The heterogeneous advantage: biodiesel by catalytic ... - CiteSeerX

141

Topics in Catalysis Vol. 40, Nos. 1–4, November 2006 ( 2006) DOI: 10.1007/s11244-006-0116-4

The heterogeneous advantage: biodiesel by catalytic reactive distillation Anton A. Kiss, Florin Omota, Alexandre C. Dimian, and Gadi Rothenberg* van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

The incentives for using biodiesel as renewable fuel and the difficulties associated with its production are outlined. The pros and cons of manufacturing biodiesel via fatty acid esterification using solid acid catalysts are investigated. Finding a suitable catalyst that is active, selective, and stable under the process conditions is the major challenge. The most promising candidates were found to be the sulfated metal oxides that can be used to develop a sustainable esterification process based on catalytic reactive distillation. KEY WORDS: biodiesel; reactive distillation; esterification; solid acid catalysts; mixed metal oxides.

1. Introduction A major concern of the modern society is the sustainable energy management. The automotive industry uses a large amount of global energy, making the implementation of sustainable fuels a crucial issue worldwide [1]. Biodiesel is a renewable fuel (figure 1, left) that can be manufactured from vegetable oils, animal fats or even recycled greases from food industry [2]. Remarkably, it is the only alternative fuel currently available that has an overall positive life cycle energy balance, producing 3.2 units of fuel product energy per unit of fossil energy consumed in its life cycle, compared to barely 0.83 units for petroleum diesel [3]. In spite of not containing any petroleum products, biodiesel can be blended with regular diesel to provide a ready-to-use bio-fuel. Blends of 80% petroleum diesel and 20% biodiesel (known as B20) can be used in unmodified diesel engines. For instance, using B20 in trucks and buses would completely eliminate the black smoke released during acceleration [4]. Unblended biodiesel (B100) can be also used, but may require minor engine adjustments to avoid maintenance problems [5,6]. Biodiesel is a green fuel that has many advantages over conventional diesel fuel [2]: it is safe, renewable [7], non-toxic and biodegradable [3,6], it contains insignificant amounts of sulfur and its increased lubricity extends the life of diesel engines. In addition, it has a high cetane number (above 60 compared to only 40 for regular diesel), a high flash point (>130 C) and it emits 70% fewer hydrocarbons, 80% less carbon dioxide, and 50% less particles. Table 1 shows the average biodiesel emissions compared to conventional petroleum diesel [3,6]. As a result of recent legislation changes that require a decrease of vehicle emissions, as much as less than * To whom correspondence should be addressed. E-mail: [email protected]

10 ppm sulfur content, the interest in biodiesel as an alternative fuel has accelerated tremendously [2,4,6–9]. Taking into account the previously mentioned advantages of biodiesel, it comes as no surprise the rapid increase in biodiesel production in United States and Europe (figure 1, right), with Germany, France and Italy among top producers. Biodiesel, a mixture of mono-alkyl esters of fatty acids, is currently manufactured by trans-esterification of triglycerides with methanol using NaOH or KOH as liquid base catalyst [8]. This catalyst is corrosive to the equipment, but this is easily overcome with little cost penalty, by constructing the reaction vessels out of stainless steel. However, the main drawback is that the liquid base catalyst has to be neutralized afterwards—typically using HCl—thus producing waste salt streams. Moreover, due to the presence of free fatty acids it reacts to form soap as unwanted by-product hence requiring more expensive separation. In this process the biodiesel composition depends heavily on the content of the raw fatty material, namely on the types of fatty acid groups building the triglyceride. Another method to produce these fatty esters is the batch-wise esterification of fatty acids using H2SO4 as catalyst. Tailor made fatty esters can be obtained using this method, making it suitable for the production of a large range of fatty esters with applications in cosmetics, pharmaceutics or food industry. Unlike raw materials containing a diverse mixture of triglycerides, the use of fatty acids allows the manufacturing of fatty acid alkyl esters with a specific content (e.g. only C12–C16 or C18–C22 esters). However, the problem here is the batch operation mode that is not suitable for very large-scale production and again it involves costly neutralization and separation of the homogeneous catalyst [9,10]. Removal of sulfuric acid is imperative due to the EU restrictions on sulfur content in fuels [9]. During the last decade many industrial processes shifted towards using solid acid catalysts [11,12]. The 1022-5528/06/1100–0141/0 2006 Springer Science+Business Media, Inc.

