Hydrogenation of Furfural with a PtâSn Catalyst: The Suitability to

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Full Paper

Hydrogenation of furfural with a Pt-Sn catalyst: The suitability to sustainable industrial application Áine O'Driscoll, Teresa Curtin, Willington Y. Hernandez, Pascal Van Der Voort, and James J. Leahy Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00228 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Organic Process Research & Development is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Organic Process Research & Development

Hydrogenation of furfural with a Pt-Sn catalyst: The suitability to sustainable industrial application Áine. O’Driscoll1,2,*, Teresa. Curtin1,2, Willington. Y. Hernández3, Pascal. Van Der Voort3, James. J. Leahy1,2, 1

Carbolea Research Group, Department of Chemical and Environmental Sciences, University of

Limerick, Limerick, Ireland 2

Material and Surface Sciences Institute, University of Limerick, Limerick, Ireland

3

Department of Inorganic and Physical Chemistry, Center for Ordered Materials,

Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium *Corresponding Author: Áine O’Driscoll; [email protected]

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: furfural, furfuryl alcohol, hydrogenation, Pt-Sn/SiO2 catalyst, protic & aprotic solvents, catalyst reuse and regeneration, Abstract The liquid phase hydrogenation of furfural to furfuryl alcohol was carried out using a Pt-Sn/SiO2 catalyst synthesised by co-impregnation to investigate the impact of synthesis and reaction conditions. The results showed that calcination of the catalyst at 450°C gave the highest furfural conversion. An investigation on the reaction conditions found that furfural conversion increased with temperature and hydrogen pressure. Reuse of the catalyst was shown to be as effective as catalyst regeneration with a 3% loss in furfural conversion observed with each repeated use. The use of protic and aprotic solvents showed that furfural conversion was lower using an aprotic solvent such as toluene or propanone but selectivity to furfuryl alcohol remained close to 100%. The two protic solvents, 1-propanol and 2-propanol, formed several additional products including 2-methyl furan, 2-propoxy methyl furan, 2-furaldehyde dipropyl acetal, difurfuryl ether and 2-furaldehyde diethyl acetal. Introduction The level of fossil fuels is diminishing and energy demands are continuing to rise. Biomass is currently the only renewable source of carbon and this has led to increased interest and research in its potential for chemicals, energy and fuel production. The necessity to substitute the energy demands currently satisfied by fossil fuels with biomass derived materials is aided by directives such as the Renewable Energy Directive (RED). This directive requires EU members to fulfil at least 20% of total energy needs and 10% of transport needs with renewables by 2020.1 Additional projects such as the European sustainable process industry SPIRE initiative also support fossil energy substitution. SPIRE is a public-private partnership which aims to reduce


Page 2 of 47

Page 3 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fossil energy intensity by 30% coupled with a 20% reduction in the use of non-renewable resources by 2030 (SPIRE 2013).2 The increase in biomass utilisation, as a result of participation in such initiatives, has augmented research and development of alternative biobased routes for the production of fine chemicals from biomass sources. Furfural (FF), which is obtained from the C5 sugars in lignocellulosic biomass, is a platform chemical which may be further processed to produce furfuryl alcohol (FA), furan, tetrahydrofurfural and tetrahydrofurfuryl alcohol among others.3 The desired product, furfuryl alcohol is formed by the catalytic hydrogenation of furfural however, selective hydrogenation is necessary due to the array of possible furfural derivatives. This process requires careful catalyst selection together with optimisation of the reaction conditions to ensure maximum economical product delivery. The industrial catalyst for the hydrogenation of furfural to furfuryl alcohol is copper chromite and is well known to be environmentally toxic, promoting extensive research to develop suitable alternatives.4 Green chemistry for industry addresses all aspects of the manufacturing process to promote industrial environmental sustainability. Bourne et al5 highlighted twelve principles of industrial green chemistry which include prevention of wastes, use of ambient temperature and pressure together with using a reduced number of steps in a process. The disposal of solvents from reactions and purification processes account for the bulk of industrial waste. These principles underpin this work by targeting furfuryl alcohol as the sole product and maintaining the single step process of furfural hydrogenation. The ability to reuse a catalyst is a necessity for an economical process. Catalysts may deactivate over time and generally, regeneration of the catalyst is required. Several techniques are used to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

achieve this with the regeneration technique applied based on the cause of deactivation.6 The most common technique employed for the extraction of organics from a solid catalyst matrix at laboratory scale is soxhlet extraction. It has been used effectively for over a century and is often used as the benchmark.7 There are some issues with soxhlet extraction as it is energy demanding and time consuming. An alternative method is ultrasound assisted extraction using a sonicator. This method significantly reduces the process duration and further reduces the quantity of solvent required. An additional important factor is the ability to conduct the process at room temperature.8-9 This work focuses on identifying the optimum synthesis and reaction conditions for a 0.7%Pt0.3%Sn/SiO2 catalyst synthesised by co-impregnation. Reuse and regeneration of the catalyst using soxhlet extraction and sonication were also investigated in this work. The results were compared to those from leading liquid phase based literature for the hydrogenation of furfural to furfuryl alcohol. Materials and Methods Furfural, furfuryl alcohol, ethanol, toluene, 1-propanol and 2-propanol and propanone, all of analytical reagent quality, were obtained from Sigma-Aldrich. Metal salts for catalyst preparation; platinum (II) acetylacetonate and tetrabutyl tin together with the catalyst support SiO2 were also obtained from Sigma-Aldrich.


Page 4 of 47

Page 5 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental Catalyst Preparation The catalyst was synthesised by co-impregnation by adding 0.0407 g of platinum (II) acetylacetonate and the required amount of tetrabutyl tin to 50 ml of ethanol and 2.0 g of the silica support. This suspension was placed in a sonicator for 5 minutes prior to the addition of the support. The suspension was then stirred for 16 hours at room temperature followed by rotary evaporation of the solvent. The catalyst was calcined in air with a temperature ramp of 5°C min-1 from ambient temperature to 450°C and maintained at this temperature for 5 hours. Catalytic Tests The liquid phase hydrogenation of furfural was carried out in a 600 ml autoclave (Parr 4560) equipped with a mechanical stirrer. For a typical experiment, 1.0 g of catalyst was placed in the reactor and reduced at 300°C and 20 bar of hydrogen pressure for 4 hours. The reactor was then flushed with nitrogen and charged with 25 ml of furfural and 175 ml solvent. The reactor was heated to 100°C and the pressure was rapidly increased to 20 bar of hydrogen with a stirring speed of 600 rpm. Homogeneous mixing was assumed and samples were taken at regular intervals for analysis by gas chromatography (Agilent Technologies 7820A) equipped with an FID detector. A Restek Stabilwax® 10623 30 m x 0.25 mm x 0.25 µm column was used, with nitrogen as the carrier gas at a flow rate of 12 ml min-1 and a split ratio of 1:25. The injector and detector were operated at 300°C and 250°C, respectively. A selection of samples were analysed using gas chromatography-mass spectroscopy (Agilent Technologies 7890A) under the same conditions as for the GC-FID for product identification purposes. Formulae



