Catalytic Conversion of Sodium Lignosulfonate to Vanillin

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Catalytic Conversion of Sodium Lignosulfonate to Vanillin: Engineering Aspects. Part 1. Effects of Processing Conditions on Vanillin Yield and Selectivity Andrzej W. Pacek,*,† Ping Ding,† Mark Garrett,‡ Gary Sheldrake,‡ and Alvin W. Nienow† †

School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, U.K. School of Chemistry and Chemical Engineering, Queens University, Belfast BT9 5AG, U.K.

‡

ABSTRACT: The commercial production of vanillin from sodium lignosulfonate under highly alkaline conditions, catalyzed by Cu2+ at elevated temperature and pressures up to ∼10 bar, has been simulated in a 3-L stirred reactor. Initially, the process was operated in the presence of nitrogen in dead-end mode, and it was shown that vanillin and vanillic acid were formed by hydrolysis at temperatures of 120, 140, and 160 °C. At the two higher temperatures, the amount of vanillin produced was the same. Subsequently, experiments were conducted at the same elevated pressures and temperatures with addition of air or oxygenenriched air once the temperature in the reactor had reached temperatures similar to those used when only hydrolysis occurred. In this case, the concentration of vanillin at 140 and 160 °C was equal to that due to hydrolysis, and the subsequent 2-fold increase was due to oxidation. In addition, both vanillic acid and acetovanillone (which has rarely been reported) were produced, as was hydrogen. Thus, for the first time, it has been shown that the production of vanillin (and other compounds) from sodium lignosulfonate at elevated temperatures involves hydrolysis and oxidation, with hydrolysis starting at just above 100 °C, that is, much lower than has previously been reported. Approximately 50% is produced by each mechanism. In addition, the orders of the reactions of the different steps were estimated, and the reaction mechanisms are discussed. of world production) such as lignosulfonate or Kraft lignin.1 The main advantage of the first process is that it is relatively straightforward and the separation of vanillin from the reaction mixtures is cost-efficient; the serious disadvantage is the petroleum origin of the reactants. The latter point is particularly important in terms of sustainability and the recent strong political/economic pressure to replace traditional refineries with biorefineries for both energy and fine-chemicals productions.5 Whereas the oxidation of lignosulfonate to vanillin under strongly alkaline conditions using a transition metal as a catalyst is relatively simple and meets the demands for cheap, environmentally friendly, and sustainable manufacturing, the separation of vanillin after oxidation is rather complex and is often perceived as a serious bottleneck in the overall manufacturing process. The starting point for such a process at the commercial scale is the production of water-soluble lignosulfonate (LS) during the reactions that occur with the sulfite pulping of cellulose to make paper (Figure 1).6 This byproduct of paper making is often used as a fuel supplement. It is inexpensive and plentiful, and in that regard, it is ideal as a starting material for valorization through the production of vanillin or other valuable organic molecules. Whereas the basic molecular formula of lignosulfonate is as shown in Figure 1, its structure and shape, despite extensive research, are still not well established. Lebell7 suggested that it occurs as spherical lignosulfonate molecules forming microgels in aqueous solutions, whereas Goring et al.8 argued that, because

1. INTRODUCTION Vanillin (4-hydroxy-3-methoxybenzaldehyde, C8H8O3) is a very important commercial compound used in a variety of industries. In the food industry, it is used mainly as a flavoring agent in ice creams, soft drinks and chocolate, baked confectionery, and cheap brandy and whisky and also as a food preservative in sausages, seasonings, and so on. In the cosmetic industry, it is used as a fragrance ingredient in perfumes and creams; in the chemical industry, as an antifoaming agent, vulcanization inhibitor, and chemical precursor; and in the pharmaceutical industry, as an odor-masking agent. Recently, it has also been suggested that vanillin might have certain antioxidant and cancer prevention properties and that it is involved in bacterial cell-tocell signaling, which potentially greatly increases its importance.1−3 Traditionally, vanillin was produced by simple extraction from the bean or pod of tropical vanilla orchids growing mainly in Indonesia, Madagascar, and China, but currently, this source of supply is less than 1% of the global production of vanillin. The growing, harvesting, and preparation of vanilla plants for extraction is labor-intensive; therefore, the cost of vanillin obtained from natural resources is more than 100 times higher than the cost of vanillin synthesized by chemical processes.4 As the demand for natural vanillin is still rather high, biotransformation of caffeic acid and veratraldehyde has been investigated as a possible source of vanillin. The great advantage of this method is that it produces numerous chemical components (or their precursors) that constitute the natural vanilla flavor. However, such processes are still at early stages of development.4 At the industrial scale, vanillin is either synthesized by a petrochemical route based on guaiacol (85% of the world supply) or manufactured by oxidation of biomass containing lignin (15% © XXXX American Chemical Society