142

Biodieselproduction / [million liters]

A. A. Kiss et al./Biodiesel by catalytic reactive distillation

CO2+ light No global warming Biomass

CO2release to atmosphere

Refining /Synthesis

Use in cars and trucks

1.55 kg fossil CO2 per1 liter diesel burned

Exploration

2000 1600

EU USA

1200 800 400 0 1999

2000

2001

2002

2003

2004

Figure 1. Life cycle of biodiesel as environmentally-friendly fuel (left). Annual biodiesel production in EU and USA (right) (in million liters).

key benefit of using solid acid catalysts is that no polluting by-products are formed, and the catalysts do not have to be removed since they do not mix with the biodiesel product. In addition to lower separation costs, less maintenance is needed since these catalysts are not corrosive. In contrast to liquid acids that possess welldefined acid properties, solid acids may contain a variety of acid sites [13]. Usually they are categorized by their Brønsted or Lewis acidity, the strength and number of sites, and the textural properties of the support. Therefore, to solve the problems associated with using homogeneous catalysts we aim to develop a sustainable biodiesel production process, based on continuous reactive distillation (RD) using solid acid catalysts [14,15]. The RD design integrates reaction and separation into the same unit, thus intensifying the mass transfer while simplifying the process flowsheet and operation. By combining reaction and separation into one unit, one can shift the esterification equilibrium to the desired components by continuous removal of reaction products. The RD process has less separation steps, produces no waste salt streams as water is the only by-product, and could use a part of the produced biodiesel as source of energy. The low residence time of the liquid phase inside the RD column (20–60 min) requires a highly active catalyst. Unlike a batch reactor or a CSTR, a RD column has some hydraulic constrains that limit the maximum

residence time. In addition, the production rate is increased when the residence time is short. The non-ideal nature of the mixture may lead to liquid segregation into an aqueous and an organic phase. Water may easily deactivate the solid acid catalysts hence a water-tolerant catalyst is required [16]. Agitation may prevent the segregation, but this is feasible only at lab-scale or using a CSTR. However, no mixing devices are used in distillation columns and typically any moving part is avoided in chemical industry due to the increased energy consumption and higher maintenance costs. Fatty acid esterification using solid acids is not yet well established in industry, as it is much more difficult to find a suitable solid acid catalyst for long-chain acids esterification compared to shorter acids such as acetic acid. Biodiesel is a mixture of fatty acid alkyl esters, derived typically from short chain alcohols. Methanol is more suitable for biodiesel manufacturing, but we also studied other alcohols to examine the applicability of a range of alcohol types in this process. The actual applications would depend on the feedstock at hand. We screened various materials with potential solid acids catalyst applications. The catalyst development was integrated in the process design at an early stage. In this article we present the key features of our approach, and discuss the possible applications of this new process.

2. Experimental work Table 1 Average biodiesel emissions compared to conventional diesel Emission type Total unburned hydrocarbons Carbon monoxide (CO) Carbon dioxide (CO2)—life cycle production Particulate matter Nitrogen oxides (NOx) Sulfur oxides (SOx) Polycyclic Aromatic Hydrocarbons (PAH) Nitrated PAH’s (nPAH)

B20

B100

)20% )12% )16% )12% +2% )20% )13% )50%

)67% )48% )79% )47% +10% )100% )80% )90%

The experimental results are presented on the use of solid catalysts in esterification of dodecanoic acid (C12H24O2) with methanol (CH4O), propanol (C3H8O) or 2-ethylhexanol (C8H18O). Reactions were performed using a system of six parallel reactors (100 ml)—STEM Omni-Reacto Station 6100, with modular design and interchangeable heating blocks, glassware, and reflux heads). Reaction progress was monitored by gas chromatography (GC). GC analysis was performed using an InterScience GC-8000 gas chromatograph with a DB-1 capillary column (30 m · 0.21 mm). GC conditions:


isotherm at 40 C (2 min), ramp at 20 C min)1 to 200 C, isotherm at 200 C (4 min). The injector and detector temperatures were set at 240 C. Reaction profiles were measured for both non-catalyzed and catalyzed reactions, at several temperatures exceeding 100 C (below 100 C and in absence of mixing, the liquids separate before equilibrium is reached). The catalyst concentration in the reaction mixture was varied from 0 to 5 wt%. The initial reactant molar ratio used was varied from alcohol:dodecanoic acid = 1:1 up to 5:1. Table 2 shows the matrix of experimental conditions. In a typical reaction, 1 equivalent of dodecanoic acid and 1 equivalent of 2-ethylhexanol were reacted at 160 C in the presence of 1 wt% solid acid. Scheme 1 O C10H23

