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Novel bifunctional palladium-dodecatungstophosphoric acid supported on titania nanotubes: One pot synthesis of n-pentyl tetrahydrofurfuryl ether from furfuryl alcohol and n-pentanol Manishkumar S. Tiwari, Tanya Jain, and Ganapati D. Yadav Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00078 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

Ind. Eng. Chem. Res. Manuscript ID: ie-2017-00078c R3

Novel bifunctional palladium-dodecatungstophosphoric acid supported on titania nanotubes: One pot synthesis of n-pentyl tetrahydrofurfuryl ether from furfuryl alcohol and n-pentanol

Manishkumar S. Tiwari, Tanya Jain and Ganapati D. Yadav* Department of Chemical Engineering Institute of Chemical Technology Nathalal Parekh Marg, Matunga, Mumbai-400 019, India Email: [email protected] Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020

*Author to whom correspondence should be addressed

1 ACS Paragon Plus Environment


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ABSTRACT n-Pentyl tetrahydrfurfuryl ether

has

desired properties to be used as a diesel

component. In this work, one pot synthesis of n-pentyl tetrahydrofurfuryl ether from the condensation of n-pentanol and furfuryl alcohol followed by hydrogenation was studied using a novel metal-acid bifunctional catalyst containing palladium and dodeca-tungstophosphoric acid (DTP) supported on titania nanotubes (TNT). The synthesized catalysts were well characterized by various techniques. Effect of different loadings of DTP on TNT was studied and 20% (w/w) loading showed the best result. TNT supported DTP catalyst shows better activity as compared to TiO2 supported one which is due to high dispersion of DTP and acidity of former as revealed in characterization. The best catalyst 20% (w/w) DTP/TNT was then impregnated with different Pd loading and used for hydrogenation to get desired product. The catalyst is active, selective and reusable and can be employed in one pot synthesis of several industrially relevant products.

Keywords: One pot synthesis; biomass; heteropoly acid; furfuryl alcohol; n-pentanol; npentyltetrahydrofurfuryl ether; titania nanotube; palladium; hydrogenation; green chemistry, sustainable chemistry.


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1. INTRODUCTION Titanium dioxide (TiO2) has been explored in a number of applications such as photocatalysis, solar cells, catalyst and support.1,

2

Low specific surface area of titanium

dioxide (50 m2/g) and incompatible mechanical properties are a few problems that need to be overcome before it could be used as a supporting material.3,4 Titania nanotubes (TNT) prepared from TiO2 using a simple alkaline hydrothermal methods1,5 is proving to be a very efficient and rapid method to overcome these disadvantages. Nanotubes possess a very high surface area along with high pore volume and inner diameter around 7 nm, making it suitable for the dispersion of the catalytic materials and metals.6 So far a very few literature reports have been published on the use of titania nanotubes in the field of catalysis.6-8 Hence it was thought interesting here to exploit the advantages offered by TNT for the synthesis of a bifunctional catalyst which could be exploited in an industrially relevant reaction. Heteropoly acids are reported to possess more acidity than the mineral acids and hence used in different industrially important reactions.9-15 Amongst the various structures of heteropoly acids (HPA), Keggin structures are rampantly used. Dodecatungstophosphoric acid (H3W12O40, DTP)10-15 and cesium substituted modification (Cs2.5H0.5W12O40, CsDTP)10,16-18 supported on K-10 clay have attracted considerable attention having been used in a spectrum of reactions by our laboratory. The problems associated with HPAs such as low surface area, thermal stability and high solubility in the polar solvent have been overcome by proton ion replacement with metal ions and/or supporting on different supports without affecting their acidity.10,19 We have reported a novel approach of supporting DTP, cesium salt of DTP (Cs-DTP),

10, 16-18,20

aluminium substituted DTP (Al-DTP)21 on the acidic support

such as K-10 clay which results in the enhancement in the acidity and the activity of the catalyst.



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Biomass derived furfural (FA) is industrially produced by hydrolysis and dehydration of hemicelluloses.22 It can be efficiently converted to a number of industrially relevant products from different processes such as acetalization, decarbonylation, hydrogenation, aldol condensation and oxidation.22,24 Hydrogenation of furfural yields two important products, i.e., furfuryl alcohol (FAL) and 2-methylfuran (MF); MF is used as an intermediate to synthesize fuels and different value added products.23-25 Various approaches have been reported to obtain fuels or fuel additives from the FA, FAL and MF.25-27 Alkanes and alkyl tetrahydrfurfuryl ether (ATE) obtained from MF and FAL, respectively are considered to be