Received: March 9, 2013 Revised: May 20, 2013 Accepted: May 21, 2013

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dx.doi.org/10.1021/ie4007744 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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(engineering aspects) such as mixing intensity, residence time, flow rates, temperature, and pressure. The work reported here focuses on these engineering aspects and investigates the effects on the vanillin yield of processing parameters such as temperature, oxygen partial pressure, air flow rate, specific energy dissipation rate (intensity of mixing), and concentration of lignosulfonate.

2. EXPERIMENTAL SECTION Experiments were carried out in a pressurized, stainless steel, jacketed 3-L Büchi stirred reactor fitted with four removable baffles (which were present in all experiments except where noted) and a 0.075-m-diameter Rushton turbine impeller (Figure 2). A condenser was fitted at the outlet and connected

Figure 1. Transformation of lignin to lignosulfonate during sulfite pulping (from http://en.wikipidia.org/wiki/lignosulfonates) (Accessed May 2013).

of the position the lignosulfonate macromolecules take on any interface, it has to have a disk-like shape. Recently, it has been suggested9 that the lignosulfonate molecule forms a randomly branched polyelectrolyte that folds into a spherical shape once the charges have been removed. The average molecular weight of lignosulfonate depends on the temperature at which the digestion of lignin is carried out10 and the type of wood that has been pulped. In general, it is lower for hardwood lignosulfonate than for lignosulfonate from softwoods.6 The number-average molecular weight varies between 3000 and 7000 g/mol, whereas the mass-average molecular weight varies between 36000 and 64000 g/mol.11 The fact that the structure and molecular weight of lignosulfonate are not well established means that even the determination of the concentration of lignosulfonate in water is not trivial and is still a subject of research in its own right.6 In addition, there are no simple or even moderately advanced analytical techniques enabling the routine and rapid measurement of the concentration of lignosulfonate in reacting mixture. As a result, it is virtually impossible to develop precise equations describing the kinetics of the oxidation of lignosulfonate to vanillin. Not only does the complexity of the lignosulfonate molecule make the measurement of its transient concentration during oxidation to vanillin very difficult or even impossible, there are also other factors adding to this inherent problem. For example, Bjorsvik10 identified simultaneous oxidation and alkaline hydrolysis at higher temperatures, and the sensitivity of the distribution of products and intermediates to such conditions as additional factors making this process practically intractable by standard methods. Therefore, he modeled the oxidation of lignosulfonate using statistical methods based on the results of oxidation (vanillin yield) at different processing conditions available at the industrial-plant and laboratory scales at the Borregaard biorefinery in Norway. Despite those difficulties, Borregaard has manufactured vanillin from lignosulfonate on the industrial scale for more than 50 years and is currently the second largest vanillin manufacturer in the world and the main supplier to Europe.1 In the process used by Borregaard, lignosulfonate is catalytically oxidized at very high pH with a transition-metal catalyst at elevated pressure and temperature to give a rather low vanillin yield between 5% and 7% with respect to lignosulfonate.10 Given the availability of lignosulfonates as a byproduct from pulping, an increase in this yield to vanillin (or other valuable aromatic organic compounds) seems an appropriate research goal. In principle, the yield of multiphase catalytic reactions can be increased either by improvement of the chemistry of the process, which is typically done by optimization/modification of the catalyst, or by selection/modification of the process parameters