160 ºC

n-Bu

OH

first step consists in the hydroxylation of zirconium, titanium and tin complexes. The second step is the sulfonation with H2SO4 followed by calcination in air at various temperatures. The calcination was performed in a West 2050 oven and the temperatures were achieved with a heating rate of 240 C h)1, and then hold for 4 h at given calcination temperatures. The airflow was regulated with an Analogic flow regulator. 2.2.1. Preparation of sulfated zirconia catalyst ZrOCl2 Æ 8H2O (50 g) was dissolved in water (500 ml), followed by precipitation of Zr(OH)4 at pH = 9 using a 25 wt% NH3 soln. The precipitate was washed with water (3 · 500 ml) to remove the chloride salts (Cl– ions were determined with 0.5 N AgNO3). In the second step, O

HO +

143

C10H23

120 min

O

n-Bu + H O (1) 2

Scheme 1.

2.1. Chemicals and catalysts Double distilled water was used in all experiments. Unless otherwise noted, chemicals were purchased from commercial companies and were used as received. Dodecanoic acid 98 wt% (GC), methanol, propanol and 2-ethylhexanol 99+ wt% were supplied by Aldrich, zirconil chloride octahydrate 98+ wt% by Acros Organics, 25 wt% NH3 solution and H2SO4 97% from Merck. Most of the solid acid catalysts were provided readyto-be-used by commercial suppliers. Zeolites beta, Y and H-ZSM-5 were provided by Zeolyst, and ion-exchange resins (Nafion NR50 and Amberlyst-15) by Alfa. 2.2. Preparation of mixed metal oxides The sulfated metal oxides (zirconia, titania and tin oxide) were synthesized using a two steps method. The Table 2 Matrix of experimental conditions Catalyst

H2SO4 H3PW12O40 Cs2.5H0.5PW12O40 Zeolites (Y, b, ZSM5) Amberlyst-15 Nafion-50 Nb2O5 Æ 5H2O Sulfated carbon ZrO2/SO42) (SZ) TiO2/SO42) SnO2/SO42) a

Catalyst amount (wt%)

Temperature (C)

Reactants ratioa

0 (non-catalyzed) 0.5b 1 2 3 5b 10b

120 130 140 150 160b 180b

1:1 2:1 3:1 5:1

Reactants ratio is expressed as initial molar ratio of alcohol to acid. These experiments were performed only for sulfated zirconia.

b

Zr(OH)4 was dried for 16 h at 140 C, impregnated with 1N H2SO4 (15 ml H2SO4 per 1 g Zr(OH)4), and calcined in air for 4 h at 650 C.

2.2.2. Preparation of sulfated titania catalyst SO42)/TiO2 was prepared from Ti[OCH(CH3)2]4 (Acros, > 98%). HNO3 (35 ml) was added to an aqueous solution of Ti[OCH(CH3)2]4 (42 ml in 500 ml H2O). Then 25% aqueous ammonia was added until the pH was raised to 8. The precipitate was filtered, washed and dried for 16 h at 140 C .The product was impregnated with 1N H2SO4 (15 ml H2SO4 per 1 g Ti(OH)4). The precipitate was filtered, washed and dried for 16 h at 140 C, then calcined in air for 4 h. 2.2.3. Preparation of sulfated tin oxide catalyst Sn(OH)4 was prepared by adding a 25% aqueous NH3 solution to an aqueous solution of SnCl4 (Aldrich, > 99%, 50 g in 500 ml) until pH 9–10. The precipitate was filtered, washed, and then suspended in a 100 ml aqueous solution of 4% CH3COONH4. The precipitate was filtered, washed and dried for 16 h at 140 C. Then, 1N H2SO4 (15 ml H2SO4 per 1 g Sn(OH)4) was added to prepare SO42)/SnO2 and the precipitate was filtered, washed and dried for 16 h at 140 C. The calcination was performed in air for 4 h. 2.3. Preparation of Cs2.5 catalyst [Cs2.5H0.5PW12O40] Cs2CO3 (1.54 g, 10 ml, 0.47 M) aqueous solution were added dropwise to H3PW12O40 (5 ml, 10.8 g, 0.75 M aq. sol.). Reaction was performed at room temperature and normal pressure while stirring. The white precipitate was filtered and aged in water for

144


60 hours. After aging, the water was evaporated in an oven at 120 C. White solid glass-like particles of Cs2.5H0.5PW12O40 (9.0375 g, 2.82 mmol) were obtained.

