ZnO-Based Catalysts: Influence of

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Hydrogenolysis of Glycerol over Cu/ZnO-Based Catalysts: Influence of Transport Phenomena Using the Madon−Boudart Criterion Dominique Jean,† Bendaoud Nohair,† Jean-Yves Bergeron,‡ and Serge Kaliaguine*,† †

Département de Génie Chimique, Université Laval, Québec City, Québec G1V 0A6 Canada Oleotek, Thetford Mines, Québec G6G 0A5 Canada

‡

ABSTRACT: Batch hydrogenolysis of concentrated glycerol has been conducted over different Cu/ZnO-based catalysts prepared by the coprecipitation method. The catalysts were characterized by X-ray diffraction, H2 temperature-programmed reduction, and the N2O titration technique of measurement of the metallic Cu surface area. Results show that the reaction system is affected by hydrogen pressure, temperature, glycerol concentration, and the surface area of metallic copper. Our results tend to show that the reaction scheme is more complicated than the commonly accepted dehydration−hydrogenation mechanism. Tests conducted with varying hydrogen pressure indicate that the mechanism may begin with the dehydrogenation of glycerol to glyceraldehyde. Tests conducted with varying water content tend to show that high water content favors ethylene glycol (EG) formation. The selectivity to 1,2-propanediol (12PG) versus ethylene glycol is a function of the relative reaction rates where the glyceraldehyde can react to either yield 12PG or go through a retro-Claisen route to yield EG. Finally, multiple catalytic tests conducted with a constant amount of copper surface area show that, according to the Madon−Boudart criterion, the catalytic system is heavily hampered by transport phenomena limitations.

1. INTRODUCTION Recent advances in biofuel technology and dwindling fossil fuel resources have led to an increase in biodiesel production as the worldwide demand for fuel has not shown signs of decrease. For example, worldwide car production was 39.8 million cars in 1999, and the 2012 data show an increase to 59.9 million cars. Biodiesel is usually produced from the transesterification of vegetable oil, and such a process yields about 10 w/w% of raw glycerol.1−4 The market, being flooded with a large quantity of glycerol, reacted in such a way that the price of glycerol plummeted. A glycerol market analysis conducted by ABG for the U.S. Soybean Export Council Inc. in 2007 projects that by 2020 biodiesel production will contribute up to 13 million tons of crude glycerol.6 However, glycerol can be transformed into a wide variety of high-value products such as acrolein, 1,2propanediol (12PG or propylene glycol), and 1,3-propanediol (13PG), among others. Propylene glycol has many uses as a commodity fluid and finds applications in pharmaceuticals, antifreezes, food, and agricultural adjuvants.1,3,5 Its use as an antifreeze is of particular interest as it is much less hazardous to human health than the currently used ethylene glycol. Propylene glycol is usually produced by the hydration of propylene oxide or chlorohydrins.3 However, propylene oxide comes from petroleum and as such, its price is increasing. One alternate route, the hydrogenolysis of glycerol to obtain 12PG, has been suggested to proceed via a two-step process: the first one involves the dehydration of glycerol to acetol, which is hydrogenated in the second step to yield propylene glycol.4,7−9 This green process is however not widespread in the industry as many challenges are yet to be overcome, namely, high hydrogen pressures, high temperatures, and generally low conversions and selectivities. The process must meet both environmental and economic criteria,2 and the economic criterion can be met through a better understanding of the © XXXX American Chemical Society

hydrogenolysis reaction of glycerol to propylene glycol. Once the experimental conditions are understood, work can then proceed to meet the economic aspect of the process. Numerous groups have studied different catalytic systems. Noble-metal-based catalysts such as Pt, Pd, Ru, Ir, and Re10−24 and supported Ni-,25−27 Co-,28,29 and Cu-based catalysts9,30−49 received much attention. Despite their high activity, noble metals and Ni readily cause the cleavage of C−C bonds, thus reducing the yield of the reaction toward 12PG. Moreover, noble metals are expensive making them less desirable from an economic perspective. Cobalt-based catalysts have relatively low activity and little work has been done on these. On the other hand, Cu-based catalysts offer excellent selectivity toward 12PG because of their inherent property to selectively cleave the C−O bonds,35,39,42,43,50 making them inherently superior to noble metals for glycerol hydrogenolysis. However, their lower activity and tendency for sintering44,51 mean longer reaction times are necessary to achieve targeted conversions while preserving the catalyst’s integrity, and much work has to be done to make them attractive from an industrial standpoint. Copper-based catalysts have been extensively studied on various supports such as SiO2,35,39,40,44−46,48,49 ZnO,30,33,36,40,42 Al2O3,38,40,41,43,48 Cr2O3,9,31,34,37,40 or ZrO247 with varying performances. Dasari et al.31 have shown, using a Cu/Cr2O3 catalyst, that glycerol hydrogenolysis proceeds via a two-step reaction involving the dehydration of glycerol to hydroxyacetone or acetol and the subsequent hydrogenation of acetol to 12PG. Copper-chromite catalysts show promising Special Issue: Ganapati D. Yadav Festschrift Received: February 28, 2014 Revised: May 23, 2014 Accepted: May 27, 2014