potential

alternatives for the synthesis of bio-diesel.26,28 The high cetane number of the alkanes produced by the MF and carbonyl compound enables them to be used as diesel.27 Ethyl tetrahydrfurfuryl ether (ETE) obtained from FAL is reported to have a cetane number ~80-90 and is also used as a diesel component.29 Synthesis of ATE from FAL is more atom efficient than the synthesis of linear alkanes from MF because it requires only 3 moles of hydrogen and removal of only one oxygen; whereas in the case of alkane preparation 6 moles of hydrogen are needed along with removal of all oxygen atoms.28 The synthesis of ATE form FAL is a two-step reaction; in which the first step is the etherification of FAL and second step is the hydrogenation of the ether to ATE. CaO et al.28 have reported synthesis of ETE and MTE from FAL using two different reactors. Firstly, the etherification reaction was conducted and product was isolated; and subsequently it was hydrogenated using Raney nickel in a second reactor. The overall process requires two different reactors and additional separation step for the isolation of products. Besides have reported a very low yield of the ATE. Hence, a single pot reaction using a multifunctional catalyst could help achieve a more economical process and the yield of product can be increased. ATE can be synthesized by direct etherification of tetrahydrofurfuryl alcohol (i.e. hydrogenated product of FAL) using acid catalyst but requires very high temperature.28 This can be explained by the steric effect


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caused by the tetrahydrofuran ring. It leads to relatively low reactivity of the hydroxyl group in tetrahydrofurfuryl alcohol (TFA). Therefore, current route of ATE synthesis is believed to be feasible because the hydroxyl group in FA shows higher reactivity than that in TFA and FAL can form a stable oxonium ion.28 As reported earlier, ATE with longer alkyl chain has a more potential to increase the cetane number.28,29 Hence we thought of n-pentanol as a linear alcohol to synthesize n-pentyl tetrahydrofurfuryl ether (PTE). One pot synthesis or cascade engineered route provides a better route of preparation as it helps to avoid the multiple purification steps and thus reduces the cost of the operation 24-27

. Further, it also helps to reduce the energy consumption needed for various purification

steps and decreases the waste resulting in green and sustainable process for the synthesis.30-33 The use of multifunctional catalyst having duel sites makes this process more attractive and thus we have focused on the development of a new multifunctional catalyst for the single pot synthesis of PTE from FAL and n-pentanol. There is no published information about supporting DTP on titania nanotube (TNT). The high surface area of the nanotubes as compared to TiO2 had helped to get better dispersion of the catalytic sites on the support and hence resulted in better activity.1,4 Thus, we have used here for the first time TNT as a support for DTP and further to synthesize a bifunctional catalyst required for the one pot synthesis, we have used Pd as a metal site. Pd is very well explored in hydrogenation reactions.23,34,35 The present study deals with the synthesis of titania nanotubes (TNTs) using the hydrothermal method and the optimization of parameters to get appropriate surface morphology of nanotubes to load DTP and Pd in order to achieve a highly active and selective catalyst in the one pot synthesis of the PTE from furfuryl alcohol (FAL) using n-



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pentanol. The catalysts were well characterized by different techniques. Kinetic modelling also reported. 2. EXPERIMENTAL SECTION: 2.1. Chemicals and catalysts

All chemicals were procured from firms of repute and used without further purification. Furfuryl alcohol (synthesis grade), titanium dioxide, sodium hydroxide, hydrochloric acid, toluene, dodecatungstophosphoric acid hexahydrate, methanol, n-pentanol, n-decane (AR grade) were procured from S.D. Fine Chem. Ltd, Mumbai, India. Palladium acetate was purchased from Alfa Aesar, Mumbai. 2.2. Preparation of Titania nanotube (TNT) and Pd-DTP/TNT Overall procedure for synthesis of catalyst is shown in Figure 1. Titania nanotubes (TNT) were synthesized by using alkali-hydrothermal method. The details of synthesis can be easily found in the literature.1,3-6 In brief, 2.85 g TiO2 powder was taken in a Teflon bottle containing 100 ml of 10 N NaOH solution and stirred for 1 h. The resulting suspension was heated at 140 °C for 20 h in a Teflon lined bomb reactor. The precipitated solid was collected and rinsed with the 1 L 0.2 M HCl solution 3 times. The prepared nanotubes were oven-dried at 100 °C for 24 h. DTP supported on TNT was prepared by a simple wet impregnation method. The loading of DTP was varied from 10 to 25 % (w/w). A calculated amount of DTP was added drop wise to a beaker containing TNT suspended in water. The mixture was stirred for 12 h and water evaporated in a vacuum evaporator. The solid material was subjected to drying for 12 h at 170 oC and calcined at 300 oC for 1 h. Samples containing different loading of DTP


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were prepared and denoted as xDTP/TNT (Where x is 10-25% (w/w) DTP supported on TNT).

Figure 1: Preparation of 2.5 (w/w) Pd-20DTP/TNT catalyst 20% (w/w) DTP/TNT (20DTP/TNT) catalyst was then subjected to support palladium as a metal source. The detailed method can be found elsewhere.37,38

1 g

20DTP/TNT was taken in 20 ml dry toluene and 10 ml solution of palladium acetate in toluene slowly added at 30 o C. The solution was stirred for 4 h in order to ensure complete loading of Pd. Decoloration of toluene solution was observed as reported earlier, indicating formation of Pd (II) salt of DTP. Toluene was evaporated thereafter by using rotary evaporator. During evaporation Pd (II) was readily reduced to Pd (0) which resulted in black material indicating the reduction of palladium.