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The conversion of furfural, selectivity to furfuryl alcohol, reaction yield and molar conversion were calculated as follows;

FF Conversion (%):

FA Selectivity (%):

Yield (%):

x

x

x

Molar Conversion:

= ()

where x = weight of catalyst used

= initial moles of furfural ! = moles of furfural in sample " = moles of furfuryl alcohol Catalyst Regeneration Regeneration of the catalysts was carried out using two methods; soxhlet extraction and sonication. Soxhlet extraction was conducted over a 24 hour period using 50 ml of toluene per gram of catalyst. Sonication using an ultrasonic bath was performed over 12 hours using 30 ml of toluene per gram of catalyst. Catalyst characterisation Scanning electron microscopy (SEM) & energy-dispersive x-ray analysis (EDAX) were used for measurement of metal content using a Hitachi SU-70 SEM operating at 20 kV equipped with an


Page 6 of 47

Page 7 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EDAX detector. Samples were placed directly onto carbon tape on the specimen stub with no further pre-treatment performed. Surface area analysis was used to establish surface area, pore volume and pore radius of the catalysts by nitrogen physisorption using a Quantachrome Autosorb AS-1. For each analysis, 10 mg of sample was outgassed under vacuum at 150°C for 12 hours to remove water and other atmospheric contaminants followed by analysis consisting of measurement of adsorption and desorption of nitrogen gas on the sample conducted at -196°C. X-ray photoelectron spectroscopy (XPS) data was obtained using a Kratos Axis 165 spectrometer equipped with an Al Kα (1486.6 eV) X-ray source and a fixed pass energy of 20 eV. The intensities were estimated by calculating the integral of each peak after smoothing and subtraction of the S-shaped background. The experimental curves were fitted with Lorentzian and Gaussian lines of variable proportion. Binding energies (BE) were referred to the C 1s line at 284.8 eV. The catalyst was not reduced prior to XPS analysis. H2-TPR measurements were performed in an Autosorb iQ TPX device, from Quantachrome. In a typical experiment, around 80 mg of sample was pretreated at 140ºC (heating rate 20ºC min-1) for 1 hour in a flow of He (30 ml min-1). Subsequently, the sample was cooled to 80ºC under the same flow of He. The reduction analysis was performed from 80 to 900ºC (10ºC min-1) in a 50 ml min-1 flow of 5% v/v H2 in Ar. Hydrogen consumption was calibrated using CuO as a reference. The morphology of the catalysts was determined by transmission electron microscopy (TEM) using a JEOL JEM-2011F electron microscope, operating at an accelerating voltage of 200 kV. The samples were prepared for analysis by drop-casting a suspension of catalyst in iso-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

propanol on copper TEM grids. The images obtained were analysed using Digital Micrograph® software to measure the particle size from which the distribution graphs were generated by measuring at least 100 particles. Fourier transform infrared spectroscopy (FTIR) was used to identify the presence of organic substances on the catalyst. The spectrum for each sample was collected using an Agilent Technologies Cary 630 FTIR equipped with an attenuated total reflectance (ATR) accessory. The samples were placed directly onto the diamond and collected spectra were averaged over 32 scans from 4000 cm-1 to 650 cm-1. Thermogravimetric Analysis (TGA) was used to investigate the deposition of organics on the catalyst during the reaction by measuring the difference in weight loss of a fresh catalyst and a catalyst after hydrogenation. A TA Q50 TGA was used and for analysis, 10 mg of catalyst was heated to 550°C at a ramp of 10°C min-1 in a nitrogen atmosphere with a flow rate of 60 ml min1

.

Results and Discussion This research focuses on the influence of the reaction parameters on the hydrogenation of furfural to furfuryl alcohol. A 0.7%Pt-0.3%Sn/SiO2 catalyst synthesised by co-impregnation was used for all reactions. The reaction conditions considered to be the standard conditions for this work were; 25 ml furfural, 175 ml solvent (toluene), 20 bar hydrogen pressure, a reaction temperature of 100°C and a reaction duration of 300 minutes. The parameters selected for investigation included catalyst calcination temperature, reaction temperature, pressure of hydrogen supplied to the reaction, solvent employed and the concentration of furfural.


Page 8 of 47

Page 9 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The pre-treatment of catalysts has been established as an effective means of enhancing catalytic performance. Calcination is an example of a pre-treatment step and was investigated in this work. Calcination was conducted in air and aimed to activate the deposited metal and decompose the precursor salt providing access for the reactant to the active sites within the pores.10 The influence of the calcination temperature on furfural conversion with respect to time was investigated. The results are displayed in Figure 1 with the optimum calcination temperature of 450°C clearly evident. Selectivity to furfuryl alcohol remained close to 100% for all calcination conditions studied. This investigation showed that calcination is necessary for the development of an active catalyst for furfural conversion. The range of activity observed may be due to the influence of temperature on metal activation and the catalyst pores. When calcination was carried out at 350°C, residues from the precursor salt may not have fully cleared from the pores or perhaps some of the deposited metal was not activated. On the other hand, at a high temperature (550°C), the support framework may be affected such as the collapse of some pore walls to form very large pores which would reduce the interaction between the catalyst and the reactant. Additionally, at a higher temperature the metal particles may agglomerate leading to a larger particle size. The uncalcined sample was also less active, most likely due to the lack of active metal.



60

50

FF Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 47

350°C 450°C 550°C Uncalcined

40

30

20

10

0 60

120

180

240

300

Time (min) Figure 1: The influence of calcination temperature on 0.7%Pt-0.3%Sn/SiO2. (Reaction Conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (toluene), #$ % =20 bar, T=100°C.) The catalysts prepared at different calcination temperatures were characterised by nitrogen adsorption/desorption and their resulting surface areas and pore dimensions are reported in Table 1. Surface analysis highlighted that the surface area was not influenced by the calcination temperature. Increased surface area is typically associated with a more active catalyst however in this work the narrow range of surface areas observed does not indicate an influence between surface area and catalyst activity. No significant changes to pore volume and pore radius were observed.