Figure 2. Schematic diagram of the experimental rig: (1) jacketed stirred reactor fitted with a Rushton turbine, (2) thermocouple, (3) condenser, (4) pressure gauge and regulator, (5) water vapor separator filled with silica gel, (6) oxygen analyzer, (7,8) gas flow meters, (9) water bath, (10) oil bath, (11) computer.

to the water bath. The temperature in the reactor was measured and set to values from 120 to 180 °C controlled by a jacket through which dimethyl polysiloxane oil from an oil bath was circulated. The pressure in the reactor was also measured and regulated. The following temperatures were continuously measured and recorded: the temperature in the reaction mixture, the water temperatures at the inlet and outlet of the condenser; and the oil temperatures at the inlet and outlet of the jacket. The gas phase (air, nitrogen, oxygen, or a mixture) was fed to the reactor from gas cylinders through a sparger placed beneath the agitator. Gas flow rates were measured and controlled by flow meters. In flow-through mode, the gas phase from the condenser passed through a separator where the moisture was adsorbed by silica gel before gas was directed to an oxygen analyzer (Rapidox 1100E, Cambridge Sensotec Limited, St. Ives, Cambridgeshire, U.K.). Samples of the gas were also analyzed using a mass spectrometer (Prima δB MS, Thermo Fisher Scientific, Waltham, MA).12 In dead-end mode, only the oxygen consumed by the reaction was continuously added to the reactor to maintain a constant preset pressure. Most of the experiments were carried out at a total pressure of 11.5 bar and temperatures between 120 and 180 °C. B



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The concentrations of vanillin, vanillic acid, and acetovanillone were measured by high-performance liquid chromatography (HPLC; G1367A, Agilent, Palo Alto, CA) following the procedure described in refs 10 and 12

3. MATERIALS Sodium lignosulfonate, NaLS, was supplied by Borregaard as spray-dried powder from the acidic sulfite cooking of Scandinavian spruce. The powder was dissolved in 3 M NaOH solution, and copper sulfate at a concentration of 4.6 g L−1 was added as catalyst. The concentration of NaLS was 220 g L−1 unless specified otherwise. The solutions were Newtonian liquids with viscosities measured by rheometer (RA 1000, TA Instruments, Elstree, Hertfordshire, U.K.) at room temperature that were only weakly dependent on pH, being 8.2 mPa s at pH 6 and 12.3 mPa s at pH 14. The interfacial tension was also measured at room temperature by the pendant drop method, and it also showed a weak dependency on pH, varying from 28−29 mN m−1 at pH 6 to 22 mN m−1 at pH 14. Both viscosity and interfacial tension at temperatures above 100 °C (reaction temperatures) are difficult to measure; therefore, where necessary, the values at room temperature were used. Figure 3. Transient concentrations of (a) vanillin and (b) vanillic acid during hydrolysis at (●) 120, (○) 140, and (▼) 160 °C. At time zero, the reactor contents reached the specified temperature.

4. RESULTS AND DISCUSSION The industrial production of lignin from lignosulfonate or Kraft vanillin is carried out at highly alkaline conditions (pH ∼14) and at elevated temperature (between 100 and 200 °C). It has been well established that under such conditions, oxidation of lignosulfonate10 to vanillin is associated with hydrolysis. However, the literature does not report any attempt to separate those two reactions; or to determine what proportion of vanillin comes just from hydrolysis and what proportion from consecutive/simultaneous oxidation and hydrolysis. Such an attempt has been made here, and the results are discussed in the following sections. 4.1. Hydrolysis of Lignosulfonate to Vanillin. The solution of NaLS in 3 M NaOH (pH 14) was charged into the reactor at room temperature and sparged for 10 min with nitrogen with the impeller running at 500 rpm. The concentrations of vanillin and vanilic acid in the solution were measured and were equal to 0.021 and 0.029 g L−1, respectively. Next, the vessel was sealed, and after being heated, the total pressure was adjusted to 11.5 bar by addition of nitrogen. Although, in principle, it is not necessary to investigate hydrolysis at elevated pressure because the kinetics of homogeneous liquidphase reactions (here, hydrolysis) are independent of pressure, it was decided to do so because, in the industrial process, the whole operation is carried out at a constant pressure of 11.5 bar. A number of different experiments were conducted. In the first, the liquid was heated to 120, 140, or 160 °C, and at each temperature, hydrolysis was carried out for 2 h. The concentrations of vanillin and vanillic acid were measured in small samples of liquid withdrawn from the reactor at certain time intervals, and the results are summarized in Figure 3. Traces of acetovanillone were also detected after hydrolysis, and in general, this was the case in all experiments reported in this article. However, as acetovanillone production is not related to oxidation,25 the details are not discussed further. Figure 3 clearly shows that vanillin and vanillic acid were produced by hydrolysis in the absence of oxygen. Hydrolysis of lignosulfonate to vanillin has been reported in the literature, but

