Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Hydrogenolysis Reactions...

12

HYDROGENOLYSIS REACTIONSUSING NANOCATALYSTS

Aziz Fihri and Vivek Polshettiwar

NANOCATALYSIS FOR GLYCEROL HYDROGENOLYSIS

Introduction

Dependence on fossil fuels to meet our energy needs has increased significantly duringthe last few decades, with exponentially increase in emission of greenhouse gas CO2.More than 33 billion tons of carbon in the form of CO2 was emitted into the atmospherein 2010. Due to rapid development worldwide, the demand for energy is increasingand there will be no significant change in the coming years in terms of the source ofthis energy. Although known reserves of fossil fuels are gradually decreasing, they willcontinue to be main source for satisfying this energy demand.1

Therefore, there are extensive efforts underway to develop nonconventional energysources such as solar, nuclear, or biofuel, which may not replace fossil fuel but canreduce the dependence on them. In this regard, attention has been paid to biofuels,where renewable biomass-related raw material is converted to valuable chemicals andfuel.2, 3 Although biofuels seem a renewable and environmentally benign alternative tofossil fuels, their sustainability is a major concern. One of the concerns is glycerol, themain by-product during biodiesel production through transesterification of vegetable oilsand animal fats, which made large quantities of glycerol due to the rapid developmentof biodiesel processing. Although glycerol has many uses in various industries, but its

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

443

444 HYDROGENOLYSIS REACTIONS USING NANOCATALYSTS

HO

OH

OH

HO OH

HO

OH

1,3-Propanediol

1,2-Propanediol

Glycerol

HO

Propane

1-Propanol

OH

2-Propanol

HO

OH

OH CH3OH CH4

C C hydrogenolysis (degradation) products

Scheme 12.1. Some major products of the hydrogenolysis of glycerol.

surplus is intensely increasing and therefore new applications need to be developed, byconverting them into valuable chemicals, which could replace petroleum derivatives.4–6

Glycerol is a saturated compound with a higher O/C content than most commoditychemicals. The catalytic hydrogenolysis of glycerol, which involves chemical bond dis-sociation and simultaneous addition of hydrogen to resulting molecular fragments, is animportant category of processes for glycerol conversion into commodity chemicals andis based on the heterogeneous catalysis.7, 8 Scheme 12.1 shows the different pathwaysand products of the C–O hydrogenolysis of glycerol. Among the products in glyc-erol hydrogenolysis, 1,3-propanediol (1,3-PD), 1,2-propanediol (1,2-PD), 1-propanol(1-PrOH), 2-propanol (2-PrOH), and ethylene glycol (EG) are industrially important.1,2-PD is used mainly in the synthesis of pharmaceuticals, polymers, liquid detergents,cosmetics, antifreeze, plastics, and transportation fuels.9 The commercial route to pro-duce 1,2-PD is by the hydration of propylene oxide derived from propylene by either thechlorohydrin process or the hydroperoxide process.10–12 However, the 1,3-PD is a veryfavorable target because of the high cost of traditional processes of 1,3-PD productionand its use in large-scale production of polyester and polyurethane resin from 1,3-PD.5,8

The development of an efficient and low-cost process to convert glycerol into 1,3-PDwill make the biodiesel process more profitable. Although glycerol hydrogenolysis hasbeen intensively studied in recent years, the main products are less valuable such as EGand propanols in most cases. These products have not been regarded as main targets inthe conversion of glycerol due to the lower market prices and poorer atom efficiency inthe production from glycerol than from 1,3- and 1,2-PD. The aim of this section of thechapter is to review the main research work published concerning the hydrogenolysis ofglycerol into value-added chemicals using nanocatalysis.

NANOCATALYSIS FOR GLYCEROL HYDROGENOLYSIS 445

Figure 12.1. TEM images of Cu/MgO catalysts.19 (Reproduced with permission, Copyright

Elsevier.)

First-Row Transition Metal Catalysts

Monometallic Catalysts. Metals such as copper (Cu), nickel (Ni), and cobalt(Co) are known to lead to good activity and selectivity in a series of reactions such assteam reforming,13,14 oxidation of CO and hydrocarbons,15 ester hydrogenolysis,16 andhydrogenation reactions.17,18 Although the hydrogenation activity of these three metalsis generally lower than that of noble metals, their lower prices and higher resistance topoisoningmake themeconomically attractive. These non-noblemetal catalysts have beenintensively studied in the hydrogenolysis of glycerol. In this context, copper has takencenter stage because of its high selectivity to 1,2-PD. Yuan and coworkers immobilizedcopper on MgO by impregnation and coprecipitation at 180◦C and 30 bar of H2 pressurewith different copper loading amounts of 10, 15, 20 wt%.19 The shape and averageparticle sizes of Cu and MgO in these catalysts were revealed by transmission electronmicroscopy (TEM) analysis (Figure 12.1). It was found that Cu/MgO catalysts wereshaped like cross-linked rods with 10–35 nm diameters, whereas the calculated copperparticle sizes were 4–23 nm. Among the catalysts tested, the Cu (15 wt%)/MgO wasfound to be more active in terms of selectivity for 1,2-PD as well as for glycerolconversion (Scheme 12.2). The addition of small amounts of NaOH in the reactionmixture promoted the dehydration step, and this increased the conversion of glycerol to82–95.8% but decreased the selectivity for 1,2-PD from 97.4 to 95.8% (Table 12.1).

In a subsequent study, the same research group prepared a more selectivecatalyst for glycerol hydrogenolysis by dispersing copper on a hydrotalcite solid(Cu0.4/Mg5.6Al2O8.6). The X-ray diffraction (XRD) and TEM measurements showedthat the average size in reduced bifunctional highly dispersed Cu–solid base was almost


Glycerol

Dehydra

tion

Hydrogenation

H21,2-PropanediolAcetol

Aci

d cata

lyst

- H2O

HO

OH

OH

HO

O

HO

OH

OH

OHO

OH

O

OH

OHH2

Alkali

1,2-Propanediol2-HydroxyacroleinGlyceraldehyde

Scheme 12.2. Reaction mechanism for conversion of glycerol to 1,2-PD.

the same (∼0.8 nm) (Figure 12.2).20 Using 8.0 ml aqueous solution, 75 wt% glycerol,1.0 g of reduced catalyst, at 30 bar pressure with 20 h reaction time at 180◦C, the conver-sion of glycerol and the selectivity for 1,2-PD reached 80.0% and 98.2%, respectively.Importantly, a total glycerol conversion was reported by doubling the reaction time,without a significant loss in 1,2-PD selectivity. In addition, a little amount of addedNaOH further increased the activity for the desired reaction without obvious cleavageof carbon–carbon while only slightly decreasing the 1,2-PD selectivity.