Page 11 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: Surface characterisation for the 0.7%Pt-0.3%Sn/SiO2 catalyst with different calcination conditions. Surface Area

Pore Volume Pore Radius

(m2 g-1)

(cc g-1)

(Å)

Silica Support

682

1.3

43.2

Uncalcined

644

1.5

42.9

350°C

680

1.3

43.3

450°C

640

1.2

41.1

550°C

670

1.3

43.2

Catalyst

XPS analysis of the catalyst calcined at 350°C, 450°C and 550°C was conducted to determine the chemical states of the constituent elements of the catalysts resulting from calcination at different temperatures. The binding energies (BE) of the Pt 4f and Sn 3d peaks for the catalysts are listed in Table 2 while the Pt 4f core level spectra and Sn 3d core level spectra can be observed in Figure 2 (a) and (b) respectively. The spectra are noisy due to the low metal content present in the catalyst. Table 2: Binding energy of the selected catalysts. Binding Energy (eV) Catalyst Pt 4f7/2

Pt 4f5/2

Pt 4f7/2

Pt 4f5/2

Sn 3d5/2

Sn 3d3/2

350°C

71.3

74.6

74.4

77.7

487.3

495.7

450°C

71.6

74.9

74.1

77.5

487.1

495.6

550°C

71.4

74.8

74.2

77.5

487.2

495.6


Page 12 of 47

350°C 450°C 550°C 80

75

Binding Energy (eV)

70

(a)

350°C Intensity (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (-)


450°C 550°C

500 498 496 494 492 490 488 486 484 482

Binding Energy (eV)

(b)

Figure 2: (a) Pt 4f core level spectra and (b) Sn 3d core level spectra of 0.7%Pt-0.3%Sn/SiO2 calcined at the indicated temperatures. Figure 2(a) shows the Pt 4f spectra of the catalyst calcined at different temperatures. The Pt 4f region contains two pairs of doublets. The intense doublet observed for the catalyst at 71.6 and 74.9 eV is a signature of metallic platinum or PtO in this instance. The second doublet observed for the monometallic catalyst at 74.1 and 77.5 eV can be attributed to Pt(IV), present in the form


Page 13 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of PtO2. The shift from the literature value of 71.0 eV for the bulk metal to a value of 71.4 eV for a monometallic Pt/SiO2 catalyst suggests the possible influence of metal-support interactions. The increased shift to 71.6 eV for the bimetallic catalyst calcined at 450°C highlights the influence associated with the addition of tin to create a bimetallic system.11 12 The studied catalyst contain some Pt(IV) present as PtO2 however no significant changes were observed in the oxidation state of platinum as a result of the calcination temperature. This confirms the stability of the platinum in the catalyst at high temperatures. The region of binding energy in the high resolution Sn 3d core level spectra signifies the presence of Sn(IV) which is present as SnO2. The peaks for Sn 3d5/2 and Sn 3d3/2, shown in Figure 2(b), were detected with the binding energies falling in the range of 487.1–487.3 and 495.6–495.7 eV which shows the stability of tin in the catalyst after high temperature treatment as tin remains in the Sn(IV) form. XPS analysis did not reveal any significant differences in the chemistry of the metal interactions as a direct result of the calcination temperature. Therefore, additional characterisation of the catalysts was conducted. Temperature programmed reduction (TPR) analysis was carried out with H2-TPR profiles of the bimetallic Pt-Sn/SiO2 catalysts shown in Figure 3. The results indicate that the calcination temperature has a strong influence on the reduction profiles of the evaluated catalysts, principally in the temperature interval from 350 to 800ºC. In the low temperature (LT) reduction area, below 350°C, all catalysts were characterised by a peak centred at 290ºC which can be attributed to the co-reduction of Pt oxides and Pt-O-Sn type species.13-14 14 15

In addition, all catalysts display a shoulder at lower temperatures on this peak (∼ 218 ºC)

which may be due the reduction of platinum species in addition to weak interactions with the support and/or the reductive decomposition of any remaining platinum precursor used in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

preparation of the catalysts.16 In fact, for calcination at the lowest temperature (350ºC), the hydrogen consumption began earlier, at around 140ºC, possibly indicating the presence of the un-decomposed organometallic precursor. This is also evident from the calculated hydrogen consumption displayed in Table 3 which, for this catalyst, is higher than 100% most likely due to the reduction decomposition of the Pt and/or Sn precursors.

Figure 3: H2-TPR profiles of the prepared 0.7%Pt-0.3%Sn/SiO2 catalyst, calcined at the indicated temperatures. The peaks observed at reduction temperatures higher than 350ºC (high temperature (HT) reduction area) were very heterogeneous in their position and intensity depending on the thermal treatment applied. In general, the high temperature peaks can be associated with the reduction of Sn species with the reduction of Sn4+ to Sn2+ observed at an intermediate temperature (539, 548


Page 14 of 47

Page 15 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 565 for the catalyst calcined at 350, 450 and 550ºC, respectively), while the highest temperature peak is most likely due to the reduction of Sn2+ to Sn0.13-15, 17-19 The peaks observed at 410 and 473ºC for the catalyst calcined at 350 and 450 ºC respectively could be associated with the reduction of different Pt-O-Sn species having a strong interaction with the silica support.19 It is evident that the reduction peaks occurring in the HT reduction area tend to be shifted to higher temperatures depending on the thermal treatment performed on the catalysts. Thus, it is likely that the increase in the calcination temperature of the catalyst caused a stronger interaction of those species with the support.18 This situation will also decrease the reducibility capacity of the sample calcined at the highest temperature (550 ºC), as observed in Table 3. Table 3: Hydrogen Consumption Calcination

µmol H2 g-1

Reducibility

Temperature (°C)

catalyst.

(%)*

350

132

108.2

450

110

90.2

550

106

86.9

*Considering a theoretical value of 122 µmol H2 g-1 catalyst., assuming 0.7%Pt/0.3%Sn composition

From the H2-TPR results presented, a definitive conclusion on the influence of calcination temperature on the activity of the catalyst for furfural conversion was not evident. Therefore, it is most likely that the thermal treatment performed on the catalyst at 450 ºC provided a good compromise between the complete decomposition of the organometallic precursors used in the preparation of the catalyst together with the metal-support interactions generated on the material.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TEM analysis was also conducted to identify the particle size distribution with the results displayed in Figure 4. It was observed that calcination of the catalyst at the higher temperature of 550°C resulted in a larger metal particle size and wider distribution while the lower calcination temperatures of 350°C and 450°C gave a narrower metal particle size distribution and smaller average particle size. The average particle size of the catalyst calcined at lower temperatures was 2.5 nm however the frequency of this particle size at 450°C was 51% while it was 34% at 350°C. The difference in particle size due to the influence of temperature is most likely resulting from agglomeration of the metal particles at 550°C resulting in a larger particle size while at 350°C a lower frequency of the 2.5 nm active particles was observed. Calcination at 450°C presents the optimum catalyst calcination temperature with a smaller particle size and a smaller variation in the frequency distribution of the particles. It seems that particle size may influence catalyst activity as the lowest catalyst activity was observed with the largest particle size (550°C) while the higher frequency of the smaller particle size (450°C) gave the best catalyst activity.


Page 16 of 47

Page 17 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Frequency distribution for the catalyst calcined at a) 350°C, b) 450°C and c) 550°C. Overall, it is most likely that the differences seen in furfural conversion were due to structural changes from calcination at different temperatures together with less active metal species present on the support. The influence of reaction conditions on furfural conversion The influence of the reaction temperature on the catalyst activity, using the catalyst calcined at 450°C, was investigated at temperatures in the range of 25–150°C. The results are displayed in Figure 5 with respect to time. For each temperature studied, the conversion increased over time together with an increase in furfural conversion for each temperature increase. The temperature had no significant influence on the selectivity to furfuryl alcohol remaining close to 100% at all temperatures.