Different Pd loaded catalysts were so

prepared with metal loading ranging from 1 to 4% (w/w) and designated as yPd-20DTP/TNT. (Where y= 1, 2.5 and 4% (w/w) Pd.) 7 ACS Paragon Plus Environment


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2.3 Catalyst characterization All catalysts xDTP/TNT and yPdxDTP/TNT were characterized by different methods and the details could be found elsewhere.20,37,38 In brief, FTIR spectra of different samples were acquired by making KBR pellets with catalyst samples in (100:1) w/w ratio approximately on the Perkin Elmer instrument. The prepared catalysts were characterized by powder XRD analysis using diffractometer (Bruker AXS, D8 Discover instrument) with Cu Kα radiation (1.5406 A°) from 2θ=10 to 80°, at a scan rate of 0.5s and 40kV and 100mA. The surface area of different samples was analyzed using N2-adsorption at -196°C by the multiple point BET method on ASAP 2010 instrument, Micromeritics, USA. Prior to analysis, samples were dried at 150°Cunder vacuum for 6 h. Acidity and metal dispersion of the prepared samples were measured by using NH3TPD method and H2–chemisorption method in Autochem II 2920 TPD/TPR (Micromeritics, USA). TEM micrographs were collected on PHILIPS CM200 transmission electron microscope instrument with electron acceleration energy of 200kV. SEM-EDX analysis of prepares catalyst was performed on the JEOL–JSM-6380 LA (Japan) instrument. 2.4. Reaction procedure and analysis method 2.4.1. Synthesis of PFE (Step1) Etherification of furfuryl alcohol was performed in a batch reactor provided with overhead stirrer coupled with PID controller to maintain the temperature. Appropriate amount


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of furfuryl alcohol and pentanol along with the internal standard (n-decane) was placed in the reactor and catalyst was added once the desired temperature was reached. The reaction was performed under autogenous pressure (ranging from 1-2 bar). Conversion and selectivity were calculated by using the internal standard method using GC (Chemito 1000) provided with BP-1 Column and FID detector. The injector and detector were both kept at 280 °C and the oven temperature programmed from 60 ° C (1 min) up to 280 °C with a ramp rate of 10 °C/ min. Nitrogen was used as the carrier gas. The products were also identified by using GC-MS (Perklin Elmer with RTX-5 mass column). 2.4.2 One pot synthesis of PTE One pot synthesis of PTE was performed in 100 mL autoclave (Amar Equipments, Mumbai). The temperature was maintained by using PID controller. The set amount of furfuryl alcohol and pentanol along with the internal standard (n-decane) was placed and the catalyst (yPd-xDTP/TNT) was added. The reactor was flushed 3 times with nitrogen, temperature raised to set point and initial sample taken. The reaction was conducted at the set temperature and the sample was taken for analysis to ensure that the furfuryl alcohol was 100% converted. Typically it was 2 h experiment. After that the autoclave was pressurized with hydrogen at 4 MPa and temperature lowered to 120 °C. The reaction was further continued for 3 h. Then the reaction mixture was collected and centrifuged to separate the catalyst and analysed by GC (Chemito 1000) provided with BP-1 column and FID detector. The GC programme was same as used above. The products were also identified by using GCMS (Perklin Elmer with RTX-5 mass column). 3. RESULT AND DISCUSSION 3.1 Effect of thermal treatment on the surface morphology of titania nanotube (TNT)



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It is well known that the thermal treatment affects the surface morphology of titania nanotube (TNT).1-5 Hence, calcination of TNT was studied in temperature range of 300-600 °C. The BET surface area of all the samples were recorded (Table S1). The TNT uncalcined and calcined at different temperatures are mesoporous in nature as evidenced by the values of pore size (in the range of 2-50 nm) and the H3 hysteresis loop. There is hardly any change in the surface area and pore sizes of the nanotubes up to 300 °C whereas beyond 300°C the surface area decreases with increase in temperature. Further, the pore volume also decreases from 0.54 cm3/g to 0.27 cm3/g from 300 to 600°C and the pores become narrow in size. These characteristics are due to the transformation of TNT into the TiO2 crystallite as concluded in several literatures.1,5 Thus it is concluded that the high surface area needed for the better dispersion of catalyst was achieved by using calcination temperature of 300°C and it should be accounted while synthesising a multifunctional catalyst. Therefore DTP loaded catalyst was prepared by calcining the solid at 300° C for 1h. 3.2. Characterization of catalysts 3.2.1 XRD The XRD patterns of different samples are compared in Figure 2. Diffraction pattern shown in Figure 2 (a) illustrates that TiO2 precursor used here has major anatase phase.1,3 Peaks at 2θ = 25.8, 37.01, 37.9, 38.8, 48.2, 54.01, 55.2, 62.9, 68.9, 70.6 and 75.2° can be attributed to the 101, 103, 004, 112, 200, 105, 211, 204, 116, 220 and 215 crystalline structures of anatase structure, respectively.1,3 However, TNT shows significant decrease in the intensities of the peaks related to the parental precursor (Figure 2 (b)).This indicates the formation of nanotubes as reported earlier.1,3 As shown in Figure 2 (c), the slight increase in the intensities of the peaks is mainly due to the dehydration of TNT (calcined at 300° C) and due to the crystalline nature of the DTP. However, the peaks related to DTP are not clearly seen which