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dx.doi.org/10.1021/ie5008773 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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ASDI Inc. The catalyst was loaded in a tubular reactor and calcined at 350 °C for 1 h under a flow of 20 cm3/min of 20% O2 in helium. The reactor was then cooled to room temperature under vacuum. A mixture of 5% H2 in Ar was then allowed inside the reactor. The flow was passed through an ethanol/solid CO2 bath to trap water and sent to a thermal conductivity detector (TCD). H2-TRP profiles were obtained by heating at 5 °C/min to 350 °C under a flow of 10 cm3/min, and the total hydrogen consumption, expressed in moles, was noted as X. N2O titration was then performed.42,54 First, the reactor was evacuated to remove the remaining H2 then heated to 60 °C. A mixture of 5% N2O in Ar was circulated at 40 cm3/ min for 30 min at 60 °C. The system was then purged under vacuum; another TPR experiment was carried out, and hydrogen consumption was noted as Y. Dissociative adsorption of nitrous oxide on surface copper denoted Cu(s):

results with good activity and stability, but there are concerns about the health and environmental issues related to chromium; thus, their application is limited. Cu/ZnO catalysts have been extensively studied as they show great activity and selectivity toward 12PG. Chaminand et al.30 compared multiple catalysts, and the best selectivities to 12PG were obtained over Cu/ZnO catalysts. Wang et al.42 prepared Cu/ZnO catalysts by a coprecipitation technique and showed the dependence of activity on the Cu/Zn ratio as well as the dependence of the final catalyst performance on the hydroxycarbonate precursors obtained during the coprecipitation step. Bienholz et al.44 obtained good performance with pure glycerol but showed that water, either used as solvent or produced by the dehydration step of the reaction, and high temperature caused heavy sintering of the Cu particles. One major problem encountered is that it is difficult to compare the different works as the reaction parameters were varied between the different groups, namely, different glycerol concentration, liquid volume and gas volume in a batch reactor, hydrogen pressure, and catalyst weight. The aim of our study is to understand the physical and chemical factors influencing the hydrogenolysis of glycerol Cu/ZnO-based catalysts under welldocumented experimental conditions. The Koros−Nowak criterion developed by Madon and Boudart52 will be used to verify whether or not the catalytic system is controlled by the surface reaction rate or by transport phenomena.

2Cu(s) + N2O(g) → Cu(s)2 Oads + N2(g)

(1)

Bulk and surface copper reduction: CuO + H 2 → Cu + H 2O,

hydrogen consumption = X (2)

Surface copper reduction only: Cu(s)2 Oads + H 2 → 2Cu(s) + H 2O, hydrogen consumption = Y

2. EXPERIMENTAL SECTION 2.1. Material. Ultrahigh-purity hydrogen (UHP 5.0) was purchased from Praxair. Glycerol ACS (99.5%) grade was purchased from Anachemia. Copper nitrate (98%), zinc nitrate (98%), and barium nitrate (98%) were purchased from SigmaAldrich. Sodium bicarbonate (99.7−100.3%) was purchased from Sigma-Aldrich and EMD. Methanol HPLC grade was purchased from Fisher Scientific. n-Butanol (HPLC grade) was purchased from Sigma-Aldrich. All chemicals were used as received without further purification. 2.2. Catalyst Preparation. Catalysts were prepared by the coprecipitation method. Appropriate amounts of metal nitrate salts were dissolved in deionized water under thorough stirring. A solution of sodium bicarbonate was prepared and heated to 70 °C under thorough mixing. The metal nitrates solution was then heated to 70 °C and then delicately added to the bicarbonate solution to prevent foaming. A precipitate was instantaneously formed. The solution was aged for 3 h at 70 °C. After being aged, the solution was filtered and the precipitate washed many times with water to remove any trace of sodium and nitrate. The removal of the nitrate ions improves the copper resistance to sintering.53 The precipitate was dried overnight at 80 °C, ground, and calcined for 6 h under a ramp of 0.5 °C/min. Catalysts will be denoted C-(x/y/z)-T where x, y, and z correspond to the composition in w/w % of CuO, ZnO, and BaCO3, respectively, and T stands for the calcination temperature. For example, C-(30/58/12)-350 means that the catalyst has a composition of 30% CuO, 58% ZnO, and 12% of BaCO3 calcined at 350 °C. 2.3. Catalyst Characterization. Catalyst composition was confirmed by atomic absorption using a PerkinElmer 110B atomic absorption spectrophotometer. Scanning electron microscopy was performed using a JEOL JSM-840-A with an operating voltage of 15 kV. Temperature-programmed reduction (TPR) profiles and N2O copper titration were obtained using a RXM-100 catalyst characterization machine by