70

25°C 50°C 80°C 100°C 120°C 150°C

60

FF Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 47

50 40 30 20 10 0 50

100

150

200

250

300

Time (min) Figure 5: The influence of the indicated reaction temperatures on furfural conversion with time. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (toluene), #$ % =20 bar.) The influence of hydrogen pressure on the reaction was investigated by conducting the furfural hydrogenation reaction at 100°C and pressures between 10 and 30 bar. A different batch of catalyst, referred to as 0.7%Pt-0.2%Sn/SiO2, was used for this investigation. The results showed that increased pressure resulted in furfural conversion of 33, 53 and 73% observed after 300 minutes using 10, 20 and 30 bar hydrogen pressure as shown in Figure S-1. The concentration of reactant used was also investigated. Many researchers use a significantly lower reactant volume per gram of catalyst than the 25 ml of furfural per gram of catalyst typically employed in this work. This fact is highlighted in Table 4 which displays the reaction


Page 19 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conditions and conversion for a selection of relevant reports from the literature. The selected catalysts together with the highest catalyst activity reported for each are presented in the table for comparison with the catalyst synthesised in this work. A varied range can be observed in the choice of solvent and metals used for catalyst synthesis. One chromium containing catalyst20 is included for comparative purposes. Table 4: Literature based catalysts selected for comparison with the synthesised 0.7%Pt0.3%Sn/SiO2 catalyst. Reaction Catalyst

Solvent

Conversion

Reactant &

Selectivity

Solvent (ml).

(%)

2-Propanol,

100

Conditions: Catalyst

Amount

Source

Temperature, (g) PH2, 100°C, PtSn/SiO2

Ir-ReO/SiO2

CuZnCrZr

Co-Cu/SBA-15

Pd-Cu/MgO

Pt-Sn/SiO2

21

0.25 10 bar

2, 50

96

30°C,

Water,

>99 22

0.05 8 bar

0.25, 3

>99

170°C,

2-propanol

100 20

1.50 20 bar

12.5, 87.5

96

130°C,

2-propanol

96.7 % 23

1.00 30 bar

5,40

FA Yield

110°C

Water,

100 24

0.05 6 bar,

0.5, 10

98.6

100°C

Toluene,

63

Current

20 bar,

25, 175

>99

Research

1.00



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The published research generally tends to use less than 10 ml of reactant per gram of catalyst. Using the information outlined in the literature, a selection of furfural concentrations including 5, 10 and 25 ml per 200 ml total volume were selected to investigate the influence of reactant concentration on furfural conversion with respect to time. This information is displayed in Figure S-2 which shows that increasing the ratio of reactant to catalyst results in lower conversion. It was observed that the reaction which employed 5 ml of reactant achieved 100% conversion after 240 minutes while the 10 ml and 25 ml samples reached 85% and 53% conversion respectively after 300 minutes. No conversion was observed in the absence of a catalyst. The relationship between the reactant volume and catalyst quantity was further studied by presenting the data in molar conversion as moles of furfural converted per gram of catalyst over time with the results presented in Figure 6. From this figure it can be seen that the molar conversion per gram of catalyst is higher using 25 ml of reactant.


Page 20 of 47

Page 21 of 47

0.18

Moles of FFconverted / gram catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.16

5 ml FF 10 ml FF 25 ml FF

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 60

120

180

240

300

Time (min) Figure 6: Moles of furfural converted per gram of catalyst over time. (Reaction conditions: 1 g catalyst, FF= volume as indicated, 175 ml solvent (toluene), #$ % =20 bar, T=100°C.) The performance of the literature based catalysts highlighted in Table 4 was compared to the 0.7%Pt-0.3%Sn/SiO2 catalyst in terms of molar conversion of furfural per gram of catalyst. These results are presented in Figure 7 with the result from this work representing the use of 25 ml of furfural. The comparison highlights the higher conversion of the 0.7%Pt-03%Sn/SiO2 catalyst synthesised in this work achieving a molar conversion of 0.18 moles converted per gram of catalyst used with the best literature catalyst (Pd-Cu/MgO)24 giving a conversion of 0.12 moles converted per gram of catalyst used. It must also be noted that the reaction parameters including the reaction time (minutes), which is included in parentheses, varied among the published reports.



0.20

Molar conversion of FF / gram catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 47

(300)

0.18 0.16 0.14

(80)

0.12

(360) 0.10 0.08

(180)

0.06

(210)

0.04 0.02

(480)

0.00 PtSn/SiO2 Co-Cu/ SBA-15 [21] [23]

CuZnCrZr Ir-ReO/SiO2 Pd-Cu/MgO Pt-Sn/SiO2 [20] [24] Current [22] Research

Figure 7: The molar conversions reported in leading publications and the present work. The duration of each reaction in minutes is reported in parentheses. The 0.7%Pt-0.3%Sn/SiO2 catalyst investigated in this work did not achieve 100% conversion after 300 minutes when 25 ml of furfural was used. Therefore, the hydrogenation process was continued past 300 minutes and Figure 8 shows furfural conversion over 36 hours reaction time.


Page 23 of 47

100 90 80

FF Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 0

5

10

15

20

25

30

35

40

Time (h) Figure 8: Catalyst activity over time using 0.7%Pt-0.3%Sn/SiO2. (Reaction Conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (toluene), #$ % =20 bar, T=100°C.) Figure 8 shows that the catalyst remained active beyond the 5 hour reaction period which is typically used in this work. A reaction time of 24 hours was required to convert 96% of the furfural while 100% conversion was achieved after 36 hours. Although this is longer than the times reported in other studies which typically required 1.5–8 hours for total conversion, the reactions carried out for this work are highly selective to furfuryl alcohol and converted close to 25 ml of furfural in 24 hours using just one gram of the catalyst. Overall, the ability of the catalyst to achieve complete conversion is positive in terms of an industrial commercial catalyst.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalyst reuse and regeneration The reuse and regeneration properties of a catalyst play a crucial role in its selection for commercial application. The 0.7%Pt-0.3%Sn/SiO2 catalyst presented in this work was subject to repeated hydrogenation. The catalyst was first reused without regeneration to identify if activity was reduced with each reaction cycle. The catalyst was filtered after each reaction, washed with approximately 50 ml of solvent followed by oven drying at 80°C. The results of the reused catalyst are displayed in Figure 9 which shows a small decrease in furfural conversion with each hydrogenation cycle. The fresh catalyst was a light beige colour while the used catalyst was a rust orange colour which deepened with each hydrogenation cycle. The used catalyst turned black following oven drying in excess of five minutes. It was considered possible that the colour changes observed were resulting from the presence of deposited organics on the catalyst.