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may be due to very low loading of DTP, and also because of the high dispersion of DTP on TNT. Further loading of Pd also does not show any change in the diffraction pattern shown by DTP loaded TNT. The high dispersion of Pd particles can be attributed to the fact that there is no change in the overall XRD pattern. The reused catalyst shows the same diffraction patterns as that of virgin indicating the retention of structure on reuse.

Figure 2 :XRD patterns of (a)TiO2, (b) Titania nanotubes (TNT), (c) 20DTP/TNT, (d) 2.5Pd- 20DTP/TNT (e) Reused 2.5Pd- 20DTP/TNT 3.2.2 BET Figure 3 shows the N2 adsorption desorption isotherm of different samples. The surface properties are listed in Table 1. Surface area of nanotubes are very high (16 times increase) as compared to TiO2 precursor. BET surface area of TNT decreases after loading DTP. The decrease in pore volume indicates the successful dispersion of DTP. Further, when Pd is loaded, the surface area further decreases but the pore volume remains almost constant indicating the dispersion of the metal exclusively on the surface and negligible migration of



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Pd in the finer pores of the nanotubes. The reused and virgin catalyst shows no difference indicating highly stable and reusable nature of catalyst.

Figure 3 N2 adsorption-desorption isotherms of (a) TiO2, (b) Titania nanotubes (TNT), (c) 20DTP/TNT, (d) 2.5Pd- 20DTP/TNT Table 1: Surface area, pore volume and pore diameter analysis

Catalyst

Surface area Pore diameter (m2/g) (nm)

Pore Volume (cm3/g)

1

TiO2

12.8

16.6

0.053

2

TNT

192.8

12.1

0.55

3

20DTP/TNT

147.5

9.5

0.35

4

2.5Pd-20DTP/TNT

112.3

9.1

0.34

5

Reused2.5Pd-20DTP/TNT

111.2

9.2

0.34

No.


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3.2.3 TEM The TEM analysis reveals the formation of nanotubes with 100 nm length (Figure 4). There is no change in the length and morphology of the nanotubes after loading DTP and Pd. Small particles of Pd can be clearly seen in TEM images. The average particle diameter was found in the range of 4-7 nm.

Figure 4: TEM images of (a, b) 20DTP/TNT, (c) 2.5Pd- 20DTP/TNT 3.2.4 FT-IR The IR images of different samples are shown in Figure 5. The formation of nanotubes can be confirmed only on the basis of the increase in the intensity of the peaks at 3463 and 1626 cm-1 due to –OH stretching.6,7 Supporting DTP on TNT shows different peaks at 1069, 968, 891 and 790 cm-1 which are the fingerprints of the Keggin structure of DTP. 19,20

This also confirms that DTP is well dispersed on the support. Further introduction of Pd

does not change the IR spectrum. The reusable and fresh catalyst shows no difference and thereby confirming that the catalyst is reusable.



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Figure 5: FT-IR spectra of catalysts (a)TiO2, (b) Titania nanotubes (TNT), (c) 20DTP/TNT, (d) 2.5Pd- 20DTP/TNT (e) Reused 2.5Pd- 20DTP/TNT 3.2.5 SEM SEM images are shown in Figure 6. The formation of very small particles is evident from the micrographs. The elemental analysis (Table S2) confirms the presence of different elements related to DTP along with Pd metal. No distinct changes were seen in the SEM image after 3rd reuse of catalyst. ICP-AES analysis shows the presence of 2.45% (w/w) of Pd on the surface of the catalyst.

Figure 6: SEM Images of (a) 2.5Pd- 20DTP/TNT (b) Reused 2.5Pd- 20DTP/TNT 14 ACS Paragon Plus Environment

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3.2.6 Ammonia -TPD Acidity of different samples was measured by ammonia-TPD up to 300°C (Table 2). TNT also possesses some acidity which enhances after loading of DTP. TPD profiles of 20 DTP/TNT and 2.5Pd-20 DTP/TNT are shown in Figure S1. With the increase in DTP loading the acidity also increases but after loading of 20% w/w DTP there is no distinct difference in the acidity of the catalyst which may be due to the extra layer deposition of DTP on the previous layer. The loading of Pd does not affect the amount of acidity shown by the catalyst confirming that there is no interaction of Pd and DTP. Analysis of reused catalyst shows negligible change in overall acidity which confirms the reusable nature of catalyst. Table 2: Acid strength distribution of different catalyst

No.