it has been postulated that it starts at 170−200 °C.10 Here, hydrolysis was observed just above 100 °C, which implies that, in industrial processes, it might start during heating of NaLS in heat exchangers before the NaLS is charged into the reactor and contacted with air. Figure 3 also shows that the kinetics of hydrolysis is strongly dependent on temperature. At 120 °C, hydrolysis was relatively slow, and the concentrations of vanillin and vanillic acid gradually increased throughout 120 min to reach 5 and 0.25 g L−1, respectively. At a temperature of 160 °C, the concentration of vanillin went through a weak maximum at 20 min. This vanillin concentration profile implies that nearly all of the lignosulfonate that could be converted by hydrolysis to vanillin and vanillic acid was, in fact, converted and that, during hydrolysis, vanillin was also degraded, probably to vanillic acid or other lower-molecular-weight compounds. As expected, between those two temperatures, at 140 °C, the concentration of vanillin increased much faster than at 120 °C, and after 100 min, it reached the same maximum as at 160 °C, confirming the concept that all of the lignosulfonate that could react to vanillin by hydrolysis had done so. In fact, the presence of vanillic acid reported here is surprising and questions the literature that suggests vanillic acid results from the oxidation of vanillin.10 Clearly, both components can be produced without oxygen, and under these conditions, it appears that the reactions occur simultaneously directly from NaLS. A similar experiment was conducted to test whether the catalyst itself was important. As can be seen in Figure 4, the yield of vanillin by hydrolysis in the absence of oxygen did not increase upon the addition of the catalyst. In the next experiment, the approach was to assess the impact of following changes in temperature alongside other parameters in a different way. The temperature was raised to 120 °C (time zero), and then it was held for 20 min at a set agitation speed of 500 rpm. After 20 min, it was raised to 140 °C and, after another 20 min, to 160 °C. This procedure was then repeated at 750 rpm C



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detailed analysis requires transient concentrations of lignosulfonate, vanillin, vanillic acid, and NaOH. Whereas the concentrations of vanillin and vanillic acid could be measured accurately, the transient concentrations of NaLS could not. Therefore, the kinetics of hydrolysis were related to the initial concentration of NaLS and the operating temperature plus the transient concentration of vanillin, as shown in Figure 5b. In this case, the experiment was conducted using different concentrations of NaLS and monitoring the concentration of vanillin as it changed with increasing temperature after it started being produced at 100 °C (time zero in Figure 5b). To use these results to estimate the kinetics of hydrolysis of NaLS to vanillin, it was assumed that vanillin is not consumed during hydrolysis. This assumption is supported by the literature, which shows that keeping the vanillin solution at pH ≫ 9.2 at T = 130−170 °C for extended periods of time under nitrogen did not lead to a reduction of vanillin concentration.15 It was also assumed that the effect of the NaOH concentration on vanillin yield would be negligible because of its very high concentration and the fact that the pH was greater than 12 during the experiment.10 With those assumptions, the vanillin production rate from hydrolysis (rHV ) is given by

Figure 4. Effect of the presence of catalyst on the vanillin concentration during hydrolysis at T = 160 °C and N = 500 rpm. At time zero, the reactant reached 160 °C.