Table 3 Catalyst characterization Catalyst sample

2.4. Preparation sulfated carbon-based catalyst Activated mesoporous carbon (7 g) and H2SO4 (98%, 70 ml) were heated at 250 C for 17 h. Reaction was performed under N2 atmosphere and normal pressure. Suspension was filtered and washed with water until testing with BaCl2 showed that no SO42) ions were present. The resulting black powder was dried in air for 15 h, when 8.1798 g of catalyst was obtained. 2.5. Catalyst characterization Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomization (ICP-AES) on a CE Instruments Sorptomatic 1990. NH3–TPD was used for the characterization of acid site distribution. Sulfated zirconia (0.3 g) was heated up to 600 C using He (30 ml min)1) to remove adsorbed components. Then, the sample was cooled at room temperature and saturated for 2 h with 100 ml min)1 of 8200 ppm NH3 in He as carrier gas. Subsequently, the system was flushed with He at a flowrate of 30 ml min)1 for 2 h. The temperature was ramped up to 600 C at a rate of 10 C min)1. A thermal conductivity detector (TCD) was used to measure the ammonia desorption profile of NH3. The textural properties were established from the nitrogen adsorption isotherm determined after degassing at 200 C under vacuum at 5–10 mbar. Surface area was calculated using the BET equation and the pore volume was determined at a relative pressure of 0.98. The pore size was calculated using the Barrett-JoynerHalenda (BJH) method. The characterization results are given in Table 3. Values are in very good agreement with literature data [17, 18]. As expected, higher sulfur content corresponds to higher acidity of the catalyst and consequently higher catalytic activity. In addition, the pore size plays an important role as the reactants and the products must be able to fit inside the catalyst to take full advantage of the total surface area available. The pore sizes of sulfated metal oxides are sufficiently large (> 2 nm) to facilitate the mass transfer into and from the catalyst pores. This compensate for their lower acidity compared to Amberlyst-15 or H3PW12O40.

Cs2.5H0.5PW12O40 ZrO2/SO42)/650 C TiO2/SO42)/550 C SnO2/SO42)/650 C

H2SO4 H3PW12O40 Cs2.5H0.5PW12O40 Zeolites (Y, b, ZSM5) Amberlyst-15 Nafion-50 Nb2O5 Æ 5H2O SO42)/ZrO2

Surface area (m2/g)

163 118 129 100 Acidity (meq g)1) 20.4 1.0 0.15 0.52–1.12 (depending on Si/Al ratio) 4.70 0.80 0.31 0.20

Pore volume (cm3/g)

Pore Sulfur diameter content (%) max./ mean/calc. [nm]

0.135 0.098 0.134 0.102

2/5.5/3 4.8/7.8/7.5 4.1/4.3/4.2 3.8/4.1/4.1

N/A 2.3 2.1 2.6

The suspension was then filtered and treated with a BaCl2 solution to test for SO42) ions. In a second experiment, the catalyst was added to an equimolar mixture of reactants. After 3 h at 140 C, the catalyst was recovered from the reaction mixture, dried at 120 C and finally stirred in 50 ml water. The pH was measured and the suspension titrated with a diluted solution of KOH after 24 h. Sulfate ions in the suspension were determined qualitatively with BaCl2 (at 140 C the reaction mixture does not split into two liquid phases, because the water evaporates). In a third experiment, the same procedure was repeated at 100 C when the reaction mixture segregates and a separate aqueous phase is formed. From the leaching tests it can be concluded that sulfated zirconia is not deactivated by leaching of sulfate groups when water is present in the organic phase but it is easily deactivated in pure water or aqueous phase. Other solid acid catalysts are also deactivated by the presence of water or in aqueous phase. There are several methods to prevent aqueous phase formation and leaching of acid sites: (1) use an excess of one reactant, (2) work at low conversions, and (3) increasing the temperature to a value exceeding the boiling point of water (to remove water)—this preserve the catalyst activity and drive reaction to completion.

2.6. Catalyst leaching The reaction mixture may segregate into two liquid phases, leading to possible leaching of sulfate groups. The leaching of catalyst was studied in an organic and an aqueous phase. First, a sample of fresh sulfated zirconia catalyst (0.33 g) was stirred with water (50 ml) while measuring the pH development in time. After 24 h, the acidity was measured by titration with KOH.

2.7. Selectivity and side reactions Typically, the alcohol-to-acid ratio inside an industrial reactive distillation unit may vary over several orders of magnitude [14,15]. Especially for stages where an excess of alcohol is present, the use of an acid catalyst may lead to side reactions such as etherification or dehydration. The selectivity was assessed by testing the

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formation of side products in a suspension of catalyst in alcohol (e.g. sulfated zirconia in pure 2-ethylhexanol) under reflux for 24 h. Also, no ethers or dehydration products were detected by GC analysis using the solid acids presented in this study, under the reaction conditions.