(3)

Both hydrogen consumptions can then be used to calculate the copper dispersion D, the specific copper surface area SCu, and the average copper volume−surface diameter dv,s. Specific surface area and average volume−surface diameter are obtained under the assumption that copper particles are spherical. The average surface occupied by a copper atom is 0.0711 nm2/atom, or 1.4 × 1019 copper atoms/m2. Thus, the following equations are obtained: D = 100·2·Y /X(%)

(4)

SCu = 2·Y ·Nav /(X ·MCu ·1.4 × 1019) ≅ 1353·Y /X (mCu 2/g Cu)

dv , s = 6/(S ·ρCu ) ≅ 0.5·X /Y (nm)

(5) (6)

where Nav is Avogadro’s number, MCu atomic weight of copper, and ρCu copper density (8.92 g/cm3). A disadvantage of that method is that isolated surface copper without a neighboring surface copper atom would not be counted, which may underestimate the real values of the measured parameters. 2.4. Catalytic Test. All catalytic tests were performed in a 50 mL Microclave reactor by Autoclave Engineers. Agitation was assured by a MagnetDrive system. The amount of catalyst was varied to keep the amount of surface copper constant (wi fa,i) in all systems unless noted otherwise. Prior to reactions, the catalyst was loaded in the reactor, which was then purged with hydrogen to remove air. The catalyst was then reduced in situ at 300 °C under hydrogen flow for 2 h, after which the reactor was cooled to room temperature. Glycerol was added in the reactor under a slightly positive pressure to maintain a reductive atmosphere. Hydrogen pressure was increased to operating conditions, and once the desired pressure was reached, the hydrogen feed was closed and the reactor was B



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heated to the operating temperature. Stirring speed was set to 1500 rpm. Some catalytic tests were conducted in a custom autoclave by Autoclave Engineers provided with a hole at the bottom to which a microvalve with 1/16” tubing was connected, allowing liquid sampling from the bottom of the reactor. A frit prevents the loss of catalyst. The pressure over the liquid phase pushes down the sample drop by drop, allowing for minimal liquid sampling. A fan is used to cool the valve and sample. The operating conditions used are the same as those in the closed batch system. This sampling system has the advantage of minimizing the volume sampled, thus preventing volume variations in the catalytic system. 2.5. Chemical Analysis. The samples were first centrifuged in a safety centrifuge (Fisher Scientific) to separate the catalyst. Analysis was performed on a CP-3800 gas chromatograph (Varian Inc.) equipped with a flame ionization detector. A Stabilwax column (30 m × 0.53 mm × 1 μm) coupled with a 5 m guard column by Restek was used for separation. Methanol was used as the solvent with the hypothesis that only a very small amount of methanol is produced via hydrogenolysis. This hypothesis is supported by many researchers, who found small amounts of methanol (under 2%) in their products.15,16,20,36 Conversion is defined as the amount of glycerol consumed to the glycerol that was initially present. Selectivity is defined, on a carbon basis, as the amount of product formed compared to the amount of glycerol that reacted.

One then has to plot the following: ln(r ) = s ln(fa )

where the reaction rate r = mol/(s gcatalyst) as illustrated in Figure 1.

Figure 1. Application of the Madon−Boudart criterion.

If the slope s = 1, then there’s no transport phenomena limitations and the system is therefore limited by the surface reaction on the catalyst. A system with a slope of s = 1 indicates that it is possible to obtain reliable kinetic data. A slope s = 0.5 indicates that the reaction is severely limited by transport phenomena. A case where 0.5 < s < 1 indicates a situation where there are more or less severe transport phenomena limitations. The use of eq 10 is valid for f w, fa, or f m. However, one limit of the method is that it cannot pinpoint the type of transport phenomenon that limits the reaction in case of such limitations. It is established that for Cu/ZnO-based catalysts, surface metallic copper is the active phase,44 so f will refer to surface copper that was calculated using the N2O titration technique; thus, fa was used in the present work. A relatively short reaction time of only 2 h (see results) was chosen to make sure hydrogen availability was not limiting the reaction, and stirring speed was set to 1500 rpm to reduce to a maximum the possible mass-transfer limitations at the surface of the catalyst.