Page 24 of 47

Page 25 of 47

60

50

FF Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

20

10

0 1

2

3

Hydrogenation Cycle

Figure 9: The effect of repeated hydrogenation cycles on furfural conversion after 300 minutes reaction time. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (toluene), #$ % =20 bar, T=100°C.) A number of analyses were applied to the used catalyst to determine if the pores of the catalyst were blocked after hydrogenation causing the observed reduction in catalyst activity. The catalyst which completed three hydrogenation cycles was characterised and compared to a fresh catalyst. Thermogravimetric analysis was conducted to investigate if organics were present on the catalyst with the results presented in Figure 10 together with the derivative curve for the used catalyst. A weight loss of 7% was observed for the fresh catalyst which can be attributed to water, as silica is a hygroscopic material and the loss occurred in the ambient–100°C range. A weight loss of 17% was observed for the used catalyst. This loss was most likely due to organics and other materials present on the catalyst as the loss occurred in the temperature range of 50– 550°C. It was noted that the rate of the weight loss varied across this temperature range. The



derivative curve illustrates the trimodal nature of the weight loss from the used catalyst. A narrow peak is present at 40°C with two clear peaks present at 110–160°C and 400–520°C. The first peak is due to loss of adsorbed moisture due to the hydroscopic properties of silica. The second peak is most likely from the loss of adsorbed reaction and product mixture toluene, furfural and furfuryl alcohol due to the location of the peak. The final peak (~450°C) is broad which, coupled with the shape of the TGA curve, indicated that the weight loss was slow. Due to the temperature range of this peak it is most likely representative of the pyrolysis of resins or char.25

100

0.06

96 94

0.05

92 0.04

90 88

0.03

Deriv. Weight (% min-1)

Derived Weight (Used) 0.07 Used Fresh

98

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 47

86 0.02 84 82

0.01 0

100

200

300

400

500

Temperature (°C)

Figure 10: TGA analysis of the fresh and used 0.7%Pt-0.3%Sn/SiO2 catalyst. Thermal analysis confirmed the presence of deposited organics on the used catalyst however more information was required to understand the effects of this deposition on the catalyst in terms of its surface characteristics and activity when reused. Textural analysis investigated the


Page 27 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


influence of potential organics on the surface area and porosity. The surface analysis indicated that the repeated cycles had a significant impact on all aspects of the catalyst surface characterisation. This influence was seen in the comparison of the nitrogen adsorption/desorption isotherms as shown in Figure S-3. The isotherms highlighted a significant decrease in the volume of nitrogen adsorbed onto the surface of the used catalyst indicating a reduction in surface area. This is confirmed by the surface area and pore volume data shown in Table 5. The reduction in surface area and pore volume is possibly due to adsorption of organics on the catalyst during use, with further analysis of the catalyst required for confirmation. Table 5: Surface area porosity analysis for fresh and usedcatalysts. Surface Area

Pore Volume

Pore Radius (r)

m2 g-1

cc g-1

Å

Fresh

640

1.2

43.3

Used

440

0.8

37.2

Sample

The catalyst also underwent FTIR analysis with the results presented in Figure S-4 showing the 4000-800 cm-1 region which highlights peaks originating from the silica support. Additional peaks visible on the spent catalyst in the 1800–1300 cm-1 region are detailed in Figure 11 and compared to a fresh catalyst.



Fresh 1508 1543 1525

1458

1627

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 47

Used 1394

1503 1568

1367

1337

1462

1663 1800

1700

1600

1500

1400

1300

Wavenumber (cm-1) Figure 11: FTIR spectra of the fresh and used (3 cycles) 0.7%Pt-0.3%Sn/SiO2 catalyst. A small number of peaks present on the fresh catalyst are due to residuals from the synthesis process with the additional peaks present on the spent catalyst due to deposition of organics during the reaction sourced from the reactant, the solvent or products.26-27 The peak assignment is outlined in Table 6 however given the similarities in the structures of these molecules the source of some peaks is ambiguous.


Page 29 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6: Identification of FTIR peaks on the used catalyst Wavelength (cm-1)

Functional Group

Source

1337

Twisting of C-H2

Toluene

1367

Rocking of C-H2

Furfural / Toluene

1394

Stretching of C-C

Furfural

1462

Bending of C-H2

Furfural / Toluene Furfural / Furfuryl Alcohol

1503

Stretching of aromatic C=C / Toluene

1568

Stretching of C=C

Furfural / Furfuryl Alcohol

1663

Stretching of C-H=O group

Furfural

Soxhlet extraction and ultrasonic extraction were investigated as regeneration techniques for the Pt-Sn/SiO2 catalyst. The results of the applied regeneration techniques were compared to the catalyst which was reused with the results shown in Figure 12. A fresh catalyst was used for each test and following the reaction the catalyst was filtered and dried. No additional treatment was applied to the catalyst which was reused. Sonication was carried out for 12 hours while soxhlet extraction was conducted using toluene for 24 hours.



70

Cycle 1 Cycle 2

60

Furfural Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50 40 30 20 10 0 Soxhlet Extraction

Sonication

Reuse

Figure 12: Furfural conversion after 300 minutes for fresh, used and regenerated catalysts. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (toluene), #$ % =20 bar, T=100°C.) The furfural conversion observed from this comparison indicated that catalyst activity did not vary significantly, with only a small loss in activity observed for both regeneration techniques investigated. It was noted that the reduction in activity for the reused catalyst is similar to that of the regenerated catalysts which indicates that regeneration had no effect on the catalyst. A significant factor in the decrease in conversion for the reused catalyst was the handling of the catalyst during the filtration process. A small quantity of catalyst was lost due to the filtration process which is likely to contribute to the loss in activity in cycle two. Overall, the presence of organics does not appear to inhibit the catalyst activity. This may indicate that the organics are not present on the catalyst surface during hydrogenation or they may not be deposited on the


Page 30 of 47

Page 31 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


active sites. Therefore, this catalyst is suited to industrial application as it displays satisfactory reuse properties for the desired reaction. Solvent Effect With a view to increasing furfural conversion, a variety of different solvents were tested. Solvents which are frequently used in published literature for the hydrogenation of furfural to furfuryl alcohol include toluene, 1-propanol, 2-propanol and ethanol. In this study, protic and aprotic solvents were investigated. The alcohol functional group and its location within the solvent molecule were investigated with protic solvents 1-propanol and 2-propanol while propanone was selected as an aprotic solvent for comparison to the previously used toluene. Although ethanol is frequently selected in literature it was not used to test the catalyst in this work as it was shown previously to favour the production of additional products.28 The ability of the solvent to act as a hydrogen donor was also considered with the structure and polarity of the selected solvents outlined in Table 7.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 7: Solvents investigated in this work Solvent

Structure

Polarity

Toluene

Nonpolar Aprotic

1-propanol

Polar Protic

2-propanol

Polar Protic

Propanone

Polar Aprotic

The influence of the solvent on furfural conversion is shown in Figure 13. The catalyst activity varied with the solvent employed although it was noted that overall, higher conversion was observed when a protic solvent was used. At all times, 2-propanol showed better conversion in comparison to the other solvents studied.