3. Results and discussion 3.1. Experimental results On the large scale processes, an esterification catalyst must fulfil several conditions that may not seem so important on laboratory scale. The catalyst should be very active and selective (as by-products formed in secondary reactions are likely to render the process uneconomical), water-tolerant (water by-product may deactivate the catalyst) and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. Considering these conditions, we searched for a strong Brønsted acid with increased hydrophobicity, and high thermal stability (up to 200–250 C). Hydrophobic surfaces are preferable for conducting organic reactions in water to avoid the covering with water of the solid acid surface and prevent the adsorption of organic materials. In the next figures, conversion is defined as: X½% ¼ 100 ð1 ½Acidfinal = ½Acidinitial Þ

ð2Þ

and the amount of catalyst used is normalized to the total amount of reactants: Wcat ½% ¼ Mcat =ðMacid þ Malcohol Þ

ð3Þ

In these equations, X[%] is the conversion of the fatty acid, [Acid]initial and [Acid]final are the molar concentrations of fatty acid before and after reaction, Wcat [%] is the weight percent of catalyst used in reaction and Mcat, Macid and Malcohol are the amounts of catalyst, acid and alcohol, respectively.

100

H2SO4 1%wt H3PW 12O40

80

Amb

SZ

C/SO4

60

2-

Cs2.5

Nafion Cfiber/SO4

40

Non-catalyzed

20

Conversion / [%]

Conversion / [%]

100

In our experiments, we screened zeolites, ionexchange resins, sulfated carbon-based catalysts, heteropoly compounds and mixed metal oxides. Several alcohols were used to show the larger range of applicability. Under the reaction conditions, no by-products were observed. For all of the catalysts described here, the selectivity was assessed by testing the formation of side products in a suspension of catalyst in alcohol (e.g. sulfated zirconia in pure 2-ethylhexanol) under reflux for 24 h. No ethers or dehydration products were detected by GC analysis. Three types of zeolites were investigated: H-ZSM-5, Y and Beta. Zeolites showed only a small increase of conversion (1–4%) compared to the non-catalyzed reaction. However, this is in agreement with previous findings suggesting that the reaction is limited by the diffusion of the bulky reactant into the zeolite pores, and the reaction takes place only on the external surface of the catalyst [16]. Zeolites contain Si, Al, and oxygen in their framework and cations, water and other molecules within their pores. The SiO2/Al2O3 ratio can be used to control the acid strength and hydrophobicity of zeolites [19]. While acid strength increases at lower SiO2/Al2O3 ratio, hydrophobicity increases at higher SiO2/Al2O3 ratios. This means that a compromise is needed for optimal performance [16]. In our experiments, negligible differences in activity were observed even for large variations of the SiO2/Al2O3 ratio. We tested then two ion-exchange organic resins: Amberlyst-15, a styrene-based sulfonic acid and NafionNR50, a copolymer of tetrafluoroethene and perfluoro2-(fluorosulfonylethoxy)-propyl vinyl ether [20–22]. Both catalysts showed high activity (figure 2, left) but only for a short time—2 h (Amberlyst) and 4.5 h (Nafion). This makes them unsuitable for our industrial applications. In addition, Nafion-NR50 had a lower activity than our prepared sulfated metal oxides and Amberlyst-15 is not stable at temperatures exceeding 150 C. In fact, the manufacturer of Amberlyst-15 recommends 120 C as the maximum temperature.

10%

80

5

3 1.5

60

0.5%w

40 Non-catalyzed

20 Alcohol:Acid = 2:1 T=130 C, 2 %wt cat.

0

SZ catalyst

0 0

30

60

90

Time / [min]

120

0

30

60

90

120

Time / [min]

Figure 2. Esterification of dodecanoic acid with 2-ethylhexanol: (left) fatty acid conversion at 130 C, using liquid and solid acid catalysts (2 wt%), (right) non-catalyzed and catalyzed (0.5–10 wt% SZ catalyst) reaction profiles.