Catalyst

Acidity (mmol/g)

1

Titania nanotube(TNT)

0.12

2

10% (w/w) DTP/TNT (10DTP/TNT)

0.34

3

15% (w/w) DTP/TNT(15DTP/TNT)

0.48

4


0.68

5


0.65

6

2.5Pd-20 DTP/TNT

0.70

7

Reused 2.5Pd-20 DTP/TNT

0.69

3.2.7 H2-Pulse chemisorption H2-pulse chemisorption of Pd loaded catalyst (Pd-20DTP/TNT) was conducted at 50° C to evaluate the active metal dispersion. Considering the stoichiometry Pds/H=1, metal dispersion was found to be 43.9% with metallic surface area 4.9 m2/g sample and 195 m2/g 15 ACS Paragon Plus Environment


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metal, respectively. Cubic crystallite size was calculated to be 2.1 nm. The overall results showed good dispersion of active metal on the support. 3.3 Synthesis of n-pentyl furfuryl ether (PFE) As shown in Scheme 1, the overall process can be easily divided in two parts, namely, Step1: etherification of furfuryl alcohol and Step 2: hydrogenation of the PFE to PTE using Pd and hydrogen. Initially screening of the prepared acid catalysts was done as regards total acidity for the first step using different loadings of DTP 3.3.1 Effect of different loading of DTP on TNT for the etherification reaction (Step1): To study the effect of loading of DTP on TNT on the conversion of furfuryl alcohol (FAL) and selectivity of PFE, the amount of DTP was changed from 10% to 25 % ( w/w). It was found that TNT itself catalyzed the reaction and 20% conversion and 18% yield of PFE after 2 h. This is due to the acidity of titania particle. Loading DTP on nanotubes resulted in efficient and increased conversion of FAL (Table 3). With increase in the loading of DTP, there was a significant increase in the conversion of the FAL and the selectivity of PFE also changed. Low loading of DTP gave less conversion, but somewhat higher PFE selectivity. Increase in the loading of DTP resulted in enhancement in the acidity of the catalyst as shown by the NH3 –TPD analysis, and hence the conversion of FAL also increased, but the selectivity of PFE decreased due to its further conversion to PL (Table 3). 20% w/w DTP loading gave the best result with 100% conversion of FAL and 79% selectivity of PFE. Further increase in the loading of DTP from 20% to 25% (w/w) results in a decrease in activity of a catalyst which can be correlated with the decrease in acidity of the catalyst as confirmed from ammonia TPD. The reason could be multilayer adsorption on the nanotube. Hence, 20% w/w DTP loading was optimized. As reported in literature28,38 the formation of self-condensed products of two alcohol is a major challenge in this type of reaction; however, 16 ACS Paragon Plus Environment

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with the use of TNT supported DTP catalyst, no self-condensation products to produce symmetrical ethers occurred.

The absence of formation of symmetrical ethers of both

alcohols can be explained as follows. The acidity and structure of the catalyst and the reactivity of reactants are responsible for the formation of only the unsymmetrical ether, which is the desired product. The current catalyst is not super acidic and also contains a metal site. Further the temperature and mole ratio of FAL to n-pentanol (1:20) and their reactivities in the presence of each other do not promote self-etherification. Excess of linear alcohol npentanol prevents self-etherification of FAL because (a) n-pentanol is more nucleophilic than FAL, (b) furfuryl carbocation is more stable as compared to primary n-pentyl carbocation and (c) furfuryl carbon is activated preferentially in the presence of catalyst, which facilitates the attack of more nucleophilic n-pentanol on furfuryl carbon and gives the desired product. This explanation is in consonance with that offered earlier39 for the condensation of n-butanol with benzyl alcohol. Furthermore, independent experiments were also conducted with n-pentanol and FAL as single reactants and it was found that no product was formed in the case of n-pentanol whereas FAL gave only 20% conversion at the same conditions. Hence with the use of FAL and n-pentanol together, the reactivity of n-pentanol is more and hence there is no formation of self-condensed product. We have also compared the activity of DTP supported on TiO2 and TNT. DTP supported on TNT gives high activity and selectivity of PFE. This is due to high surface area of TNT which results in high dispersion of DTP (20% w/w) than TiO2. Hence 20DTP/TNT was selected for further optimization.



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Step 1

Step 2

O O

OH Furfuryl alcohol (FAL)

+

Page 18 of 38

O

Pd-DTP/TNT

C5 H11-OH

+

k1

O

C5H11 Pd-DTP/TNT

H2 O

H2 , k4

n-Pentyl furfuryl ether (PFE)

n-Pentanol

C 5 H11 O

n-Pentyl tetrahydrofurfuryl ether (PTE) PdDT

k

2

,

P/T N

T

k

O

3

O H3C

C5 H11 O

n-Pentyl levulinate (PL)

Scheme 1: Synthesis of n-pentyl tetrahydrofurfurylether from furfuryl alcohol and npentanol. Table 3: Effect of different loading of DTP on TNT for the Etherification reaction No.