Page 32 of 47

Page 33 of 47

90 80 70

FF Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Toluene 1-Propanol 2-Propanol Propanone

60 50 40 30 20 10 0 60

120

180

240

300

Time (min) Figure 13: The influence of solvent on furfural conversion with hydrogenation time. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent, #$ % =20 bar, T=100°C.) The furfural conversion achieved using the two protic solvents differed as did the conversion observed using the two aprotic solvents. In the case of the protic solvents, this difference is due to the use of a primary and secondary alcohol as secondary alcohols generally give higher activity than primary alcohols. This is due to the enhanced stabilising effect of the two alkyl groups in secondary alcohols via inductive electron donation in the dehydrogenation of the alcohol.29 Additionally, the properties of the selected solvents such as the polarity may also influence the solvent-reactant interactions. In this work, a nonpolar aprotic and polar aprotic solvent were selected. It has been reported previously in literature, from a study of furfural conversion to 2-methyl furan, that a lower solvent dielectric constant (ϵ) leads to higher activity.



30

This work is in line with these findings as furfural conversion was higher in toluene (2.38)

than in propanone (20.7). Significant differences were observed for the selectivity to furfuryl alcohol between the different solvents with the results presented in Figure 14. The selectivity is clearly influenced by the solvent chemistry with toluene and propanone achieving selectivity to furfuryl alcohol of close to 100% while the use of an alcohol solvent gave lower selectivity. Furfuryl alcohol selectivity was lower for 1-propanol as the molecule is less stable than 2-propanol facilitating additional sidereactions.29

100

90

FA Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 47

80

70

60


50

40 60

120

180

240

300

Time (min) Figure 14: The influence of solvent on the selectivity to furfuryl alcohol. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent, #$ % =20 bar, T=100°C.)


Page 35 of 47

From the data presented in Figure 13 it is evident that higher furfural conversion is achieved using a protic solvent. Protic solvents, 1-propanol and 2-propanol, readily act as hydrogen donors facilitating transfer hydrogenation which contributes to furfural conversion in addition to direct H2-hydrogenation.31-32 However, low selectivity to the desired product furfuryl alcohol is observed when an alcohol is employed as the solvent, due to the production of additional products.

80 70 60

FA Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



50 40 30 20 10 60

120

180

240

300

Time (min) Figure 15: The influence of solvent on the yield of furfuryl alcohol from furfural. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent,#$ % =20 bar, T=100°C.) The influence of the solvent on the yield of furfuryl alcohol is presented in Figure 15. The use of an alcohol solvent increases the conversion and subsequently the quantity of furfuryl alcohol produced. However, several additional products were formed when using an alcohol as solvent and these were identified by GC-MS and shown in Table 8.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 47

Table 8: The major by-products formed from the use of alcohol solvents

Structure

Name and empirical Formed with: formula

2-methyl furan C5H6O

1-Propanol

2-isopropoxy methyl furan C8H12O2

1-Propanol

2-furaldehyde dipropyl acetal C11H18O3

1-Propanol

furfuryl alcohol C5H6O2

1-Propanol 2-Propanol

difurfuryl ether C10H10O3

2-Propanol

2-furaldehyde diethyl acetal C9H14O3

2-Propanol


Page 37 of 47

Different by-products were formed using the two protic solvents which showed selectivity between 50 and 90% for furfuryl alcohol with the desired product the only common product of these two solvents. The products observed in terms of total ion count as a function of time when 2-propanol was employed as solvent are shown in Figure 16. The results are displayed in total ion counts as difurfuryl ether is not commercially available preventing the preparation of a standard calibration graph. Furfuryl alcohol is the principal product in the reaction with similar quantities of difurfuryl ether and 2-furaldehyde diethyl acetal formed.

4.5

Difurfuryl ether 2-Furaldehyde diethyl acetal Furfuryl alcohol

4.0 3.5 3.0

Counts/I.S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 2.0 1.5 1.0 0.5 0.0 60

120

180

240

300

Time (min) Figure 16: Products formed with time using 2-propanol as solvent. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (2-propanol), #$ % =20 bar, T=100°C.)



The products of the reaction with 1-propanol as solvent are displayed in Figure 17. The use of 1propanol as a solvent resulted in the production of more products than when using 2-propanol however furfuryl alcohol remained the principal product. 6.0 5.5 5.0 4.5 4.0

Counts/I.S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

2-methyl furan 2-isopropoxymethyl furan 2-furaldehyde dipropyl acetal Furfuryl alcohol

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 60

120

180

240

300

Time (min) Figure 17: Products formed with time using 1-propanol as solvent. (Reaction conditions: 1 g catalyst, 25 ml FF, 175 ml solvent (1-propanol), #$ % =20 bar, T=100°C.) The use of an alcohol as solvent in the presence of a metal catalyst has frequently been reported to result in reduction of the carbonyl group via the donation of hydrogen from the alcohol. Typically referred to as the Meerwein-Ponndorf-Verley (MPV) reduction reaction, it allows for the hydrogenation of an aldehyde to its corresponding alcohol and ketone. The use of ethanol as a solvent for furfural hydrogenation to furfuryl alcohol has previously been reported previously by this group and found that low furfuryl alcohol selectivity in the presence of ethanol may be


Page 39 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


due to the MPV reaction. 28 However, in this work no products of the MPV reduction reaction were observed after furfural hydrogenation. The production of difurfuryl ether and 2-furaldehyde diethyl acetal using an alcohol as solvent have been observed in previous work by this group studying the hydrogenation of furfural to furfuryl alcohol with the mechanisms for these reactions detailed in the corresponding publication.28 The formation of 2-methyl furan has been reported in literature by Panagiotopoulou et al 30 studying the influence of the solvent as a hydrogen donor on the hydrogenation of furfural to 2-methyl furan and by Lee et al 33 who focused on the hydrodeoxygenation of furfural to 2-methyl furan, while extensive work has been carried out by Sitthisa et al on the production of 2-methyl furan with a focus on the mechanism and kinetics of the reaction. 34 35 A proposed mechanism for the formation of 2-methyl furan based on the literature is shown in Scheme 1. The scheme outlines that 2-methyl furan is formed by hydrogenation and dehydration of furfuryl alcohol. Furfural (1) is hydrogenated to a furfuryl alcohol intermediate (2). Furfuryl alcohol is then hydrogenated to an intermediate (3) which is dehydrated to 2-methyl furan.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1: The proposed mechanism for 2-methyl furan production from furfuryl alcohol. The formation of 2-isopropoxymethyl furan is reported in the literature from investigations on the hydrogen donor influence on furfural hydrogenation and catalytic bio-oil upgrading.30, 36 This literature was used to develop the mechanism outlined in Scheme 2 which proposes the formation of 2-isopropoxymethyl furan from furfural when 1-propanol was used as solvent. The scheme highlights that the etherification of furfuryl alcohol pathway begins with the protonation of furfuryl alcohol (1), where it is proposed that the H+ ion is available from the alcohol solvent. This step results in a protonated intermediate (2) that in turn reacts with a 1propanol molecule to form 2-isopropoxy methyl furan (3).