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The tungstophosphoric acid shows high activity, close to H2SO4 used as a benchmark. Regrettably, this acid is soluble in water (200 g/100 g water.) hence not usable as a solid catalyst. However, its cesium salt (Cs2.5) is also super acidic and its mesoporous structure has no limitations on the diffusion of the reactants (Table 3). Yet, Cs2.5 exhibits lower activity per weight of catalyst, compared to the tungstophosphoric acid. The sulfated carbon-based (carbon fiber, mesoporous carbon) catalysts exhibited also lower activity compared to the other catalysts, so they cannot be seriously considered for industrial scale applications. Sulfated zirconia was chosen as a representative of the metal oxides family, after an initial screening of various metal oxides [16]. Compared to the sulfated zirconia catalyst, the other mixed metal oxides prepared and tested (sulfated titania and tin oxide) performed slightly better—several percents increase in acid conversion. However, in terms of costs sulfated zirconia is less expensive and in addition, it is readily available at industrial scale. Sulfated zirconia is well known for its industrial applications in a variety of processes [23–26]. Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions [23]. The calcination temperature has a great effect on its

100

Alcohol:Acid = 5:1 SZ Catalyst 1%wt

Alcohol:Acid = 5:1 θ T=140 C, SZ Catalyst

80

160°C

60

Conversion / [%]

Conversion / [%]

100

catalytic activity; the optimal calcination temperature being is in the range of 600–700 C. However, concentration of the sulfuric acid used for catalyst preparation had an irrelevant effect on the catalytic activity. In our experiments it showed high activity and selectivity for the esterification of fatty acids with a variety of alcohols ranging from 2-ethylhexanol to methanol. By increasing the amount of catalyst used the reaction rate, hence conversion after a certain time, can be further increased (figure 2, right), making this catalyst suitable for reactive distillation applications where high activity is required in a short time. Moreover, sulfated zirconia is also very selective and thermally stable. In a separate set of experiments, we tested the catalyst reusability and robustness. In five consecutive runs, the activity dropped to 90% of the original value, and remained constant thereafter. Re-calcination of the used catalyst restored its original activity. Considering the promising results with 2-ethylhexanol, we tested the applicability of sulfated zirconia also for 1-propanol and methanol (figures 3 and 4). The increase of conversion with the temperature is much higher in the case of the catalyzed reaction. Moreover, high conversions can be reached even at 140 C providing that an increased amount of catalyst is used. Note however that increasing the amount of catalyst to more

140°C

40 120°C

20

80 3%wt

60

1%wt

40 0%wt

20

0

0 0

20

60

40

0

Time / [min]

20

40

60

Time / [min]

Figure 3. Esterification of dodecanoic acid with 1-propanol, using an alcohol:acid ratio of 5:1 and sulfated zirconia (SZ) as catalyst. (left) 1 wt% catalyst at 120 C, 140 C and 160 C (right) non-catalyzed, 1 wt% and 3 wt% catalyst at 140 C. 100

100

150°C

Conversion / [%]

Conversion / [%]

3%wt 140°C

80

120°C

60 40 20

Alcohol:Acid = 3:1 SZ Catalyst 1%wt

0

80 1%wt

60 0%wt

40 20

Alcohol:Acid = 3:1 T=140θC, SZ Catalyst

0 0

20

40

Time / [min]

60

0

20

40

60

Time / [min]

Figure 4. Esterification of dodecanoic acid with methanol, using an alcohol:acid ratio of 3:1 and sulfated zirconia (SZ) as catalyst: (left) 1 wt% catalyst at 120, 140, and 150 C (right) non-catalyzed, 1 wt% and 3 wt% catalyst at 140 C.

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than 5% brings only a minor grow of conversion. Comparing the reactions with the three alcohols, we note that esterification of dodecanoic acid with methanol takes place at higher rates compared to 1-propanol or 2-ethylhexanol. This can be explained by the alcohols’ relative sizes, with methanol having the smallest molecular size. 3.2. Discussion Although the reaction mechanism for the heterogeneous acid-catalyzed esterification was reported to be similar to the homogeneously catalyzed one [27], there is a major difference concerning the relationship between the surface hydrophobicity and the catalyst’s activity (figure 5, left). For zeolites, the Si/Al ratio represents a compromise between the hydrophobic character of a zeolite and its acidity. Hydrophobicity is needed to avoid the absorption of water by-product that will lead to deactivation. At low Si/Al ratio, water is easily absorbed to the surface, blocking the access of the fatty acid. Then again if the Si/Al ratio is too high, the zeolite may lose its acidic properties. By increasing the Si/Al ratio, reaction pockets are created inside a hydrophobic environment, where the fatty acid molecules can be absorbed and react further. Water molecules are unlikely to be absorbed on sites enclosed in hydrophobic areas. The literature reports that highsilica zeolites were efficient catalysts for esterifying acetic acid with ethanol, but showed a decreased activity for 2-buthanol [16]. Accordingly, no significant differences were observed in our case for the various SiO2/Al2O3 ratios, most likely because the reactants are too large to fit in the pores. In the case of ion-exchage resins (Nafion and Amberlyst) the hydrophobicity is governed by the polymer backbone that is likely to have affinity towards to the tails of the reactants. Moreover, the sulfonic acid groups grafted on the chain are strong acids compared to the hydroxyl groups on zeolites. Hence, the resins are good esterification catalyst candidates, but they fail on the thermal stability test [16]. Of the sulphated metal oxides group, sulfated zirconia is shown to be a good catalyst with high thermal stability and strong acid sites [17]. In addition, its