In another study, the same research group reported the synthesis of a CuO/SiO2catalyst by the precipitation–gel (PG) technique and its catalytic activity was comparedto that of a catalyst prepared by the conventional impregnation method (IM).21 Using4 g of reduced catalyst, 80 g aqueous solution, 80 wt% glycerol, total pressure 90 bar,12 h at 473 K, the PG catalyst showed a similar selectivity toward 1,2-PD (98%) asimpregnated catalysts. However, the PG catalyst had much higher activity and betterlong-term stability than the catalyst prepared using IM technique. The high activity ofthe PG catalyst was due to themuch smaller particle size and the higher copper dispersitywith strong copper–support interactions. Also, when the catalytic tests were performed

TABLE 12.1. Glycerol hydrogenolysis over Cu/MgO catalystsa

Selectivity (%)

Catalyst conversion (%) 1,2-PD EG

CuO-10/MgO 48.6 97.4 0.8CuO-15/MgO 72.0 97.6 1.3CuO-20/MgO 58.4 96.8 0.8CuO-15/MgOb 82.0 95.8 0.3

aReaction conditions: 75 wt% aqueous solution of glycerol, 8.0 ml, 1.0 g of reduced catalyst, H2 pressure30 bar, 180◦C, 20 h.b0.125 g NaOH was added.


(a)-1 (a)-2

0.8 nm

Figure 12.2. TEM images of the reduced Cu0.4/Mg5.6Al2O8.6 prepared by coprecipitation.20

(Reproduced with permission, Copyright Elsevier.)

in a fixed-bed flow reactor, the PG showed higher activity and remarkable stability(∼80% conversion) compared to the catalyst prepared by impregnation. No noticeabledecrease in the activity was observed for PG catalysts after 200 h; however significantloss of activity was observed over IM catalyst after only 100 h of time on-stream, thusindicating that PG method is superior than IM method.

To examine the effect of residual sodium originating from catalyst synthesis on thecatalytic activity of CuO/SiO2 in the hydrogenolysis of glycerol, the research group ofXia prepared different catalysts with various amounts of sodium by the PG method.22

The average sizes of copper particles were determined by XRD and were between4.7 nm to 14.7 nm. The obtained results showed that the conversion and selectivity ofthe catalysts in glycerol reactions generally decreased with increasing sodium content.However, the presence of small amounts of sodium was found to be highly beneficialfor the CuO/SiO2 catalyst to exhibit both high catalytic activity and good stability. Theauthors indicated that the conversion of glycerol increased with increasing pH, and ahigh conversion of 41.1% was obtained in an initial solution of pH 12.

Liu and coworkers prepared various cobalt nanostructures such as nests, nanoflowers(Figure 12.3), and nanowires (Figure 12.4) and used them as hydrogenolysis catalystsof glycerol.23–26 1,3-PD was formed as a main product with moderate selectivity (57–72%) at 493 K. These results show that first-row transition metal catalysts are veryeffective for the production of 1,2-PD from glycerol, although the mechanism is still


Figure 12.3. TEM images of the Co nanoflowers.24 (Reproduced with permission from Amer-

ican Chemical Society.)

poorly understood. The high yield of 1,2-PDmeans that these catalysts have much loweractivity to overhydrogenolysis of 1,2-PD than for glycerol hydrogenolysis.

Bimetallic Catalysts. Copper-based catalysts exhibit superior performance interms of selectivity toward propanediols, but their activities are usually low. The coppercatalyst often used in the hydrogenolysis of glycerol is initially in an oxidic state. To

(a) (b)

Figure 12.4. TEM images of the Co nanowires synthesized with sodium stearate: (a) 10 h and

(b) 40 h.25 (Reproduced with permission, Copyright Elsevier.)


achieve decent activity, the catalyst often needs to be reduced in situ under reaction con-ditions with very high H2 pressure, or to be prereduced to generate copper species thatare catalytically active under mild reaction conditions.27,28 Supported Cu-containingbimetallic catalysts were prepared by impregnation and used to convert glycerol topropanediols without any reduction pretreatment.29 On all catalysts tested, the 1,2-PDwas observed as a major product and the amounts of 1,3-PD were very small. How-ever, the activities of these catalysts differed significantly. CuAg/Al2O3 was the mostactive, with a glycerol conversion of 27%, three to six times that of the CuZn/Al2O3 andCuCr/Al2O3 catalysts. Other bimetallic catalysts have been tested such as CuNi/Al2O3and CuCo/Al2O3, but the obtained results showed poor catalytic activity. Importantly,during the hydrogenolysis of glycerol, strong effects of supports on the catalytic per-formance of supported CuAg bimetallic catalysts were observed. Among the supportstested, Al2O3 was better, whereas the more acidic supports, such as zeolites HZSM-5,HY, and H�—did not generate catalysts with high activity at the same loading of Cuand Ag. In addition, the CuAg/Al2O3 catalyst had much higher activity compared with acommonly used commercial copper chromite catalyst,which is not successfully commer-cialized due to the toxicity associated with chromium. Several researchers have focusedtheir work on Cu/ZnO catalysts. In this context, Mane and coworkers reported that anon-chromium Al–Cu nano-sized catalyst showed higher catalytic activity comparedto two commercial copper chromate catalysts.30 For aqueous-phase hydrogenolysis ofglycerol, the conversion could be increased up to 76% at 513 K, while a high 1,2-PDselectivity of 89% was maintained. The highest activity of this nanocatalyst could beattributed to the higher acidic sites because the aluminium present in this catalyst can beconverted to alumina during the calcination process, which is generally responsible forhigher acidity leading to faster dehydration to form acetol. To study the reusability forglycerol hydrogenolysis reactions, this catalyst was filtered after the first reaction andwashed and then dried in oven at 383 K and regenerated under hydrogen and used forthe fresh hydrogenolysis of glycerol. The results showed significant activity even afterthe second recycle in terms of turnover frequency, which decreased from 2.96 to 2.27in the third cycle. The authors indicated that this behavior could be mainly due to thelosses of the catalyst during recovery and reuse as well as metal sintering because ofhigh temperature and long duration of the reaction.