3. THEORETICAL BACKGROUND Many criteria have been developed over the years to evaluate influence of transport phenomena on apparent reaction rates. The one developed by Madon and Boudart52 is the most easily used when working with heterogeneous systems because it does not require the user to know the number of surface sites involved in the chemical reaction. The method is based on the Koros−Nowak criterion in which rate measurements are made on different catalyst samples in which the concentration of the catalytically active material has been changed. The concentration of active material can be defined as (a) the weight fraction of the active material in the catalyst per unit weight of catalyst ( f w(gactive material/ gcatalyst)), (b) the surface area of the active material per unit weight of catalyst ( fa(mactive material2/gcatalyst)), or (c) the surface number of moles of active material per unit weight of catalyst ( f m(surface molesactive material/gcatalyst)). For a batch system, experiments are carried out in such a way that the same amount of active sites, in our case ACu, is constant throughout all experiments by varying the amount of different catalysts: w2 = w1(fa1 /fa2 )

4. RESULTS AND DISCUSSION 4.1. Catalyst Characterization. The catalysts obtained by the coprecipitation method in NaHCO3 are presented in Table 1. As previously mentioned, surface metallic copper has been identified as the active phase, thus making fa the most important Table 1. Physical Surface Metallic Copper Properties for C(X/Y/Z)-T Catalystsa

(7)

where w1 and w2 are the mass of catalyst 1 and 2 in experiments 1 and 2. The surface area of copper in a reference experiment is calculated using

A Cu = wfi ai

1 2 3 4 5 6 7 8 9 10

(8)

To determine the surface copper area for the catalyst, the following transformation is necessary: ⎛m 2⎞ ⎛m 2⎞ M fa ⎜⎜ Cu ⎟⎟ = SCu⎜⎜ Cu ⎟⎟ Cu w/w% CuO ⎝ gcata ⎠ ⎝ gCu ⎠ MCuO

(10)

(9)

a

C

catalyst

D (%)

dv,s (nm)

fa (mCu2/gcata)

C-(30/64/6)-350 C-(30/64/6)-400 C-(30/64/6)-450 C-(30/64/6)-500 C-(36/58/6)-350 C-(36/58/6)-400 C-(36/58/6)-450 C-(4.8/80.2/15)-350 C-(30/58/12)-350 C-(42/52/6)-350

31 22 11 10 36 21 16 29 23 32

3.2 4.6 9.1 10.2 2.8 4.7 6.2 3.4 4.3 3.1

51 35 18 16 71 42 31 7.6 38 73

D, dispersion; dv,s, particle diameter; fa, specific copper surface area. dx.doi.org/10.1021/ie5008773 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX


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parameter to effectively control during the catalyst’s synthesis. Selected results will be presented. H2-TPR profiles show that copper reduction starts at 150 °C and no hydrogen consumption peaks are observed above 275 °C, indicating that a reduction temperature of 300 °C is enough to ensure all copper species are reduced to their metallic state. Adding barium (Figure 2) caused a shift of the hydrogen

Figure 4. Comparison of the H2-TPR profiles after calcination by O2 and N2O for the C-(36/58/6)-350 catalyst.

reduction of surface copper is happening as a first step and that the shoulder at the beginning of the main reduction peak may be due to surface reduction whereas that the rest of the main peak represents bulk reduction which may involve diffusion resistance of hydrogen inside the catalyst particle. This is consistent with the results that show that our catalysts generally have low dispersion, which is indicative of copper being trapped in the bulk. Copper surface properties are highly dependent on both the amount of copper in the catalyst and the calcination temperature. Dispersion did not exceed 36% on all catalysts (Table 1), indicating the majority of the copper atoms remain in the bulk and cannot be used as active catalytic sites. Increased calcination temperatures caused sintering of the copper particles, as can be seen on the C-(36/58/6)-T catalysts, where specific copper surface decreased from 70.7 to 31.3 mCu2/gcatalyst over a 100 °C increase in calcination temperature. At the same time, the particle average volume−surface diameter increased from 2.8 to 6.2 nm and dispersion decreased from 36.4% to 16.1%. On the C-(30/64/6)-T catalysts, specific surface area and dispersion showed a 3-fold decrease while the average particle volume−surface diameter increased 3-fold when the calcination temperature was increased from 350 to 450 °C. However, when the catalyst was calcined at 500 °C, the changes in the physical properties of the metallic surface copper changed much less than when it was calcined at 400 °C. These results are consistent with the known fact that small copper particles sinter more easily than bigger ones.55,56 This indicates that there is a range of temperature where the effect of sintering is more pronounced and that above 400 °C most of the neighboring particles are already agglomerated into larger particles and there are no more neighboring particles to sinter with. However, no tests were conducted with a faster temperature ramp. One disadvantage of the N2O titration technique is that it does not yield information about the copper trapped in the bulk; therefore, it is impossible to determine whether there is a migration of the bulk copper atoms to the surface of the catalyst where they can either sinter with the surface copper atoms or create new surface copper atoms. However, as bulk copper atoms are not accounted for by the N2O titration technique, the decrease in dispersion and metallic copper surface area is still believed to be caused by sintering of surface copper particles. X-ray diffraction (XRD) results will help understand what happens to the crystallinity of the different components in the catalyst.