and at 500 rpm with the baffles removed. The aim here was to assess whether the hydrolysis was sensitive to mixing by changing the mean specific energy dissipation rate and/or the flow pattern. Thus, at 500 and 750 rpm, the mean energy dissipation rates were 3.0 and 10.2 kW m−3, respectively (based on a power number Po for a Rushton turbine of 4),13 and without the baffles at 500 rpm, it was lowered by a factor of about 514 and the flow pattern was changed completely. However, as can be seen in Figure 5a, the

rVH = k HC NaLSn

(1)

where kH is the hydrolysis rate constant, CNaLS is the concentration of NaLS, and n is the reaction order with respect to the initial concentration of NaLS. kH can be related to temperature using the Arrhenius equation to give the reaction rate as a function of the initial concentration of NaLS and temperature ⎛ E ⎞ n ⎟C rVH = A exp⎜ − ⎝ RT ⎠ NaLS

(2)

The reaction order and rate constant were calculated from the data shown in Figure 5b by nonlinear regression using eq 2. They were also calculated from Figure 6a, and both methods gave a reaction order, n, between 1.01 and 1.09; that is, both methods indicated that hydrolysis is first-order with respect to the initial concentration of NaLS. The activation energy, E, and the constant, A, in the Arrhenius equation were estimated from the slope of the line in Figure 6b. Thus, overall, the kinetics of hydrolysis can be related to the initial concentration of NaLS and temperature by ⎛ 15596 ⎞ ⎟C rVH = 0.001553 exp⎜ − ⎝ RT ⎠ NaLS

(3)

The activation energy E calculated from the experimental data is 15.6 kJ mol−1 K−1, which, as expected, is lower than the activation energy of the combined hydrolysis and oxidation of Kraft lignin to vanillin (E = 29.1 kJ mol−1 K−1) reported in the literature.16 The gas phase in the reactor head space after 2 h of hydrolysis at 160 °C contained 98% nitrogen, 1.63% hydrogen, and traces of CO2. It is interesting to consider this finding in the context of our previous work, 12 where, for the first time, hydrogen generation during the oxidation of NaLS to vanillin was reported. Although the mechanism of hydrogen formation could not be clearly elucidated at that time, it was postulated that either hydrogen might be generated in the liquid phase as a result of dehydrogenation of NaLS during the oxidation process or it was generated in the gas phase from CO and H2O by the water− gas shift reaction. The lack of oxygen in the system in the experiments reported here shows that hydrogen can be

Figure 5. Effects of (a) intensity of mixing and (b) initial NaLS concentration [(●) 170, (○) 220, (▼) and 270 g L−1; 500 rpm] and temperature on vanillin concentration during hydrolysis. Time zero is when the temperature in the reactor reached: (a) 120 and (b) 100 °C.

amount of vanillin produced was independent of agitation conditions at each temperature. These results show that the hydrolysis was unaffected by agitation and controlled only by the temperature, provided that the concentration of NaOH is sufficient to maintain pH > 12.10 Finally, another similar experiment was undertaken to try to establish the kinetics of the hydrolysis to vanillin. Ideally, a D



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In eq 5, the parameters a and b depend only on the concentration of NaOH (M)17 a = −0.010986M − 1.461 × 10−3M2 + 2.03528 × 10−5M3

(5a)

b = 1 − 1.34141 × 10−3M + 7.07241 × 10−4M2 − 9.5362 × 10−6M3

The correction for vapor pressure suppression due to the presence of NaOH is important; for example, the vapor pressure at 180 °C is 10.0 bar for pure water, but it is only 8.8 bar at the NaOH concentration in the reactor (based on the amount of NaOH present and neglecting the effects of the other components in the liquid phase). 4.2.1. Dead-End System: Hydrolysis Followed by Oxidation. A solution of 220 g L−1 NaLS in 3 M NaOH containing 1.17 g L−1 Cu2+ was charged into the reactor at room temperature and atmospheric pressure, and the impeller speed was set at 500 rpm. Nitrogen was sparged through the reactor for 10 min; after that time, the reactor was sealed and heated to 160 °C, and pure oxygen was introduced to give the desired partial pressure of oxygen in the reactor (between 1.3 and 6 bar). The pressure controller was set to keep the total pressure in the reactor constant by replacing the oxygen consumed during the reaction with fresh oxygen from the gas cylinder. The inflow of oxygen to the reactor was continuously monitored, and the concentrations of vanillin and vanilic acid were measured in samples taken at appropriate time intervals. Typical results are summarized in Figure 7.