Cu/ZnO/MOx (MOx = Al2O3, TiO2, and ZrO2) catalysts were prepared by a con-tinuous coprecipitation method and tested in the gas-phase hydrogenolysis of glycerol.31

The particle sizes of metallic copper ranged from 11.5 to 22.7 nm, indicating that themetallic copper in the reduced catalysts was at a nano-sized magnitude. The reducedcatalysts were in the order of Cu/ZnO/TiO2 �Cu/ZnO/ZrO2 �Cu/ZnO/Al2O3 in termsof the size of the copper nanoparticles (NPs). The large surface area of the Cu/ZnO/TiO2catalysts favored the dispersion of the reduced metallic copper, yielding the formationof small-sized metallic copper. The supports provided the acid sites for dehydration ofglycerol, and metallic copper was the active site for hydrogenation reactions. The acidi-ties of the Cu/ZnO/MOx catalysts were in an order of Cu/ZnO/Al2O3 �Cu/ZnO/ZrO2 �Cu/ZnO/TiO2 �Cu/ZnO. The type of metal oxide as the support affected the acidity andthe dispersion of the metallic copper, subsequently affecting the dehydration and hydro-genation activity. At high reaction temperatures, the Cu/ZnO/TiO2 catalyst with weak


HO

OH

OH

glycerol

CH3 C

O

CH2OHH2

Cu0CH3 CH

OH

CH2OH CH3CHCH3

OH

+ CH4CH3 CH2OH

CH3 CH2 CH2OH

- H2O

H+CH2 CH2 CHO

OH

H2

Cu0

CH3OH + CH2 CH2

OHOHH2

Cu0CH3 CH2OH

CH2 CH2 CH2

OH OHH2

Cu0CH3 CH2 CH2OH

CH3 CH2OH + CH4

H2

Cu0

H2

Cu0

- H2OH+

H2

Cu0

H2Cu0

H2

Cu0

Scheme 12.3. Possible reaction routes of glycerol hydrogenolysis on Cu/ZnO/MOx catalysts.

and medium acid sites favored the formation of 1,3-PD and EG with maximum selec-tivities of 10% and 37%, respectively, whereas the Cu/ZnO/Al2O3 and Cu/ZnO/ZrO2catalysts exhibited good selectivity toward hydroxyacetone ranging from 81 to 40% atreaction temperatures of 240–300◦C, which were higher than those for Cu/ZnO andCu/ZnO/TiO2 catalysts. The possible reaction routes of glycerol hydrogenolysis onCu/ZnO/MOx catalysts are shown in Scheme 12.3.

Noble Metal Catalysts

Monometallic Catalyst. Since hydrogenolysis uses hydrogen as a reactant, thecatalyst must have the ability to activate hydrogen molecules. Noble metals are wellknown to be able to activate hydrogen molecules and are widely used in hydrogenationcatalysts. Maris and Davis investigated the effect of the addition of a base to commercialcarbon-supported ruthenium (Ru) and platinum (Pt) catalysts in the hydrogenolysis ofglycerol using very diluted glycerol (1 wt% aq.) at 473 K and 40 bar H2.32 The charac-terization of Ru/C and Pt/C by H2 chemisorption has revealed that both catalysts havea similar metal dispersion (43%) and presumably a similar metal particle size (2.3 nm).The presence of 0.8 M NaOH or CaO enhanced the rate of glycerol hydrogenolysis overboth catalysts, and the extent of enhancement was greater over Pt/C than over Ru/C.Under neutral conditions, Ru/C was more active than Pt/C in the hydrogenolysis ofglycerol and the main products were 1,2-PD, EG, and lactates, while lactates or lacticacids were not formed under these conditions. The base-catalyzed dehydration of glyc-eraldehyde yields pyruvaldehyde, which is transformed into lactate in the presence of abase (Scheme 12.4).

Balaraju and coworkers prepared a series of catalysts with different rutheniumloadings supported on Lewis acidic TiO2 by both conventional IM and deposition pre-cipitation (DP) methods for selective hydrogenolysis of glycerol to propylene glycol.33

The results showed that the catalyst preparation method influenced the conversion and


O

O

+ NaOH O

OH

O-

Scheme 12.4. Formation of lactate from pyruvaldehyde.

selectivity during the hydrogenolysis process. Higher conversions were obtained whenthe catalysts were prepared by the DP method (Table 12.2). It is particularly noteworthythat the low ruthenium loading was sufficient to achieve maximum conversion due to thepresence of well-dispersed nano-sized ruthenium particles on TiO2. The large particlesize with high ruthenium content might be the reason for the decrease in activity. Theauthors disclosed that the catalyst was active even when crude glycerol and glycerolwith alkali salts such as sodium sulphate and other impurities were used.

Yuan and coworkers investigated the hydrogenolysis of glycerol in alkaline aque-ous solutions using platinum catalysts on various supports.34 Platinum on hydrotalciteshowed a conversion of 92% and a 1,2-PD selectivity of 93%, and it was to be superior toMgO�Al2O3 �H� ∼ HZSM-5. The XRD, TEM, and CO2-temperature programmeddesorption (TPD) characterizations concluded that the basic properties of the catalystcontributed obviously to its activity for glycerol hydrogenolysis, while smaller sized Ptparticles on the base support were more active. This study is interesting from a practicalpoint of view, as raw glycerol solutions derived from biodiesel processes are alkaline,due to its content in transesterification catalyst.

Bimetallic Catalysts. Bimetallic catalysts have been used to alter the activity andselectivity of various reactions.35 Bimetallic PtRu and AuRu catalysts were prepared bya surface redox method, and their catalytic activity was compared to their monometallicanalogues for use in the aqueous-phase hydrogenolysis of glycerol.36 TEM analysisrevealed that the average particle size of PtRu/C and AuRu/C was 3.3 and 2.3 nm,respectively (Figure 12.5). Both bimetallic catalysts favored the formation of EG over1,2-PD during glycerol hydrogenolysis in neutral pH conditions. On the contrary, thebimetallic catalysts favored the production of lactate and propylene glycol over EG inthe presence of base. The product selectivities over the bimetallic catalysts in both highpH and neutral conditions were nearly similar to those observed over the monometallicRu catalyst.

TABLE 12.2. Glycerol hydrogenolysis activity over 5 wt% Ru/TiO2 catalystsa

Selectivity (%)

Catalyst conversion (%) 1,2-PD EG Acetol Others

Ru/TiO2 (DP) 44 58 17 4 21Ru/TiO2 (IM) 31 59 24 2 15

aReaction conditions: glycerol concentration 20 wt%, 60 bar H2, 8 h, temperature 180◦C, catalyst wt. 6%.