Figure 2. H2-TRP profiles for catalysts having varying amounts of BaCO3 with constant Cu content.

consumption peak to higher temperatures, indicating BaCO3 acts as a barrier that prevents reduction of surface copper species. Moreover, reducing bigger copper particles is harder than reducing small copper particles.53 Figure 3 shows the H2-

Figure 3. H2-TRP profiles for C-(30/64/6)-T catalysts calcined at different temperatures.

TPR profiles for catalyst C-(30/64/6)-T calcined at different temperatures. The N2O titration results show that surface copper particles have sizes of 3.2, 4.6, 9.1, and 10.2 nm for samples calcined at increasing temperature, respectively (Table 1). There is no information about the size of copper in the bulk, but we can hypothesize that the higher the calcination temperature, the higher the copper particle size in the bulk. The H2-TPR profiles show that indeed big copper particles are more difficult to reduce than smaller ones with samples calcined at 450 and 500 °C as the hydrogen consumption peak shifts to higher temperatures and get very broad with low intensity. Interestinglly, considering the H2-TPR profiles after N2O (Figure 4) oxidation of surface copper, the shift in reduction temperature is not as pronounced as that in the bulk reduction. For a given calcination temperature (Figure 4), the onset of reduction is the same for both the initial complete reduction of all copper species and the subsequent reduction of surface copper. This is an indication that in both cases D



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When the amount of copper in the catalyst is increased at constant Ba content and for a constant calcination temperature (compare catalysts 1, 5, and 10 in Table 1), the specific surface of metallic copper increases from 51 to 73 mCu2/gcata but dispersion remains essentially constant (31, 36 and 32%), as does the average volume−surface diameter (3.2, 2.8, and 3.1 nm). On the other hand, it is difficult to discern what really affects the specific surface because whenever the copper content is changed, the amount of zinc and barium are changed accordingly. When zinc is replaced by barium, such as in C-(30/64/6)-350 and C-(30/58/12)-350, an increase in the amount of BaCO3 translates into decreased values of D and fa and an increase of dv,s, suggesting that the BaCO3 seems to affect the size of copper particles. XRD results in Figure 5 show Figure 6. XRD data for uncalcined and calcined C-(30/58/12).

hydrozyncite,42 were detected, meaning that a calcination temperature of 350 °C is enough to degrade all Cu and Zn precursors to their respective oxides. The C-(30/58/12) catalyst XRD patterns were compared before and after calcination (Figure 6). The main peak at 2θ = 13°, corresponding to aurichalcite, completely disappears once the catalyst has been calcined at 350 °C for 6 h, indicating both copper and zinc precursors were degraded to their oxide form. This reduction of 50 °C compared to the commonly used 400 °C for calcination means that copper sintering will be prevented to a maximum. This is especially important as the sintering behavior is dictated not only by temperature but also by their hydroxycarbonate precursors crystalline form as well as their “chemical memory” of all synthesis steps,54 meaning that the improvement of one step is quite important. For example, with the C-(36/58/6)-T catalysts, an increase in calcination temperature from 350 to 400 °C translates in a 15% decrease in copper dispersion and a decrease in specific copper surface area of 29 mCu2/gcatalyst. Calcination temperature was increased up to 500° for the C-(30/64/6)-T samples, and the results are presented in Figure 5. The copper surface properties follow the same trend as the other catalysts, and as observed with the N2O titration data, the crystallinity does not change much between 450 and 500 °C, indicating once again that most copper sintering occurs at temperatures around 450 °C. BaCO3 decomposes at temperatures over 900 °C, which is not compatible with the copper and zinc species as sintering of copper would be too severe. It therefore remains in this precursor form, which is less soluble than its corresponding oxide. Another important feature of the XRD data is that the peaks associated with the copper are broad and of low intensity for samples where dispersion is high, which is indicative of small particles, as measured by the N2O titration technique. However, as the calcination temperature increases, the copper peak becomes sharper and better defined, meaning the copper particles are bigger and have increased crystallinity. These results are consistent with the copper particle diameters measured by the N2O titration technique. 4.2. Catalytic Tests. Scheme 1 shows the reaction pathway proposed by Wang et al.42 As discussed below, this scheme was found to be coherent with the results obtained in this work. Preliminary product identification was obtained using gas chromatography−mass spectrometry (GC-MS; results not shown), and trace amounts of glyceraldehyde (GA) as well as pyruvic aldehyde (PA) were detected in addition to acetol, 12PG, and ethylene glycol indicating that acetol seems to be

Figure 5. XRD data of C-(30/64/6)-T catalyst calcined at different temperatures.