Figure 6. Initial vanillin production rate as a function of (a) NaLS concentration and (b) temperature.

generated without a water gas shift reaction. At this point, however, an explanation for the presence of H2 during hydrolysis here is still not possible. 4.2. Hydrolysis and Oxidation of Lignosulfonate to Vanillin. The results discussed in the preceding section clearly show that a significant amount of vanillin is produced from NaLS by hydrolysis at temperatures above 100 °C. At temperatures between 120 and 140 °C, hydrolysis is relatively slow, but its rate increases exponentially with temperature. As the oxidation of NaLS to vanillin at the industrial scale is carried out between 120 and 180 °C, this finding implies that the oxidation and hydrolysis reactions can occur in parallel. Investigation of the kinetics of parallel reactions in such a complex system is very difficult, if possible at all. Therefore, to investigate further the conversion of NaLS to vanillin, the combined kinetics of hydrolysis and oxidation were investigated in two flow configurations. All of the liquid reactants were added to the reactor at the start of the run in both cases, but the gas either flowed through the reactor (semibatch) or was added on demand to replenish only the amount that had reacted (dead-end mode). In both cases, the pressure in the reactor was held constant. In the first case, the mass of oxygen consumed during reaction was determined from the difference between the concentrations of oxygen in the gas at the inlet and at the outlet from the reactor, and in the second case, it was measured directly with a gas flow meter. The mass of oxygen in the reactor was expressed in terms of its partial pressure (pO2), calculated from pO = (pT − pNaOH )yO 2

2

Figure 7. Concentrations of (●) vanillin and (○) vanillic acid in the dead-end oxidation with pure oxygen at a constant partial pressure of oxygen of 1.3 bar and a constant temperature of 160 °C. The dotted line shows the time course of the oxygen flow rate into the reactor. Time zero is when the temperature in the reactor reached 160 °C.

At time zero, the oxygen flow was started, the concentrations of vanillin and vanilic acid were already 8.2 and 0.20 g L−1, respectively. Because the concentration of vanillin at time t = 0 was already close to the maximum concentration achieved during hydrolysis (as shown in Figure 3), it is reasonable to conclude that all further conversion to vanillin in this dead-end experiment was due to oxidation. During the first 50 min of oxidation after the oxygen flow commenced, the concentration of vanillin increased to 12 g L−l, and that of vanillic acid increased to 2.1 g L−1; after that, there was little further change. These results on their own might be considered to show that oxidation was completed because all of the NaLS available for oxidation had been consumed. However, the measurement of the oxygen inflow rate (dotted line in Figure 7) clearly shows that the oxygen

(4)

where pT is the total pressure in the reactor and yO2 is the oxygen mole fraction in the gas. The water partial pressure above the NaOH solution, pNaOH, was calculated as log pNaOH = a + b log p0

(5b)

(5) E



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flow into the reactor had ceased. In addition, the analysis of the composition of the gas phase in the head space showed an absence of oxygen and the presence of H2 (as previously reported12). When the initial mass of oxygen in the reactor was doubled by increasing its partial pressure to 2.6 bar, the oxidation proceeded faster, increasing the concentration of vanillin to 15 g L−1 after approximately 40 min. Further increases of the initial mass of oxygen by operating at oxygen partial pressures of 4 and 6 bar (Figure 8) increased the oxidation rate so that the maximum concentration of vanillin was reached more quickly, but its value (∼15 g L−1) became independent of partial pressure.