(a)

20nm 20nm

(b)

Figure 12.5. TEM micrographs for (a) PtRu/C and (b) AuRu/C.36 (Reproduced with permission,

Copyright Elsevier.)

Daniel and coworkers prepared bimetallic Pt–Re (rhenium) nanoparticles supportedon Norit carbon by incipient wetness impregnation.37 XRD and TEM measurementsrevealed that the metal NPs were bimetallic and highly dispersed on the carbon support,with an average particle size of less than 2 nm. Re was mostly reduced in the bimetallicsample by H2 at 473 K in presence of platinum, presumably by spillover of dissociatedH atoms from the Pt to Re. In the hydrogenolysis of glycerol using very diluted glycerol(1 wt% aq.), the selectivity to 1,3-PD at 443 K improved from 26 to 34% (at 20%conversion of glycerol) when Pt–Re/C was sintered at high temperatures. Under thesame temperature, both monometallic Pt/C and Re/C did not exhibit any detectableactivity.

It is worth noting—even if it is out of the main scope of the present part—thata highly active and selective Ru–Cu bimetallic catalyst supported on bentonite withthe aid of ionic liquid (IL) has been reported for the hydrogenolysis of glycerol to1,2-PD.38 Before the deposition of the metal, the bentonite was treated with 1,1,3,3-tetramethylguanidinium lactate to exchange the sodium cations with IL in order tostabilize the metal nanoparticles by strong coordination. The aggregation of nanoparti-cles was prohibited by strong electrostatic interactions of IL with the negative charge inthe silicate layers of the bentonite. TEM analyses revealed that the particle size in thisRu–Cu was in the range of 5–8 nm. The catalyst was most efficient with a molar ratioof Ru-to-Cu of 3 : 1, and a total glycerol conversion with 85% yield of 1,2-PD couldbe achieved at 230◦C and 8 MPa. In addition, the copper suppressed the degradationof glycerol into EG, thereby accommodating high 1,2-PD selectivity. This nanocatalystcan be readily recovered by filtration and subsequently washed with deionized waterand gently dried under vacuum before reusing five times without a marked decrease inthe conversion of glycerol and the selectivity to 1,2-PD.


Metal Oxide-Modified Supported Noble Metal Catalysts

In most studies in the hydrogenolysis of glycerol, the main product is 1,2-PD. Toobtain more valuable 1,3-PD, different approaches are needed. In this context, a bifunc-tional system has typically been used because it is composed of acid or base sites andhydrogenation sites such as Cu, Ni, Ru, and Pt. Shinmi and coworkers attempted themodification of the rhodium catalysts, which have the highest hydrogenolysis activityamong monometallic noble metal catalysts under neutral conditions, with Re, Mo, andW.39 They compared their activity toward hydrogenolysis of glycerol in water as sol-vent. Theses catalysts were prepared by impregnatingRh/SiO2 after the drying procedurewith aqueous solutions of NH4ReO4, (NH4)6Mo7O24·4H2O, and (NH4)10W12O41.5H2O,respectively, followed by calcinations in air at 773 K. The performance of the modifiedcatalysts was compared, and the results obtained showed that the addition of these met-als to Rh/SiO2 markedly enhanced the catalytic activity. Importantly, the modificationof Rh/SiO2 with Mn, V, and Zr species, which were prepared by a similar method,decreased the activity. In contract, the modification of Rh/SiO2 with Re, Mo, and Wenhanced the activity of glycerol hydrogenolysis. Varying the amount of Re, Mo, andW showed that the catalytic activity was maximum to be at M/Rh = 0.5, 0.06, and 0.13(M= Re, Mo, andW), respectively. The addition of Re enhances the activity of glycerolhydrogenolysis and the selectivity to 1,3-PD by suppressing the side reactions suchas the carbon–carbon bond breaking and the propanediol hydrogenolysis even underlow H2 pressure and high reaction temperature. With 150 mg supported metal catalyst,20 wt% glycerol aqueous solutions, 8.0 MPa initial hydrogen pressure, and after 5 h at393 K, the Rh–ReOx/SiO2 (Re/Rh= 0.5) exhibited 22 times higher glycerol conversionand 37 times higher 1,3-PD yield than Rh/SiO2. X-ray absorption near edge structureand extended X-ray absorption fine-structure studies indicated the formation of ReOxclusters attached to the surface of Rh metal particles.40 This caused synergy betweenReOx and Rh, and the glycerol hydrogenolysis proceeded on the interface between theRh metal surface and attached ReOx species (Scheme 12.5). Importantly, the XRD andTEM measurements showed that the average size of Rh particles in Rh–ReOx/SiO2was almost the same (∼3 nm) as in Rh/SiO2 and it was maintained after the reaction(Figure 12.6).

In another work, the same research group has also studied direct hydrogenoly-sis of glycerol to 1,3-PD in aqueous medium using rhenium oxide-modified iridiumnanoparticles (NPs) supported on silica (Ir–ReOx/SiO2).41 This catalyst was preparedby impregnating Ir/SiO2 with an aqueous solution of NH4ReO4 followed by calcinationin air at 773 K for 3 h. The ratio of Re to Ir was optimized to maximize the selectivity to1,3-PD in the glycerol hydrogenolysis and was determined to be Re/Ir= 1. The averageparticle size of iridium was estimated to be 2.3 and 2.0 nm by TEM and XRD, respec-tively. The following conditions were used: 150 mg of catalysts, 31 �mol of Ir, 4 g ofglycerol, Re/Ir = 1, 16 g of water, sulfuric acid (H+/Ir = 1), and 80 bar H2 at 393 K for36 h. The selectivity to 1,3-PD at an initial stage reached 67% and gradually decreased asthe conversion proceeded. This was due to the side reactions of glycerol hydrogenolysissuch as overhydrogenolysis and 1,2-PD formation. The yield of 1,3-PD reached 38%at 81% conversion of glycerol. This value was higher than the 1,3-PD yields in any


Rh Rh

Re Re

Rh Rh

ReO O

OH2

O OH

Rh Rh

Re Re

Rh Rh

ReO

OH2

O OH

OH

H

Rh Rh

Re Re

Rh Rh

ReO

OH2

O

H

OH

OH

OH

OH

Rh Rh

Re Re

Rh RhRh

ReO

OH2

O

OHRh Rh

Re Re

Rh RhRh

ReO O

H+

H+

H2O

H+

H2O

HO OH

OH

H2

HO OH

OH

H2

HO OH

H+

Scheme 12.5. Possible mechanism of glycerol hydrogenolysis to 1,3-PD and 1,2-PD over Rh-

ReOx/SiO2.