Catalyst

Conversion of FAL (%)

Selectivity of PFE (%)

1.

Titania nanotube(TNT)

20

90

2.

10DTP/TNT

42

88

3.

15DTP/TNT

66

81

4.

20DTP/TNT

100

79

5.

25DTP/TNT

94

78

6.

20DTP/TiO2

68

76

Reaction condition: FAL/ n-Pentanol (mole ratio) = 1: 20, catalyst loading= 0.04 g/cm3, speed of agitation= 800 rpm, T=140°C, t =2h. 3.3.2 Parameter optimization for the synthesis of n-pentyl furfuryl ether (PFE) After selecting the best catalyst (20DTP/TNT) for the synthesis of PFE we studied the effects of agitation speed, temperature, catalyst loading and mole ratio to find out the optimum reaction condition needed for the maximum yield of PFE.


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The agitation speed was varied from 200 to 1400 rpm in order to obtain the optimum speed of agitation to overcome the external mass transfer resistance (Figure S2). There is no basic difference in conversion and yield of PFE after 800 rpm which indicates that mass transfer resistance was eliminated beyond 800 rpm. However, 1000 was chosen as a safer agitation speed for further study. Theoretical calculation was done to confirm the absence of external mass transfer resistance. The mass transfer rates for FAL and n-pentanol were calculated and found to be 4.1 x 10-3 mol/ (cm3s) and 7.7 x 10-2 mol/ (cm3s), respectively. The observed initial rate of etherification reaction (robs) was found to be 2.2 x10-7mol/ (cm3s). Thus, the mass transfer rates are very high in comparison of the reaction rate which confirms that there is no effect of external mass transfer resistance at and beyond 800 rpm. Catalyst quantity was varied over a range from 0.01 to 0.06 g/ cm3 (Figure 7). The increase in loading increases the number of catalytic sites and hence there is an increase in the conversion and selectivity of the PFE product. Increase in catalyst sites after loading of 0.04 g/cm3 results in enhancement in further conversion of PFE to n-pentyl levulinate (PL) and hence we carried out experiments at 0.04 g/cm3 catalyst loading. Average particle size of catalyst was found to be 0.003 cm, theoretical computation was done by invoking the WeiszPrater modulus (Cwp) to see the influence of intra particle resistance on rate of reaction. The Cwp value was found to be 6x10-5 which is far less than unity and hence there is no role of intra-particle diffusion resistance on the overall rate of reaction. Hence the reaction is kinetically controlled which was confirmed by the activation energy calculation.



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Figure 7: Effect of 20DTP/TNT catalyst loading on conversion of FAL and selectivity of PFE. FAL/ n-Pentanol (mole ratio) = 1: 20, speed of agitation 1000 rpm, T=140°C, t =2h. (■) Conversion of FAL, (■) Selectivity of PFE

The mol ratio of FAL to n-pentanol was varied from 1:05 to 1:30 by keeping the total volume constant at 40 mL. The increase in mole ratio resulted in enhanced reaction rate and conversion (Figure 8). With increase in mole ratio from 1:5 to 1:20, there was an increase in selectivity of PFE while further increase to 1:30 resulted in decrease in selectivity. After 1:20 mole ratio, excess of n-pentanol resulted in increased rate of conversion of PFE to n-pentyl levulinate (PL) and hence selectivity of PFE decreased. Hence, 1:20 mole ratio, was taken as the optimum ratio.


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Figure 8: Effect of mole ratio of FAL to n-pentanol on conversion of FAL and on the selectivity of PFE. 20DTP/TNT loading=0.04 g/cm3, speed of agitation 1000 rpm, T=140°C, t=2h. (■) Conversion of FAL,(■) Selectivity of PFE Temperature was varied from 120 to 150 °C keeping other parameters at optimum condition. The increase in temperature led to increase the rate and thus increase in conversion too (Figure 9). At lower temperature, a very high selectivity of PFE was obtained but the conversion was low while, increase in temperature resulted in good conversion of FAL (almost 100%) but the selectivity of PFE decreased. It is because further conversion of PFE to PL also increases with temperature. From activation energy studies reported later it was evident that high temperature favours formation of PL having higher activation energy as compared to that for the formation of PFE. Hence, PFE selectivity decreases with increase in temperature. At 140°C 100% conversion of FAL was achieved with 81% selectivity of PFE which is better than other temperatures. Hence 140°C was selected as the optimum temperature for etherification reaction (step 1).



