Catalytic Hydrogenations, L. C dam, Stud. Surf. Sci. Catal. 27 (1986) 79. ˇ ejka, Appl. Catal. A 327 (2007) 255. [16] M. Bejblova´, S.I. Zones, J. C [17] J. Cˇejka, A. Krejcˇ´ı, N. Zˇilkova´, J. Kotrla, S. Ernst, A. Weber, Microporous Mesoporous Mater. 53 (2002) 121. [18] M.A. Keane, J. Mol. Catal. A 118 (1997) 261. [19] A. Saadi, Z. Rassoul, M.M. Bettahar, J. Mol. Catal. A 164 (2000) 205. [20] D. Haffad, U. Kameswari, M.M. Bettahar, A. Chambelan, J.C. Lavalley, J. Catal. 172 (1997) 85. [21] M.A. Vannice, D. Poondi, J. Catal. 169 (1997) 166. [22] P. Za´mostny´, Z. Beˇlohlav, Comput. Chem. 23 (1999) 479. ˇ erveny´, J. C ˇ ejka, Appl. Catal. A 296 (2005) [23] M. Bejblova´, P. Za´mostny´, L. C 169. [24] I. Chorkendorff, J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, Wiley-VCH, Weinheim, 2003, pp. 56–68. [25] R.W. Meschke, J. Org. Chem. 25 (1960) 137. [26] V. Bethmont, C. Montassier, P. Marecot, J. Mol. Catal. A 152 (2000) 133. [27] S. Jansen, M. Palmieri, M. Gomez, S. Lawrence, J. Catal. 163 (1996) 262.