Scheme 2: The proposed mechanism for 2-isopropoxy methyl furan formation from furfuryl alcohol.


Page 40 of 47

Page 41 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The production of 2-furaldehyde dipropyl acetal has been reported in the literature from studies of catalyst influence on furfural acetalisation and hydrogenation. 37-38 In the presence of 1propanol, the additional product which formed in the highest quantity was 2-furaldehyde dipropyl acetal and a mechanism adapted from literature for its formation is outlined in Scheme 3. In this scheme, the carbonyl group of furfural (1) is protonated to form an intermediate (2). The solvent, 1-propanol, reacts with this intermediate forming the hemiacetal (3). A proton is removed from the hemiacetal which subsequently re-protonates and dehydrates to form another intermediate (4). This intermediate then reacts with a second 1-propanol molecule giving an intermediate (5) which is deprotonated to form 2-furaldehyde dipropyl acetal (6). The quantity of this product was seen to decrease over the course of the reaction as it is a reversible reaction and was hydrogenated back to furfural.

Scheme 3: The proposed mechanism of 2-furaldehyde dipropyl acetal from furfural. Overall, the use of an alcohol solvent increased furfural conversion but resulted in the production of several undesirable side-products. Furfuryl alcohol was the principle product for all solvents used but selectivity was close to 100% using toluene and propanone. From an industrial perspective, the use of an aprotic solvent results in lower furfuryl alcohol yield however, it was



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

shown that with additional time the yield of furfuryl alcohol is comparable to the yield obtained using a protic solvent. Therefore, the use of an aprotic solvent removes the requirement for product separation and subsequently reduces the complexity and cost of the process at an industrial level. Conclusions Calcination was identified as necessary for increased catalyst activity. The calcination temperature was also important with 450°C found to be the optimum. Reaction temperatures in the range of 25–150°C showed that catalyst activity rose with increased temperature and hydrogen pressure, while the reactant showed complete conversion with reduced furfural concentration. Regenerative techniques were not required as catalyst activity was the same after regeneration and after direct reuse of the catalyst. The influence of a selection of solvents found that a protic solvent yielded higher conversion but selectivity to the desired product furfuryl alcohol was compromised with the use of an alcohol solvent. Several undesirable side-products were formed in the presence of an alcohol catalyst. These include 2-furaldehyde diethyl acetal and difurfuryl ether using 2-propanol with 2-methyl furan, 2-isopropoxymethyl furan and 2-furaldehyde dipropyl acetal formed using 1-propanol as solvent. Overall, the synthesised 0.7%Pt-0.3%Sn/SiO2 catalyst showed suitability for application as an industrial catalyst. In particular, the high selectivity to furfuryl alcohol displayed using an aprotic solvent, the ease of reuse properties coupled with the straightforward synthesis method


Page 42 of 47

Page 43 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicate that this catalyst is a suitable potential for replacement of the toxic copper chromite catalyst currently used. Acknowledgements The Earth and Natural Sciences Doctoral Studies Programme is funded under the Programme for Research in Third-Level Institutions and co-funded under the European Regional Development Fund. The authors would like to acknowledge the assistance of the Bernal Institute, University of Limerick for their assistance with the XPS and TEM analysis. Supporting Information Graphical representations of the influence of hydrogen pressure and reactant concentration on furfural conversion. Nitrogen adsorption/desorption isotherms and FTIR spectra from 4000-800 cm-1 of the fresh and used catalyst.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

References 1. Council Directive (EC) 2009/28/EC of 23 April 2009 on promotion of the use of energy from renewable sources; 2009. 2. EuropeanUnion Sustainable Process Industry (SPIRE). http://ec.europa.eu/research/industrial_technologies/sustainable-process-industry_en.html (accessed 09 Feb 2016). 3. Hoydonckx, H. E.; Van Rhijn, W. M.; Van Rhijn, W.; De Vos, D. E.; Jacobs, P. A., Furfural and Derivatives. In Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA: 2000. 4. Chen, X. F.; Li, H. X.; Luo, H. S.; Qiao, M. H., Liquid phase hydrogenation of furfural to furfuryl alcohol over Mo-doped Co-B amorphous alloy catalysts. Appl. Catal., A 2002, 233 (12), 13-20. 5. Bourne, R. A.; Poliakoff, M., Green chemistry: what is the way forward? Mendeleev Commun. 2011, 21 (5), 235-238. 6. Rylander, P., Catalytic Hydrogenation over Platinum Metals. Academic Press Inc.: New York, 1967. 7. Webster, G. R. B., Soxhlet and Ultrasonic Extraction of Organics in Solids. In Encyclopedia of Analytical Chemistry, John Wiley & Sons, Ltd: 2006. 8. Bendicho, C.; De La Calle, I.; Pena, F.; Costas, M.; Cabaleiro, N.; Lavilla, I., Ultrasoundassisted pretreatment of solid samples in the context of green analytical chemistry. TrAC, Trends Anal. Chem. 2012, 31, 50-60. 9. Banjoo, D. R.; Nelson, P. K., Improved ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons in sediments. J. Chromatogr. A 2005, 1066 (1–2), 9-18. 10. Al-Fatesh, A. S. A.; Fakeeha, A. H., Effects of calcination and activation temperature on dry reforming catalysts. Journal of Saudi Chemical Society 2012, 16 (1), 55-61. 11. Sen, F.; Karatas, Y.; Gulcan, M.; Zahmakiran, M., Amylamine stabilized platinum(0) nanoparticles: active and reusable nanocatalyst in the room temperature dehydrogenation of dimethylamine-borane. Rsc Advances 2014, 4 (4), 1526-1531. 12. Ramallo-Lopez, J. M.; Santori, G. F.; Giovanetti, L.; Casella, M. L.; Ferretti, O. A.; Requejo, F. G., XPS and XAFS Pt L-2,L-3-edge studies of dispersed metallic Pt and PtSn clusters on SiO2 obtained by organometallic synthesis: Structural and electronic characteristics. J. Phys. Chem. B 2003, 107 (41), 11441-11451. 13. Xu, G. Z.; Zhang, J.; Wang, S. P.; Zhao, Y. J.; Ma, X. B., A well fabricated PtSn/SiO2 catalyst with enhanced synergy between Pt and Sn for acetic acid hydrogenation to ethanol. Rsc Advances 2016, 6 (56), 51005-51013. 14. Torres, G. C.; Ledesma, S. D.; Jablonski, E. L.; de Miguel, S. R.; Scelza, O. A., Hydrogenation of carvone on Pt–Sn/Al2O3 catalysts. Catal. Today 1999, 48 (1–4), 65-72. 15. He, S.; Sun, C.; Bai, Z.; Dai, X.; Wang, B., Dehydrogenation of long chain paraffins over supported Pt-Sn-K/Al2O3 catalysts: A study of the alumina support effect. Applied Catalysis A: General 2009, 356 (1), 88-98. 16. Bertero, N. M.; Trasarti, A. F.; Moraweck, B.; Borgna, A.; Marchi, A. J., Selective liquid-phase hydrogenation of citral over supported bimetallic Pt–Co catalysts. Applied Catalysis A: General 2009, 358 (1), 32-41.