that BaCO3 (2θ = 23.9°, 24.32°) does not sinter with increasing calcination temperature, with no peak evolution between the different calcination temperatures, and could act as a physical barrier between neighboring copper particles preventing copper from sintering at the catalyst surface during the catalytic test. That barrier could also very well act as a barrier during the N2O titration technique, yielding underestimated copper surface area. This effect of BaCO3 on copper sintering was not the intended rationale for introducing Ba in the catalyst. The initial idea was to impart basicity to the Cu/ZnO surface, but this did not materialize as the carbonate was not decomposed in working conditions. For very low amounts of copper in the catalyst in C-(4.8/80.2/15)-350, copper dispersion and particle size are the same as those of a catalyst containing 30% CuO, but specific copper area is very low compared to that of copper rich catalysts. There could be an effect due to the coprecipitation technique itself as the microstructural properties of Cu/ZnO-based catalyst are closely related to each step of the catalyst synthesis, especially the hydroxycarbonate precursors precipitation step.54 It is nevertheless well-known that the coprecipitation method does not allow for a thorough control in the catalyst’s surface properties compared to other methods, such as impregnation. This lack of control is offset by the ease of synthesis, lack of time-consuming steps, and low price of starting materials. XRD results for the C-(30/58/12) (Figure 6) show that the calcined catalysts contain CuO (2θ = 35.5°, 38.8°), ZnO (2θ =31.72°, 34.56°, 36.24°), as well as BaCO3 (2θ = 23.9°, 24.32°) and no peaks at 2θ = 13°, corresponding to their hydroxycarbonate precursors such as aurichalcite and/or E



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Scheme 1. Proposed Reaction Pathway for the Hydrogenolysis of Glycerol over Cu/ZnO-Based Catalysts43

system, thus making it easier to apply and interpret the Madon−Boudart criterion. Lastly, reducing the water content helps improve the catalyst resistance to copper sintering as it is known that water, under high-temperature conditions, causes heavy copper sintering.44,57 Table 3 presents the results

formed via a dehydrogenation−dehydration−hydrogenation step as suggested by Wang et al. Preliminary experiments were conducted to determine the basic reaction conditions to be used. It was verified that ZnO had no activity for glycerol hydrogenolysis, a result already established by Wang et al.42 Other preliminary results are presented in Table 2. The experiments show that the catalyst

Table 3. Effect of Water Content on Catalytic Test with C(25/63/12)-350a

Table 2. Determination of Basic Reaction Conditionsa reduction time (min)

conversion (%)

selectivity 12PG (%)

unreduced 60 90 120

55 64 66 64

74 76 73 72

glycerol conc w/w%

PH2 (psi)

conv (%)

Sace (%)

S12PG (%)

SEG (%)

80 90 100

783 683 630

17.3 21.6 21.2

0.2 0.3 0.7

92.2 93.4 95.4

7.7 6.3 3.2

C-(25/63/12)-350; mgly = 25g; T = 200 °C; time, 2 h; stirring speed, 1500 rpm; ACu = 30.5 m2; mcata = 0.9349 g. a

a

Catalyst, C-(30/58/12)-350; mcata = 0.72 g; mglycerol = 19.28 g; T = 200 °C; PH2 = 500 psi; glycerol concentration, 80 w/w%; time, 24 h; stirring speed, 500 rpm.

obtained under these conditions. While catalyst activity and selectivity to 12PG remained mostly constant, selectivity to ethylene glycol decreased as the water content decreased. This is consistent with the retro-Claisen mechanism route which assumes a hydration of hydroxypropanedial (HPDA) followed by a retro-Claisen reaction to yield ethylene glycol (Scheme 1). This interpretation is further supported by the fact that the retro-aldol mechanism generally requires a basic reaction medium,58,59 which is not the case with our catalytic system. These results are actually very encouraging because as was mentioned earlier, the low water content circumvents many inconvenient facts associated with working in an aqueous medium. Figure 7 presents the effects of copper particle size dv,s of the C-(30/64/6)-T catalysts on activity and selectivity. The figure shows that when particle size increases, activity increases but selectivity decreases. This result is in accordance with the known fact that bigger copper particles are more active but tend to yield propanols instead of 12PG. This indicates that it is highly important to tailor the copper particle size, thus controlling sintering to yield the desired products. It is worth noting that the first step in Scheme 1 is an equilibrium dehydrogenation step leading to the formation of GA. It is a known fact that the solubility of hydrogen in liquid media is low60 and that high hydrogen pressures are necessary to achieve high solubility in glycerol so that two different contradicting phenomena are at play. On one hand, a high

reduction step is necessary to achieve higher glycerol conversion, once again indicating that metallic copper is the active phase. However, there is some activity for the unreduced catalyst, meaning that CuO may be reduced under the hydrogen atmosphere found in the catalytic system. Indeed, the catalyst has a reddish color after reaction, meaning some copper is reduced to a Cu1+ state. It was not possible to discern if all copper was in the Cu1+ form or if some of it was in the metallic form. Copper being reduced in situ means there is hydrogen consumption that is not used for hydrogenolysis. It is, however, remarkable that selectivity remains essentially constant, indicating that 12PG selectivity may be intrinsic to CuO/ZnO-based catalysts. The rather low selectivity (roughly 70−75%) may be due to overhydrogenolysis of glycerol to propanols at very long reaction times. The first set of experiments was conducted by varying glycerol concentration from 80 to 100 w/w% glycerol in water. High glycerol concentrations were used for four main reasons. First, raw glycerol obtained from batch homogeneously basecatalyzed biodiesel synthesis contains roughly 20 w/w% of water. Second, this removes the expensive water management problems that might be encountered in large-scale operations. Third, reducing the water content helps limit the number of parameters that might affect the mass and heat transfer in the F