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Figure 9: Effect of Temperature on conversion of FAL and on the selectivity of PFE. FAL/n-Pentanol (mole ratio) = 1:20, 20DTP/TNT loading= 0.04 g/cm3, speed of agitation 1000 rpm, t =2h. (■) Conversion of FAL, (■) Selectivity of PFE 3.4 One Pot Synthesis of n-pentyl tetrahydrofurfuryl ether (PTE) Since the main aim of the present work was one pot synthesis of PTE from FAL and n-pentanol, after optimizing all parameters of step 1 synthesis with 20DTP/TNT catalyst; parameters affecting the one pot synthesis of PTE were studied. 20DTP/TNT was the best catalyst for the synthesis of PFE which is a main reactant for step 2. Hence it was taken as the catalyst for the loading of palladium (metal site) for hydrogenation of PFE to PTE. To attain maximum yield of PTE, rate of acid catalyzed step of formation of n-pentyl levulinate (PL) should be low during hydrogenation. This can be done by designing the catalyst and system parameters to achieve maximum activity for hydrogenation. In particular, the amount of palladium, hydrogen pressure and temperature are a few parameters which need to be


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optimized in order to get high reaction rate for hydrogenation and hence high yield of desired product (PTE) in one pot synthesis. Other parameters such as mole ratio, catalyst loading and speed of agitation were kept the same as above. 3.4.1 Effect of Pd loading on one pot synthesis of n-pentyl tetrahydrofurfuryl ether (PTE) Three different loadings of Pd were studied to optimize the catalyst composition for the synthesis of desired product in a single pot. Pd was loaded according to w/w basis on 20DTP/TNT. In order to attain maximum selectivity of PEE and to avoid hydrogenation of furfuryl alcohol, the reaction was conducted for first 2 h in the absence of hydrogen. The results with different loading of Pd are shown in Table 4. High loading of metal sites may result in lower activity of acid sites and vice versa and hence optimum loading of palladium was decided by maintaining the maximum activity for both reactions in one pot. With 1 % (w/w) loading of palladium, even though 100% conversion of furfuryl alcohol was obtained after 2 h (step1), hydrogenation of PFE in second step gets affected because further conversion of PFE to PL dominates at this loading and a very low yield of PTE was achieved. Similarly, when Pd loading was raised to 2.5 %, 98 % conversion of FAL was achieved after 2 h and then under hydrogen pressure of 4 MPa, 69% yield of the PTE was obtained. Further increase in Pd loading from 2.5% to 4 %( w/w) resulted in a decrease in the activity of the acid catalyzed reaction (Step1). To avoid hydrogenation of FAL in the presence of metal, the reaction was continued for extra 45 min to achieve 100 % conversion of FAL over 4% Pd loaded catalyst (4Pd-20DTP/TNT). The increase in reaction time resulted in decrease in the yield of PFE (67%) as PFE further reacts to form PL (33%) after step 1 which is mainly due to high concentration of PFE than that of FAL which results in enhanced rates of formation of PL as compared to the etherification reaction. Further, when hydrogen was introduced, the final product (PTE) yield was found to be only 61%. Thus it was concluded that there was a 23 ACS Paragon Plus Environment


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need to optimize both time and catalyst sites in order to get the maximum yield of desired product (PTE). Hence 2.5% (w/w) loading of palladium was chosen for further study. Table 4: Effect of Pd loading on the activity of catalyst in one pot synthesis. Yield of products (%) Conversion No.

Catalyst

of FAL (%)

Step1

(a)

Step2

(b)

after step 1 PFE

PL

PTE

PFE

PL

PTE

1

20DTP/TNT

100

79

21

-

38

62

-

2

1Pd-20DTP/TNT

100

79

21

-

22

31

47

3

2.5Pd-20DTP/TNT

98

80

20

-

1.9

29.1

69

4

4Pd-20DTP/TNT

67

33

-

-

39

61

100

(c)

Conditions: FAL/n-Pentanol (mole ratio) = 1:20, catalyst loading=0.04 g/cm3, speed of agitation 1000rpm, (a) T=140°C, No H2 pressure, t =2h. (b) In continuation, reaction was further carried out for another 3 h at T=120°C, PH2=4 Mpa. (c) Reaction was carried out for additional 45 min to achieve 100% FAL conversion. Yield of products is reported at the end of each steps.

3.4.2 Effect of hydrogen pressure on one pot synthesis of n-pentyl tetrahydrofurfuryl ether (PTE) After finalizing the loading of the palladium, the influence of hydrogen pressure on the overall yield of PTE was studied. The results are shown in Table S3. The pressure was varied from 2 Mpa to 6 Mpa in order to ascertain the optimum hydrogen pressure needed for the highest possible yield of the PTE. Hydrogen was introduced after 2 h (after etherification reaction). Increase in pressure resulted in enhancement in rate of reaction and yield (Table 24 ACS Paragon Plus Environment