Figure 12.6. TEM image of Rh–ReOx/SiO2 (Re/Rh = 0.5) after the catalytic use.39 (Reproduced

with permission, Copyright Elsevier.)


TABLE 12.3. The effect of different supports on glycerol hydrogenolysisa

Selectivity (%)

Catalyst conversion (%) 1,2-PDO EG EtOH 1-PrOH CH4 CO2

Ru/CNTs–IM 42.3 60.2 20.4 1.9 2.3 6.6 5.1Ru/CNTs–EG 82.9 30.0 23.0 3.3 8.1 19.8 1.5Ru/AC–IM 51.6 24.4 23.2 6.9 2.2 31.7 2.6Ru/TiO2–EG 81.7 35.2 12.7 8.3 6.7 25.3 1.5Ru/Al2O3–EG 80.8 26.7 11.5 7.1 3.6 36.1 2.9Ru/graphite–IM 16.0 53.0 18.3 4.7 9.0 5.0 1.3

aReaction conditions: catalyst amount 250 mg (Ru loading = 5 wt%); 20 wt% aqueous glycerol solution20 ml; H2 pressure 4.0 MPa; 473 K; time 12 h.

previously reported catalytic systems. Importantly, the Ir/SiO2 catalyst without ReOxmodification was inactive toward the hydrogenolysis of glycerol. The authors indicatedthat the catalyst could be reused at least three times without any change of the reactionrate and selectivities.

Metal Catalysts Supported on Carbon Nanotubes

In recent years, carbon nanotubes (CNTs) have been actively studied as new supports formetal catalysts due to their small size, high chemical stability, and large surface-area-to-volume ratio.42,43 In this context, Yuan and coworkers reported that Ru-NPs supported onmultiwalled CNTs had a higher performance in the hydrogenolysis of aqueous glycerolsolution to produce glycols of 1,2-propanediols (PD) and ethylene glycol (EG).44 Theresults indicated that the glycerol conversion depended essentially on the mean size ofRu-NPs and the particles size of around 5 nm produced highest yield of glycols. Theperformance of this catalyst prepared by IM exhibited the highest selectivity to 1,2-PD (60.2%) and moderate selectivity to EG (20.4%) compared with other rutheniumcatalysts supported on several carriers such as active carbon (AC), graphite, TiO2, andAl2O3. The Ru/CNTs prepared by reduction in the liquid phase using EG showed higherglycerol conversion of 82.9% but lower selectivity of 30.0% to 1,2-PD. The catalystswith AC, TiO2, and Al2O3 as supports led to higher conversion of glycerol, but theyfavored production of methane rather than 1,2-PD and EG, whereas the Ru supported ongraphite showed higher selectivities to 1,2-PD but its activity was much lower than theother supports. The Ru/CNTs–EG, which was prepared by reduction in the liquid phaseusing EG, showed higher glycerol conversion of 82.9% but lower selectivity of 30.0%to 1,2-PD (Table 12.3). The intrinsic property of the support may influence the catalystproperties such as the dispersion of Ru particles, and hence affect its performances. TheCNTs may not take part in the construction of the active sites, and the improvement incatalyst is more probably indirect, including enhancing the adsorption/desorption andpromoting the spillover/activation of hydrogen species.

In a very recent study, Wu and coworkers reported the synthesis of CNTs decoratedwith Cu–Ru bimetallic nanoparticles for glycerol hydrogenolysis.45 These nanoparticles


were prepared by the chemical replacement method using Cu-NPs and Ru3+ ions. TEManalyses revealed that most of the Cu–Ru particles were located on the external surfaceof the CNTs without incorporating clusters into the pores. The average particle size ofCu–Ru particles was determined to be approximately 20 nm. The resulting Cu–Ru/multi-walled carbon nanotubes (MWCNTs) catalyst showed higher 1,2-PD selectivity thanthat of the Ru/MWCNTs catalyst and better activity than the Cu/MWCNTs catalyst. Thedeposition of ruthenium on the surface of copper particles reduced its size, increasing theamount of surface-active copper to promote the hydrogen activation ability of the coppermetal. Although the Ru in this bimetallic cannot catalyze the hydrogenolysis of glycerol,it acts as a route for the spillover of hydrogen toward the major catalytic component.46

In the same context, other CNTs supported RuFe bimetallic nanoparticles wereprepared through a coimpregnation method for aqueous selective hydrogenolysis ofglycerol to 1,2-PD and EG.47 TEM images revealed that the bimetallic RuFe-NPswere uniformly dispersed on the CNT surfaces and had an average size of 3 nm. TheRuFe/CNT showed higher selectivity to 1,2-PD and a lower to methane. The selectivityto 1,2-PD increased from 45.4 to 61.4% when the atomic ratio of Ru/Fe decreased from10 : 1 to 1 : 1. Importantly, the monometallic Ru/CNT catalyst with smaller Ru-NPsshowed higher activity for C−C bond cleavage, with higher selectivity for methane buta lower for 1,2-PD. The stability of the RuFe/CNT catalyst was determined throughconsecutive recycling of the catalyst. The catalyst was separated by simple filtrationand dried at 383 K for 1 h before its use in the next reaction. The results showed thatthe conversion of glycerol remained almost constant, and the selectivity to 1,2-PD andEG changed negligibly after five runs. The authors attributed the high activity of thecatalysts to the synergistic effects of the formation of Ru–Fe alloys and the interactionsbetween bimetallic Ru/Fe-NPs and iron oxides on the CNT surfaces.

Conclusions and Outlook

The availability of crude glycerol will increase in the next years due to the remarkablegrowth in the production of biofuel globally. Although the hydrogenolysis of glyc-erol is a complex reaction, the various studies described in this chapter on selectivehydrogenolysis have enabled the efficient conversion of biomass-related materials touseful chemicals. Among the potential products, 1,2-PD is most easily obtained, espe-cially using nanocatalysts made of copper. However, despite the good progress, a numberof challenges remain. The production of 1,3-PD as major product is still a challenge,and new types of nanocatalysts are necessary to overcome this problem. Other inter-esting possibilities should also be explored such as the nanocatalytic hydrogenolysis ofglycerol to lactic acid or to fuels.