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GC-MS in trace quantities (results not shown). This tends to prove that the rate-limiting step may not be the direct dehydration of glycerol to acetol but the dehydrogenation of glycerol to glyceraldehyde. Only trace amounts of glyceraldehyde and pyruvic aldehyde being observed means that both react rapidly (dehydration and hydrogenation), and it is a known fact that acetol reacts rapidly with hydrogen to yield 12PG. This could explain the relatively low observed selectivities to reaction intermediates. Moreover, experiments conducted in nitrogen by Wang et al.42 showed that there are still small amounts of 12PG formed in the absence of hydrogen in the reactants, supporting the hypothesis of a dehydrogenation step producing hydrogen in the reaction pathway. Scanning electron microscopy (SEM) results (Figure 8) show catalyst particle size and morphology of C-(30/64/6)-350 Figure 7. Effect of metallic copper particle size on conversion and selectivity. mgly = 25 g; T = 200 °C; PH2 = 630 psi; glycerol concentration, 100%; stirring speed, 1500 rpm; ACu = 30.5 m2.

hydrogen concentration moves the equilibrium toward the formation of glycerol, but if pressure is not high enough, hydrogen availability in the liquid phase may hamper the hydrogenation reaction of acetol to obtain 12PG. Results conducted with varying hydrogen pressures are presented in Table 4. When hydrogen pressure was increased, conversion

Figure 8. SEM of C-(30/64/6)-350 (a) before and (b) after the catalytic test.

Table 4. Effect of Initial Pressure on Selected Catalytic Testsa catalyst C-(35/65/0)-350 C-(36/58/6)-350 C-(25/63/12)-350

PH2 (psi) 630 1160 630 945 630 1260

mcata (g)

conv (%)

Sace (%)

S12PG (%)

SEG (%)

0.6731 0.6731 0.4305 0.4305 1.1821 1.1821

21.7 11.6 20.6 13.4 26.9 20.6

0.5 0.3 0.6 0.3 1.6 0.2

88.0 99.1 80.4 95.2 90.8 86.4

1.1 0.7 1.6 1.9 2.8 2.1

before and after a catalytic test. It can easily be seen that catalyst particles are affected by the test conditions as there is a decrease in particle size as well as a change in morphology, indicating that characterization of the particle size distribution prior to the catalytic test would not be relevant as changes are severe because of the combined effects of pressure, temperature, and mechanical stirring. 4.3. Madon−Boudart Criterion. A series of catalytic tests were conducted to determine an optimum reaction rate measurement to use the Madon−Boudart criterion. The results are presented in Figure 9. It is possible to see in these three cases that conversion evolution remains essentially linear for a few hours on some catalysts whereas it clearly decreases at high reaction time on other catalytic systems, indicating hydrogen

Glycerol concentration, 100 w/w%; mglycerol = 25g; T = 200 °C; time, 2 h; stirring speed, 1500 rpm; ACu = 30.5 m2.

a

decreased while selectivity to 12PG generally increased for the same amount of copper active sites in the reactor. Interestingly, selectivity to ethylene glycol slightly decreased. This is evidence that, in agreement with Scheme 1, the first step is probably an equilibrium dehydrogenation step. It may also be a reason why some studies show low catalyst activity despite working at high hydrogen pressure.30 Then comes an interesting problem: to promote the first reaction step, one has to work under low hydrogen pressure. However, selectivity to ethylene glycol depends on hydrogen pressure as shown in Scheme 1. Selectivity between EG and 12PG thus depends on the relative rates of the dehydrogenation of GA to HDPA and the dehydration of GA to PA. High hydrogen pressure will favor the dehydration to PA but at the same time will hamper the dehydrogenation of glycerol to GA. The following results shed some light on the reaction mechanism. Dasari et al.31 first proposed that glycerol is directly dehydrated to acetol, which to them is the rate-determining step. Acetol is then hydrogenated to yield 12PG. Our results show that this is an oversimplification of the real mechanism as glyceraldehyde and pyruvic aldehyde were both observed by

Figure 9. Determination of the optimal catalytic time with different catalysts for the Madon−Boudart criterion. G