catalytic activity can be enhanced by preparing sulfated zirconia using a chlorosulfonic acid precursor dissolved in an organic solvent, instead of the conventional sulfuric acid impregnation [28]. Sulfated zirconia has large pores (Table 3) and therefore does not limit the diffusion of the fatty acid molecules. Sulfated zirconia does not leach under the reaction conditions and does not give rise to side reactions such as etherification or dehydration. This makes sulfated zirconia a very appealing candidate for catalytic biodiesel production.

4. Reactive distilation design In this part we present the RD design for fatty acids esterification using sulfated zirconia as green catalyst. These results were accomplished by rigorous simulations that integrate the experimental findings. The complexity of the problem is relative high, as it involves chemical and phase equilibrium (CPE), vapour–liquid equilibrium (VLE) and vapour–liquid– liquid equilibrium (VLLE), catalyst activity and kinetics, mass transfer in gas–liquid and liquid–solid, adsorption on the catalyst and desorption of products [14, 15, 29–31]. 4.1. Conceptual design Figure 5(right) shows the reaction pathways and the possible products. Several secondary reactions are possible, but these can be avoided by using a selective solid catalyst such as sulfated zirconia. The reaction conditions such as reactants ratio, temperature and pressure determine the phase equilibrium. For temperatures below 100 C, only low conversions can be achieved and an excess of alcohol is required to have only one liquid phase. For stoichiometric reactants ratio, two liquid phases exist at temperatures below 100 C. If temperature exceeds 100 C and the system is closed at over pressure then three phases exist: vapours–liquid–liquid. Liquid separation is undesirable due to catalyst deactivation in the presence of water. The best solution is working at temperatures above 100 C, in a system with continuous water REACTANTS Fatty Acid

Alcohol

esterification

C O C O

H+

OH

catalyst

H+

H2O H2O

OH H2O

H+

+ H H H+ +

H+ H2O H+ + H

catalyst

dehydration dehydration

etherification

Fatty Ester Main product

Ether

Water

Alkene

Secondary products

Figure 5. Influence of the hydrophobic/hydrophilic surface on the catalytic activity (left). Reaction pathways and possible products (right).

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Table 4 Normal boiling points of chemical species involved in the process Chemical name

Chemical formula

Mw (g/mol)

Tb (K)

Tb (C)

Dodecanoic (lauric) acid Methanol Propanol 2 Ehyl hexanol Methyl dodecanoate Propyl dodecanoate 2 ethylhexyl dodecanoate Water

C12H24O2 CH4O C3H8O C8H18O C13H26O2 C15H30O2 C20H40O2 H2O

200 32 60 130 214 242 312 18

571 338 370 459 540 575 607 373

298 65 97 186 267 302 334 100

removal. By removing water as by-product the equilibrium is shifted towards ester formation. Rigorous simulations were performed in AspenTech AspenOne 2004 engineering suite. The missing parameters were estimated based on experimental results. The analysis of physico- chemical properties shows very high boiling points for dodecanoic acid and esters (Table 4). Hence, the ester will always be separated in the bottom of the reactive distillation column (RDC). Water is present as side product, and typically is removed as top product due to its lower boiling point, together with the alcohol if it is volatile (and not completely converted) or if it forms azeotropes with water. The generalized CPE diagram is shown in figure 6(left) for esterification of dodecanoic acid with methanol. Each point inside the diagram represents the liquid composition at phase and chemical equilibrium. The esterification reaction must take place in the homogeneous region. Because of low solubility of organic compounds, a mixture in the heterogeneous region splits into two homogeneous phases: one is placed on the boundary and the other one is very close to the pure water. In the heterogeneous region the boiling point is very close to the normal boiling point of water. In the case of homogeneous liquid phase (top and left of diagram) the distillation lines are presented. Using these generalized CPE diagrams, the initial molar ratio of reactants can be predicted in order to prevent two liquid phase formation.

X2 (acid+ester)

The systematic study of reaction rates under controlled process conditions (temperature, pressure, and composition) is a suitable method for comparing solid acid catalyst candidates for fatty acids esterification [33]. The hydrophobicity of the catalyst surface and the density of the acid sites are of crucial importance in determining the catalyst’s activity and selectivity.