NANOCATALYSIS FOR ALKANE HYDROGENOLYSIS

Introduction

The hydrogenolysis reaction of alkanes over transition metal catalysts has been knownfor many years.48–52 These reactions have considerable industrial importance and have

NANOCATALYSIS FOR ALKANE HYDROGENOLYSIS 457

been studied extensively due to their application in the hydroprocessing of petrochem-ical feedstocks. The hydrogenolysis of alkanes involves breakage of carbon–carbonbonds with the uptake of hydrogen and the formation of carbon–hydrogen bonds. Aseach hydrogenolysis reaction involves the rupture of one carbon–carbon bond and theformation of two carbon–hydrogen bonds, it is always exothermic. It is recognized thatin alkane hydrogenolysis, the alkane molecules first dehydrogenate to become unsat-urated hydrocarbons that adsorbed on the metal surface. Then the hydrogen-deficientspecies undergo carbon–carbon bond scission, producing smaller molecules. To date,there are relatively few examples of nanocatalysts used in the hydrogenolysis of alkanes.The aim of this section is to review the main research work published concerning thehydrogenolysis of alkanes using nanocatalysis.

Alkane Hydrogenolysis over Transition Metals

The silica-supported Rh catalysts have been used for many reactions such as cin-namaldehyde hydrogenation,53 hydroformylation,54 ring opening,55 and CO2 reform-ing of methane.56 Meriaudeau and coworkers prepared rhodium supported on TiO2 byimpregnation with RhCl3 for hydrogenolysis reactions.57 TEM analyses revealed thatthe metal crystallites were present in the form of individual spherical particles witha particle size distribution in the range of 2.3–2.5 nm. The reduction of Rh/TiO2 inthe range of 473–773 K induced a strong metal–support interaction as evidenced bythe change in the catalytic properties of rhodium. For the hydrogenolysis of n-butane,that involves central carbon–carbon bond rupture to form ethane and terminal carbon–carbon bond rupture to form methane and propane. The results showed the deactivatingeffect of H2 reduction at 773 K and the hydrogenolysis activity was almost completelysuppressed. The authors indicated that a progressive reactivation was observed withprolonged exposure to oxygen at 293 K. Schepers and coworkers also prepared sim-ilar rhodium catalysts by impregnation of TiO2 with RhCl3.58 After reduction underhydrogen at 573 K, TEM measurements revealed that the average size of the metalparticles was 2 nm. Importantly, even after high-temperature reduction (1138 K), theaverage metal particle size was not increased. Using n-hexane–H2 mixture in the ratio of1 : 16 and 3% Rh/TiO2, the authors indicated that only the highest reduction temperaturedecreased the activity of the catalyst appreciably. At 574 K, the conversion of n-hexaneand the selectivity for hydrogenolysis cracking reached 60.4% and 96.3%, respectively.However, the authors did not indicate the selectivity of each product. In addition, asmall amount of by-product was obtained by isomerization and dehydrocyclization ofn-hexane. In the same work, the rhodium catalyst supported on SiO2 was prepared by thesimilar method. This nanocatalyst was reduced at 723 K under hydrogen, and the XRDmeasurements showed a particle size of 4.5 nm. During hydrogenolysis of n-hexane at723 K, this catalyst exhibited more severe self-poisoning than with the Rh/TiO2 catalyst.Due to lower overall conversions, the hydrogenolysis selectivity decreased whereas theisomerization and the dehydrocyclization increased. The hydrogenolysis of n-hexanewas reminiscent of the observations by Sachtler and Somojai.59 These authors foundthat when Au was spread over the Pt surface by alloying, the selectivity was changedand hydrogenolysis was suppressed with increased isomerization.


+ +

, ,

C2 2 C3+C1+

2 , + C1

Scheme 12.6. The different possible pathways in the hydrogenolysis of n-hexane, MCP, and

2,2,3,3-TeMB over alumina-supported Rh.

Anderson and coworkers prepared Rh/TiO2 by impregnation with RhCl3 at pH 8.60

The average size of the Rh particles was 2.0 nm. They reported an increase by a factorof 2 to 3 in the turnover frequency for n-hexane hydrogenolysis after reduction at 773 K,but the selectivity and product distribution were unaffected. Importantly, the authorsobserved that there was no evidence of a significant particle size effect with Rh/TiO2catalysts. This catalyst had a higher turnover number and at the same time a higherselectivity for hydrogenolysis and a higher selectivity than that of Rh/SiO2 preparedby same method. The authors believed that the reason for the difference between theRh/SiO2 and Rh/TiO2 catalysts was related to the differences in the morphology of theRh crystallites on the two supports. It is perhaps significant as there is some evidencethat metal particles have a tendency to wet titania surfaces.61

Alkane hydrogenolysis was found to be a very sensitive probe for the investigationof structure effects. Coq and coworkers compared different Rh/Al2O3 catalysts pre-pared from different precursors in the hydrogenolysis of n-hexane, methylcyclopentane(MCP), and 2,2,3,3-tetramethylbutane (2,2,3,3-TeMB).62 The reactions were carried outat atmospheric pressure in a microflow reactor connected to a gas chromatograph. Thereaction temperature was 493 K and the molar ratios of the reaction mixture were asfollows: H2/n-hexane= 14.0, H2/MCP= 15.5, H2/2233TeMB= 44. The different pos-sible pathways are given in Scheme 12.6. In the hydrogenolysis of n-hexane, the authorsreported that well-dispersed catalysts (Rh particle size � 1 nm) prepared from RhCl3exhibited the same selectivity patterns as larger particles prepared from Cl−-free precur-sors. This effect was attributed to the different morphology of the Rh particles. However,the effect of chlorine was stronger in the hydrogenolysis of branched alkanes, with thespecific activity decreased by four orders of magnitude. Chlorine probably stronglyinteracted with the Rh particles, resulting in changes in the metal–support interactionand morphology of the supported clusters. In a previous study on the hydrogenolysis ofcyclopentane over Rh/A12O3 catalysts, it was reported that faceted nanoparticles, whichexhibited dense planes, were more active than small particles.63

Coq and coworkers also prepared a series of rhodium nanocatalysts supported on� -alumina by ligand exchange between protons with rhodium acetylacetonate in thetoluene.64 At 493 K and using atmospheric pressure, the hydrogenolysis of n-hexane,MCP, and 2,2,3,3-TeMB was investigated as model reactions. In the hydrogenolysis

NANOCATALYSIS FOR ALKANE HYDROGENOLYSIS 459

C3H8 + H2 C2H6 + CH4

Scheme 12.7. Hydrogenolysis of propane over Rh/SiO2.