Page 45 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Zhang, Y.; Zhou, Y.; Shi, J.; Zhou, S.; Sheng, X.; Zhang, Z.; Xiang, S., Comparative study of bimetallic Pt-Sn catalysts supported on different supports for propane dehydrogenation. J. Mol. Catal. A: Chem. 2014, 381, 138-147. 18. Zhang, Y.; Zhou, Y.; Tang, M.; Liu, X.; Duan, Y., Effect of La calcination temperature on catalytic performance of PtSnNaLa/ZSM-5 catalyst for propane dehydrogenation. Chem. Eng. J. 2012, 181–182, 530-537. 19. Bariås, O. A.; Holmen, A.; Blekkan, E. A., Propane Dehydrogenation over Supported Pt and Pt–Sn Catalysts: Catalyst Preparation, Characterization, and Activity Measurements. J. Catal. 1996, 158 (1), 1-12. 20. Sharma, R. V.; Das, U.; Sammynaiken, R.; Dalai, A. K., Liquid phase chemo-selective catalytic hydrogenation of furfural to furfuryl alcohol. Appl. Catal., A 2013, 454, 127-136. 21. Merlo, A. B.; Vetere, V.; Ramallo-Lopez, J. M.; Requejo, F. G.; Casella, M. L., Liquidphase furfural hydrogenation employing silica-supported PtSn and PtGe catalysts prepared using surface organometallic chemistry on metals techniques. Reaction Kinetics Mechanisms and Catalysis 2011, 104 (2), 467-482. 22. Tamura, M.; Tokonami, K.; Nakagawa, Y.; Tomishige, K., Rapid synthesis of unsaturated alcohols under mild conditions by highly selective hydrogenation. Chem. Commun. 2013, 49 (63), 7034-7036. 23. Srivastava, S.; Solanki, N.; Mohanty, P.; Shah, K. A.; Parikh, J. K.; Dalai, A. K., Optimization and Kinetic Studies on Hydrogenation of Furfural to Furfuryl Alcohol over SBA15 Supported Bimetallic Copper-Cobalt Catalyst. Catal. Lett. 2015, 145 (3), 816-823. 24. Fulajtárova, K.; Soták, T.; Hronec, M.; Vávra, I.; Dobročka, E.; Omastová, M., Aqueous phase hydrogenation of furfural to furfuryl alcohol over Pd–Cu catalysts. Appl. Catal., A 2015, 502 (0), 78-85. 25. Hayes, D. J.; Fitzpatrick, S.; Hayes, M. H. B.; Ross, J. R. H., The Biofine Process – Production of Levulinic Acid, Furfural, and Formic Acid from Lignocellulosic Feedstocks: Status Quo and Future Directions. In Biorefineries-Industrial Processes and Products, WileyVCH Verlag GmbH: Weinheim 2005; pp 139-164. 26. Araujo-Andrade, C.; Gómez-Zavaglia, A.; Reva, I. D.; Fausto, R., Conformers, Infrared Spectrum and UV-Induced Photochemistry of Matrix-Isolated Furfuryl Alcohol. The Journal of Physical Chemistry A 2012, 116 (9), 2352-2365. 27. Wankasi, D.; Naidoo, E. B., Furfural production from the epicrap of wild mango (irvingia species) fruits by acid catalyzed hydrolysis. American Journal of Food and Nutrition 2012, 2 (2), 47-50. 28. O'Driscoll, Á.; Leahy, J. J.; Curtin, T., The influence of metal selection on catalyst activity for the liquid phase hydrogenation of furfural to furfuryl alcohol. Catal. Today 2016. 29. Gilkey, M. J.; Xu, B., Heterogeneous Catalytic Transfer Hydrogenation as an Effective Pathway in Biomass Upgrading. ACS Catalysis 2016, 6 (3), 1420-1436. 30. Panagiotopoulou, P.; Martin, N.; Vlachos, D. G., Effect of hydrogen donor on liquid phase catalytic transfer hydrogenation of furfural over a Ru/RuO2/C catalyst. J. Mol. Catal. A: Chem. 2014, 392 (0), 223-228. 31. Wang, D.; Astruc, D., The Golden Age of Transfer Hydrogenation. Chem. Rev. 2015, 115 (13), 6621-6686. 32. Clapham, S. E.; Hadzovic, A.; Morris, R. H., Mechanisms of the H2-hydrogenation and transfer hydrogenation of polar bonds catalyzed by ruthenium hydride complexes. Coord. Chem. Rev. 2004, 248 (21–24), 2201-2237.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

33. Lee, W.-S.; Wang, Z.; Zheng, W.; Vlachos, D. G.; Bhan, A., Vapor phase hydrodeoxygenation of furfural to 2-methylfuran on molybdenum carbide catalysts. Catalysis Science & Technology 2014, 4 (8), 2340-2352. 34. Sitthisa, S.; An, W.; Resasco, D. E., Selective conversion of furfural to methylfuran over silica-supported Ni-Fe bimetallic catalysts. J. Catal. 2011, 284 (1), 90-101. 35. Sitthisa, S.; Sooknoi, T.; Ma, Y.; Balbuena, P. B.; Resasco, D. E., Kinetics and mechanism of hydrogenation of furfural on Cu/SiO(2) catalysts. J. Catal. 2011, 277 (1), 1-13. 36. Zhang, Z.; Pittman, C. U., Jr.; Sui, S.; Sun, J.; Wang, Q., Catalytic Upgrading of Bio-Oil by Reacting with Olefins and Alcohols over Solid Acids: Reaction Paths via Model Compound Studies. Energies 2013, 6 (3), 1568-1589. 37. Rubio-Caballero, J. M.; Saravanamurugan, S.; Maireles-Torres, P.; Riisager, A., Acetalization of furfural with zeolites under benign reaction conditions. Catal. Today 2014, 234 (0), 233-236. 38. Rojas, H.; Borda, G.; Rosas, D.; Martinez, J. J.; Reyes, P., Hydrogenation of furfural on Ir/Nb2O5 catalyst study kinetic. Dyna-Colombia 2008, 75 (155), 115-122.


Page 47 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic 254x190mm (96 x 96 DPI)


Hydrogenation of Furfural with a PtâSn Catalyst: The Suitability to

Recommend Documents

Hydrogenation of Furfural with a PtâSn Catalyst: The Suitability to