Top: 373 K RDC Dodecanoic acid

VLL

Methanol

0.2 0.4 0.6 0.81 X1 (water+acid)

Distillation column Water ≥ 99.9 % Acid < 0.1 %

Feed ratio 1:1

0.2

0 Alcohol, 65°C 0

5. Conclusions

VL

0.6 0.4

UNIQUAC property model was used in simulation, but UNIFAC (Dortmund modified) property model can be also successfully applied [32]. Reactants are feed in stoichiometric ratio, methanol to dodecanoic acid feed ratio = 1:1 (1 kmol/h). The RD column has 14 stages and a low reflux ratio (R = 0.01). A higher reflux ratio is not beneficial as it brings back water into the column, hence decreasing the conversion by shifting the equilibrium towards ester hydrolysis. The fatty acid is fed above and methanol below the reactive zone (mid 8 stages), respectively. In order to reduce the amount of dodecanoic acid in the final product, the fatty acid—heavier than the alcohol—must be fed in the top of reactive zone. Figure 6(right) presents the flowsheet for esterification of dodecanoic acid with methanol. High purity final products are feasible. Note however that pure fatty acid esters cannot be the bottom product of the reactive distillation column because of the high boiling points and lower thermo-stability. By allowing 1–2% of alcohol in the bottom stream, the reboiler temperature in the RD column can be kept below 200 C. An additional evaporator is used for further ester purification. When an excess of alcohol is used the maximum reaction rate is located at the top of the column, with total acid conversion in the bottom but partial conversion of alcohol in the top. For the optimal reflux ratio the maximum reaction rate is located in the centre of the column, providing complete conversion of both reactants at the ends of the column. The composition and temperature profiles in the reactive distillation column are shown in figure 7.

Acid, 298°C

Ester, 267°C 1

0.8

4.2. Simulation results

Water, 100°C

Evaporator Bottom: 473 K

Ester ≥ 99.9%

Figure 6. Generalized CPE diagram for dodecanoic acid and methanol at 1 atm (left). FAME production—flowsheet of fatty acid esterification with methanol (right).

149

Temperature / °C

Molar fraction

0.4

200

1 0.8 Methanol Acid Ester Water

0.6 0.4 0.2 0

Temperature Ester generation

180

0.3

160 0.2 140 0.1

120

0

100 0

3

6

9

12

15

0

3

Stage

6

9

Ester formation / kmol/hr


12

Stage

Figure 7. Reactive distillation column (RDC) profiles: liquid composition (left) and temperature and ester generation (right).

Catalysts with small pores, such as zeolites, are not suitable for biodiesel manufacturing because of the diffusion limitations of the large fatty acid and ester molecules. Ion-exchange resins, such as Nafion and Amberlyst, are active strong acids, but have a low thermal stability. This is problematic as the reaction must be carried out at high temperatures to get high reaction rates. The tungstophosphoric acid is very active, but this acid is soluble in water hence not usable per se as a solid acid catalyst. In order to benefit from its high activity, and at the same time avoid the solubility disadvantage, the tungstophosphoric acid could be immobilized on silica–alumina support or even mixed metal oxides. Its Cs2.5 salt has shown lower acidity and activity per weight of catalyst (Table 3). Of the mixed metal oxides family, sulfated zirconia is found to be a good candidate as it is active, selective, and stable under the process conditions. This makes sulfated zirconia the most appealing candidate for catalytic biodiesel production. Biodiesel fuel can be produced by a sustainable continuous process based on catalytic reactive distillation. The integrated design ensures the removal of water by-product that shifts the chemical equilibrium to completion and preserves the catalyst activity. Manufacturing of fatty acid esters by reactive distillation can be applied to a variety of alcohols and fatty acids, as a multifunctional reactor, the actual applications depending on the feedstock at hand [34]. The process proposed here can dramatically improve the economics of current biodiesel synthesis and reduce the number of downstream steps. The key benefits are: 1. High unit productivity, up to 6–10 times higher than of the current process. 2. Lower excess alcohol requirements (stoichiometric ratio at reactor inlet). 3. Reduced capital and operating costs, due to less units and lower energy consumption. 4. Sulfur-free fuel, since solid acids do not leach into the product. 5. No waste streams because no salts are produced (neutralization step not required).

Acknowledgments We thank M. C. Marjo Mittelmejer-Hazeleger, Jurriaan Beckers and Taasje Mahabiesing for the outstanding technical support and the Dutch Technology Foundation STW (NWO/CW Project Nr. 700.54.653) and companies Engelhard, Cognis, Oleon, Sulzer and Uniquema for the financial support.

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