of n-hexane and MCP, the authors indicated a clear dependence of turnover frequencyon Rh particle size but only slight changes of selectivities. In contrast, a remarkablemodification of activity and selectivity was observed in the hydrogenolysis of TeMB. Inthe same work, the authors chose this reaction to investigate the effects of the precursorand support on rhodium catalysts. Others nanorhodium catalysts containing Cl− wereobtained by treatment of a metallic catalyst prepared from Rh(acac)3 by HCl solution.The effect of the addition of chlorine was remarkable and reduced the turnover frequencyby a factor of six and shifted the selectivity for isobutane from 4.4 to 8-17%. The authorsattributed this to a different morphology of the rhodium particles. The authors alsoexamined the effect of the support on hydrogenolysis. No support effect was observedwhen SiO2 or ZrO2 was used as support. For Rh/TiO2 prepared from RhCl3·3H2O,the catalytic properties were similar to those of Rh/Al2O3 and Rh/TiO2 prepared fromRh(acac)3 and reduced at 573 K and 773 K.

Du and coworkers prepared a highly dispersed Rh/SiO2 catalyst.65 TEM analysesrevealed that the particle size in their Ru/SiO2 catalyst was only between 1.08 nm and1.44 nm. Theses catalysts were tested in the hydrogenolysis of propane as a modelreaction. The reactions were conducted at atmospheric pressure using propane–N2–H2mixture in the ratio of 1 : 3 : 10 with a total flow of 140 ml/min in the temperature rangeof 400–500 K. The selectivities to methane and ethane were near to unity for all thecatalysts studied. This means that the hydrogenolysis of propane on rhodium catalystsproceeded in one pathway (Scheme 12.7). In the propane there were two adjacent carbonatoms, which could be dehydrogenated and adsorbed on the Rh catalyst. This furtherimplied that Rh existed in high dispersion in the catalysts: the Rh particles were notlarge enough to adsorb all the three carbon atoms of propane simultaneously. In thisstudy, even by increasing Rh loading, the Rh particle size only increased slightly butwas still not large enough to adsorb all the three carbon atoms of propane. The authorssuggested that there was possibly a structural sensitivity of the supported Rh catalyst forthe hydrogenolysis of propane.

The research group of Sermon also studied the hydrogenolysis of propane usingplatinum supported on SiO2.66 The reactions were in a continuous-flow Pyrex microre-actor using propane–H2 mixture in the ratio of 1 : 10 with a total flow of 130 ml/min.TEM analyses revealed that the average particle size of Pt particles in Pt–SiO2 wasfrom 1.88 to 2.00 nm. The propane hydrogenolysis on Pt/SiO2 catalysts did not showany serious deactivation over periods of 8 h below 630 K. The formation of methaneby total hydrogenolysis was much more favorable than that of ethane at all reactiontemperatures,67–69 but in this study the selectivities of methane and ethane remainednear 1.00 ± 0.02. Unfortunately, the authors did not reveal more information aboutthe initial selectivities at short reaction times. It is also possible that some catalystsinitially possessed sites capable of giving multiple hydrogenolysis at short times, lowintermediate conversions, and low temperatures, but these sites then became poisoned


(a)

(c)

(b)

Figure 12.7. TEM of Pt wire/FSM-16 (a), Rh particles/FSM-16 (b), and Pt–Rh wire/FSM-16 (c).70

(Reproduced with permission, Copyright Elsevier.)

over time of use or as the reaction temperature increased, leaving only those leading tosingle carbon–carbon bond rupture.

Fukuoka and coworkers reported the synthesis of platinum nanowires by ultraviolet-visible irradiation of H2PtCl2 impregnated in FSM-16 in the presence of water and 2-PrOH vapors.70 The TEM analysis of Pt/FSM-16 revealed that the wires were observedalong the channels of FSM-16 and the size of the wires was approximately 2.5 nm(Figure 12.7). In contrast, Pt-NPs with sizes of 2.5 nm were formed by hydrogenreduction of H2PtCl6/FSM-16 at 673 K. In the same work, the authors also preparedRh-NPs in FSM-16 by calcination of RhCl3/FSM-16 at 673 K followed by hydrogenreduction and bimetallic Pt–Rh-NPs by photoreduction of the FSM-16 coimpregnatedwith H2PtCl6 and RhCl3. By using a continuous reactor operating at 606 K, a feed ofbutane/hydrogen in a molar ratio of 1 : 9, 1 bar pressure at a high space velocity of

REPRESENTATIVE EXPERIMENTAL DETAILS 461

20,000–30,000 h−1, the Rh-NPs gave the highest activity and the hydrogenolysis com-pletely proceeded to yield 96% methane. The bimetallic nanowire/FSM-16 producedisobutane as a major product. In addition, the Pt nanowire/FSM-16 had higher activitythan Pt-NPs alone and yielded propane, ethane, and methane.

The research group of Bond has studied the hydrogenolysis of n-butane and isobu-tane using Re/Al2O3.71 After calcination in air at 763 K, theses catalysts were reducedunder hydrogen at 723 K. TEM analyses revealed that the particle size in Re/Al2O3was 0.5–1.5 nm. The reactions were conducted under atmospheric pressure using analkane–H2–N2 mixture in a ratio of 1 : 10 : 3. At 515 K, with n-butane, the selectivityof ethane and propane was 85% and 15%, respectively. The hydrogenolysis of isobu-tane reaction produced 73% of ethane and 27% of propane. As a final note, Flambardand Burch prepared the Ni/TiO2 catalysts and compared their catalytic activity withNi/SiO2.72 The average particle size was determined to be 18 nm and 13 nm by hydro-gen and carbon monoxide measurements, respectively, whereas X-ray line-broadeningindicated a particle size of only 7.5 nm. In the hydrogenolysis of n-hexane, the Ni/TiO2catalyst was slightly more active than the Ni/SiO2 catalyst with nearly identical activityduring the initiation of the reactions. Furthermore, the product distributions were alsoextremely similar. When Ni catalysts were annealed at high temperatures, the rate ofhydrogenolysis changed by three to four orders of magnitude. This was due to changes inthe surface morphology. Also, the absence of any significant difference between titaniaand silica-supported catalysts strongly indicate that the Ni surfaces were identical inboth cases.