Article

As hydrogen is consumed during the reaction, pressure decreases, decreasing solubility and hydrogen availability to catalytic sites. However, because an average reaction time over 2 h is used for the Madon−Boudart criterion, it is not known if hydrogen transfer is limiting the system when its concentration is the highest in the reaction medium at the beginning of the catalytic test. On the other hand, the fact that the global reaction is exothermic may create punctual hot spots on the catalyst surface that may alter transport properties of reactants and products directly at the surface of the catalyst because it is well-known that gas solubility generally decreases as temperature increases. Further experiments will have to be conducted in order to discriminate which transport phenomenon is dominant. These results contradict the work of Chaudhari et al.,62,63 who claim that the batch hydrogenolysis reaction of glycerol, in their test conditions, is free of mass-transfer limitations. However, their claims lack, to our knowledge, observable variables and rely on data which is calculated from correlations.64 The Madon−Boudart criterion has the advantage of relying only on the observed conversion and active sites concentration, both of which are measurable quantities. It may be noted that there is a spread of the data points in Figure 10 for a given set of catalytic test temperatures, which is highly apparent at 200 °C, where most experiments were conducted. This is explained by the fact that many factors are not controlled in the batch system. For example, it is impossible to be certain that ACu is exactly equal to 30.5 m2 in the reaction conditions (see Figure 8) and thus be certain of the reaction rate. We also use an average reaction rate over 2 h on the basis that the rate of reaction does not change over a 2 h reaction time which could also be a source of error. This error is lessened, however, by the fact that all systems are tested over that same reaction time. Moreover, mass-transfer resistance should be more significant in larger catalyst particles. The observed decrease in particle size in reaction conditions does not result in the absence of mass transfer in the reaction medium, which is in line with our conclusion that the glycerol hydrogenolysis reaction is controlled by mass-transfer phenomena in our experimental conditions.

availability or catalyst deactivation may become an issue. A reaction time of 2 h was then selected as optimal for obtaining observable conversion without suffering from hydrogen shortage or catalyst deactivation, as all catalytic systems show linear conversion evolution for a 2 h reaction time. In order to use the Madon and Boudart criterion, the amount of catalyst used in each experiment has to be pre-established. C(35/65/0)-350 was arbitrarily chosen as the reference catalyst, and Acopper of this catalyst in a reference experiment was established. The same surface of metallic copper of ACu = 30.5 m2 was then chosen for all further experiments. Catalyst weights in following experiments were adjusted according to eq 7 in order to obtain an amount of surface copper that was the same as that in the reference experiment. Results obtained at 200, 210, and 220 °C are plotted in Figure 10. Each point represents one experiment. Except for

Figure 10. Madon−Boudart criterion for different temperatures.

temperature, all test conditions are the same. From this figure, the slopes of the lines for all three operating temperatures are very close to 0.5, meaning that in the set of experimental conditions tested, severe limitations due to transport phenomena (heat or mass) make it impossible to obtain reliable kinetic data. Unfortunately, with the data obtained, we cannot pinpoint which phenomenon or combination of phenomena is dominant. Possible causes could be due to, but not limited to, the local transfer of global heat of reaction, which is known to be exothermic;61 the heat transfer to vaporize products at the test temperature, which is higher than the vaporization temperature of most of the reactants, intermediates, or reaction products (Table 5); hydrogen transfer in the liquid phase; or competition around the active sites. We believe hydrogen solubility may be the dominant factor because this solubility is directly proportional to pressure.

5. CONCLUSION Hydrogenolysis of glycerol has been performed using different Cu/ZnO-based catalysts. Catalysts were characterized in order to obtain the surface properties that would be used with the Madon−Boudart criterion to determine whether transport phenomena influence the reaction with metallic copper specific surface area as the active phase. Results show that copper-based catalysts are extremely sensitive to calcination temperature as temperatures higher than 350 °C lead to heavy sintering of the copper particles. Catalytic test results support the scheme proposed by Wang et al. in which the first reaction step is the dehydrogenation of glycerol to obtain glyceraldehyde, which can either yield ethylene glycol through the retro-Claisen mechanism or propylene glycol. The selectivity between propylene glycol and ethylene glycol can be tailored by controlling the relative reaction rates of glyceraldehyde dehydration (yielding propylene glycol) and dehydrogenation (yielding ethylene glycol). Finally, applying the Madon− Boudart criterion has shown that transport phenomena heavily hamper the glycerol hydrogenolysis reaction in a batch reactor. That problem may be circumvented using a continuous flow reactor, such as a trickle bed reactor. Future studies will be

Table 5. Boiling Points of Selected Compounds Present in Scheme 1 compound

TB (°C)

glycerol 1,2-propanediol ethylene glycol acetol glyceraldehyde pyruvic aldehyde hydroxypropanedial

290 188 197 146 140−150 at 0.8 mmHg 72 274 H



Article

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conducted in such a reactor to compare batch and continuous flow reactor performances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 418-656-2708. Notes

The authors declare no competing financial interest.

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REFERENCES

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J


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