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S3). The concentration of hydrogen in the liquid increases with increasing pressure and hence concentration of hydrogen on metal surface increases. However, when the pressure was increased from 4 to 6 MPa, almost the same yield was obtained indicating that the optimum amount of pressure needed for the system is 4 MPa. 3.4.3 Effect of temperature on one pot synthesis of n-pentyl tetrahydrofurfuryl ether (PTE) In order to suppress the formation of the n-pentyl levulinate (PL), the temperature was lowered from 140°C (needed for step1) to 120°C. Since it was noticed in the parameter optimisation part of step1 (Figure 9), the formation of PL was greatly influenced by the reaction temperature. Thus, the temperature was changed from 100°C to 140 °C and the results are shown in Table 5. Lowering temperature from 140°C to 100°C resulted in a decrease in the yield of PL. Also, the rate of hydrogenation of PFE also decreased on decreasing the temperature. However, at 120° C 69% yield of PTE was achieved. Hence 120°C was considered as an optimum temperature for the hydrogenation step. 3.4.4. Activity of physical mixture A physical mixture of 20DTP/TNT and 5%Pd/C was also used with same reaction condition as above. The quantities were used in such a way that the amount of Pd and DTP were the same as with 2.5Pd-20DTP/TNT. There was no distinct change seen in the conversion of FAL; however, the yield of PFE decreased to 74% with 26% of PL. Further, with the introduction of hydrogen and lowering the temperature of reaction, enhancement in the rate of hydrogenation was achieved, but the overall yield of PTE decreased significantly to 58%. It is because the rate of formation of PL is not affected by the hydrogenation step as there are two different catalysts present in the system. In case of 2.5Pd-20DTP/TNT, access of acidic sites was controlled because of the dispersed metal site. Hence the prepared catalyst 25 ACS Paragon Plus Environment


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2.5Pd-20DTP/TNT is much better than the physical mixture of the two catalysts (i.e. acid and metal). Table 5: Effect of hydrogenation temperature on the yield of PTE in one pot synthesis

Yield of different products (%) Conversion of

After Step 2

Temperature

FAL (%) after

No.

(°C)

step 2

PFE

PTE

PL

1

100

100

62

17.5

20.5

2

110

100

36

39.4

24.6

3

120

100

1.9

69

29.1

4

130

100

---

61.2

38.8

5

140

100

---

50.2

49.8

Conditions: FAL /n-Pentanol (mole ratio) =1: 20, Catalyst- 2.5Pd-20DTP/TNT, catalyst loading = 0.04 g/cm3, speed of agitation 1000 rpm, Hydrogen Pressure =4 MPa, reaction time= 3h. Yield of products is reported at the end of each steps.

Comparison of rate data and selectivity was also done by conducting experiments that the physical mixture of Pd/C and DTP/TNT is less reactive and selective than when the catalyst containing both metal and acidic sites are embedded in the same matrix. The first step reaction using DTP/TNT was conducted for formation of unsymmetrical ether and the acid catalyst was filtered off. Thereafter the reaction mixture was used again for hydrogenation reaction using Pd/C. The end product results shows that isolated yield of final product was marginally low (65%). Thus, having both metal and acidic sites on the same matrix is advantageous.


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3.5 Catalyst reusability Catalyst reusability was studied 4 times. After the reaction, the catalyst was separated and washed with methanol and acetone mixture at the reflux condition and dried in vacuum at 80 °C for 5 h. The loss of catalyst during filtration was made up by adding afresh catalyst. The same procedure was followed each time. There was a slight decrease in the yield of product after 4th use since 66% yield of PTE was obtained with 2.5% yield of PFE and the remaining was PL. Hence the catalyst is reusable. Further to support above finding, a systematic characterization of spent catalyst was done after reuse and results were already discussed above. The characterization results further confirmed reusability of catalyst. Catalyst stability was also accounted by using a hot filtration test. The reaction was stopped after 45 min and the catalyst was filtered and further reaction was carried out for another 2 h. Conversion and selectivity remained practically the same which confirmed the heterogeneous nature of the reaction. Hydrogen pressure was also supplied and there was no hydrogenation reaction occurs supporting the stable nature of catalyst. The ICP-AES analysis of reaction mixture shows the presence of 0.09 ppm of Pd. This can be considered excellent.

4. Mathematical model Development: 4.1 Mathematical equation development for first step: Assuming S1 as acidic and S2 as metal site. For first step reaction, the rate of reaction of furfuryl alcohol (A) with n-pentanol (B) on acidic sites to n-pentyl furfuryl ether (C) can be give as;  k1 K AC A K B C B − k 1' K C CC KW CW  w r1 = 2 [1 + K AC A + K BCB + KC CC + K D CD + KW CW ]

(1)



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Further the reaction of PFE to PL (D) cab also be given as:  k2 K C CC KW CW − k2' K D CD  w r2 = 2 [1 + K AC A + K B CB + KC CC + K D CD + KW CW ]

(2)

The detail derivation is provided in ESI. Assuming reaction is far from equilibrium and using solver we have solved the above equation and found that the adsorption constants are very low (Ki

Novel Bifunctional Palladium ... - ACS Publications

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