Conclusions and Outlook

The hydrogenolysis of alkanes has received much attention to date, because themajor challenge to the process industry is the utilization of remote natural gas andthe transformations of crude oil into hydrocarbons with different carbon numbers,which are industrially important. In spite of the considerable improvements made tothe procedure of the hydrogenolysis of alkanes by using new catalysts and reactionconditions, it is rather surprising that the practical applications of these procedureshave been so limited. The deactivation of catalysts and the use of high temperaturesand pressures remain a scientific challenge and an aspect of economical and ecologicalrelevance. Although there have been many advances dealing with using nanocatalystsin the recent years, there is still a long way to go before the ideal procedure is achieved.With all these challenges still present and with the growing interest in the nanocatalysis,it is certain that the hydrogenolysis reaction will still continue to be a fast-moving topicfor the next several years.

REPRESENTATIVE EXPERIMENTAL DETAILS

Biodiesel-Derived Glycerol Hydrogenolysis to 1,2-PD on Cu/MgOCatalysts19

A calculated amount of CuCl2·2H2O and MgCl2·6H2O was dissolved in distilled waterunder stirring, and then an aqueous Na2CO3 with a concentration of 1.0 mol/l was added


at a rate of one drip per second under vigorous stirring at room temperature until thepH of the mixed solution reached 10.5. After that, the precipitate was aged at roomtemperature for 12 h followed by filtration and thoroughly washed with deionized wateruntil the total elimination of the chloride in the filtrate. This obtained solid was driedin ambient air at 110◦C overnight and calcined at 550◦C for 4 h. After the reductionof CuO/MgO catalysts under H2 at 350◦C in a stainless steel autoclave, an aqueoussolution of glycerol (75 wt%, 8.0 ml) was added. The autoclave was purged with H2 andpressured to 3.0 MPa, put in an oil bath preheated to the required temperature. Afterthe reaction, the reactor was cooled to room temperature; the vapor phase was collectedand analyzed using a gas chromatograph whereas the liquid phase was centrifuged toremove the solid catalyst powder and analyzed using a gas chromatograph.

Hydrogenolysis of Glycerol over Hydrotalcite Cu0.4/Mg5.6Al2O8.6Catalysts20

The hydrotalcite Cu0.4/Mg5.6Al2O8.6 was prepared by coprecipitation. Required amountsof Cu(NO3)2·3H2O, Al(NO3)3·9H2O, and Mg(NO3)2·6H2O were dissolved in 400 mldistilled water under stirring (Al3+/(Cu2+ + Mg2+ + Al3+) = 0.25 and (Cu2+ +Mg2+ +Al3+) = 0.2 mol/l). An aqueous solution of NaOH and Na2CO3, in whichthe concentration of Na2CO3 and NaOH was 0.25 mol/l and 0.8 mol/l, respectively,was added slowly under vigorous stirring at room temperature. After filtration andwashing with distilled water, the resulting suspension was then dried at 80◦C for 12 hfollowed by calcination at 300◦C for 4 h. After reduction of 1 g of catalysts under H2at 300◦C for 1 h, an aqueous solution of glycerol (8.0 ml, 75 wt%) was added to thereactor and then was purged and filled with H2 to 3.0 MPa and placed in an oil bathat 180◦C. The reaction mixture was cooled to room temperature and the vapor phasewas collected by a gas bag and analyzed using gas chromatograph, whereas the liquidphase was centrifuged to remove the solid catalyst powder and analyzed using the gaschromatograph. All products detected in the liquid phase and gas phases were quantifiedvia external calibration methods.

Hydrogenolysis of Glycerol over Titania-Supported Ruthenium33

Titania-supported ruthenium catalysts were prepared by conventional IM and DP meth-ods using aqueous solutions of RuCl3·nH2O. In the IM, a calculated amount of aqueousmetal precursor solution was added to TiO2 and excess water was evaporated by ovendrying at 120◦C for 12 h. In the DP method, the support was suspended in the aqueoussolution of RuCl3·nH2O; Ru(OH)3 was precipitated on the support by the slow additionof the Na2CO3 solution until the pH of the solution reached a value of 10.5. Afterfiltration, the resulting solid was washed several times with deionized water to removeall chloride and dried at 120◦C for 12 h. An aqueous solution of glycerol at 20 wt% andreduced catalysts was added to the autoclave. The reactor was purged repeatedly withhydrogen to remove air and then the reactor was heated to the reaction temperature of180 ◦C and pressurized to 60 bar. After 8 h, the reactor was cooled to room temper-ature; the liquid products were separated from the catalyst by filtration and analyzed

REFERENCES 463

by gas chromatography using a flame ionization detector. The gas-phase products wereanalyzed using a gas chromatograph equipped with a thermal conductivity detector.

Hydrogenolysis of Glycerol over Cu–Ru/CNT45

In a typical experiment, the freshly prepared Cu/MWCNTs were added to 0.01 mol l−1

RuCl3 solution under flowing argon gas (40 cm−3 g−1 m). After replacement, the samplewaswashed thoroughly until noCl−was detected in the filtrate. The collected samplewasdried under argon flow at 80 ◦C for 12 h and then treated in a 20% H2/Ar flow at 300◦Cfor 1 h. The glycerol hydrogenolysis reaction was carried out in a 100 ml stainless steelautoclave at a stirring speed of 800 rpm. 16 g glycerol diluted with 4 mL deionized water(80 wt.% glycerol in the aqueous solution) and 0.8 g catalyst was added to the autoclave.The reactor was purged with hydrogen to remove air. Then, the reaction mixture washeated to reaction temperature of 200◦C and pressurized to 4.0 MPa. After 6 h, thereactor was cooled to room temperature. The liquid-phase products were analyzed bya gas chromatograph equipped with a capillary column and a flame ionization detector.The gas products were detected by using a thermal conductivity detector.

Hydrogenolysis of n-Hexane over Titania andSilica-Supported Nickel72

The catalysts were prepared by the wet impregnation method, with the nickel depositedonto the surface of the support using a solution of nickel nitrate by rotary evaporationat 340 K. The impregnates were dried in air at 390 K for 16 h, and stored in a vacuum.The catalysts were activated in situ by temperature programmed heating over a period of1.5 h to a temperature of 723 K, in flow of hydrogen, and holding at this temperature for1 h. After activation, the catalysts were cooled down in flowing hydrogen to the requiredreaction temperature. The hydrogenolysis of n-hexane was carried out under dynamicconditions at 548 K in a glass microreactor, with n-hexane injected into the hydrogencarrier gas by means of a motor-driven syringe. Gas samples were extracted with a gassyringe from the outlet of the reactor and analyzed using a gas chromatograph fittedwith a flame ionization detector.

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