this observation is contrary to the literature, where it has been suggested that vanillic acid results from the oxidation of vanillin, whereas our results indicate that these two components might also be produced simultaneously. The results discussed previously also show that this vanillin yield during a dead-end process (hydrolysis followed by oxidation) depends on the oxygen partial pressure (which impacts the oxygen concentration in the liquid phase), temperature, and reaction time. Although hydrolysis was very fast at these temperatures, the rate of production of vanillin was found to depend strongly on the oxygen partial pressure and to increase approximately monotonically from 0.08 g L−1 min−1 at pO2 = 1.3 bar to 0.36 g L−1 min−1 at pO2 = 6 bar. The batch (dead-end) mode of processing is not used for the oxidation of NaLS at the industrial scale. Yet, as shown already, the kinetics of oxidation of NaLS to vanillin with the sparged gases in flow-through mode cannot be determined in a dead-end reactor because the gaseous hydrogen produced during the reaction lowers the partial pressure of oxygen by preventing further oxygen from entering the reactor to replace what has been consumed. However, most previous studies have used this deadend mode. Here, all further experiments reported were carried out in flow-through mode. 4.2.2. Flow-through System: Hydrolysis Followed by Oxidation. The effects of temperature, oxygen partial pressure, mixing intensity, concentration of NaLS, and gas flow rate on the vanillin yield and kinetics of hydrolysis and oxidation were investigated. In each case, a solution of NaLS in 3 M NaOH containing 1.17 g L−1 Cu2+ as a catalyst was charged into the reactor at room temperature and atmospheric pressure, and the impeller speed was set. During continuous agitation, the temperature was increased to a predetermined value under nitrogen, and when that temperature had been reached, air or an oxygen/nitrogen mixture was introduced into the reactor at a preset flow rate with the temperature, oxygen partial pressure, and total pressure in the reactor kept constant at a range of different values. Effects of Temperature When Held Constant at 140, 160, or 180 °C. The transient concentrations of vanillin and vanillic acid during reaction are compared in Figure 9. At 140 °C, the vanillin concentration at time t = 0 (time when the temperature in the reactor reached the specified temperature and when air was introduced) was less than the maximum vanillin concentration from hydrolysis (approximately 8 g L−1; see Figure 3), probably because the time spent at temperatures during which hydrolysis could occur was too short. After air had been introduced, vanillin was produced at a rate of 0.4 g L−1 min−1 by simultaneous oxidation and hydrolysis. However, after approximately 20 min at 140 °C (by which time, based on the earlier dead-end runs, the rate of hydrolysis had decreased), the vanillin production rate also decreased to 0.05 g L−1 min−1. At 160 and 180 °C, the heating time before air was introduced was sufficient to enable the maximum vanillin yield from hydrolysis (again as determined during the dead-end experiments; Figure 4) to be reached before air was introduced to the reactor. Consequently, the production of vanillin after that was by oxidation only, again with a rate of ∼0.05 g L−1 min−1. The change of kinetics from fast in the case of simultaneous hydrolysis and oxidation to slow in the case of oxidation only might suggest that the two reactions occur in parallel. The relatively low production rate of vanillin by oxidation (after hydrolysis was completed) might result from (1) the concentration of NaLS in solution being reduced by the

Figure 8. Concentrations profiles of (a) vanillin and (b) vanillic acid during oxidation of NaLS in dead-end mode at 160 °C and pO2 = (●) 1.3, (○) 2.6, (▼) 4.0, and (Δ) 6.0 bar. Time zero is when the temperature in the reactor reached 160 °C.

The profiles of vanillic acid followed the vanillin profile quite closely, with each compound reaching a maximum concentration at roughly the same time for each of the three higher partial pressures and then decreasing. As expected, the rates of all reactions, increased with increasing oxygen partial pressure, which suggests that vanillin was oxidized to vanillic acid. This sequence of reactions is in good agreement with that proposed for the oxidation of Kraft lignin to vanillin reported by Fargues et al.16 These results clearly indicate that, if there is enough oxygen in the gas phase, the maximum vanillin concentration produced is independent of processing conditions (i.e., oxygen partial pressure, temperature, and agitation). Indeed, as reported previously, the maximum vanillin yield depends mainly on the type of lignosulfonate utilized in the process. Figure 8 also shows that vanillin production was always associated with the production of vanillic acid, although the amount of the latter was often quite small. To a certain extent, F



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Figure 9. Effects of temperature on (a) vanillin and (b) vanillic acid concentrations: (●) 140, (○) 160, and (▼) 180 °C; pT = 11.5 bar; pO2 = 1.3 bar; air flow rate = 4.5 × 10−3 m3 min−1.

Figure 10. Effects of oxygen partial pressure on the concentration of (a) vanillin and (b) vanillic acid in the flow-through reactor: pO2 = (●) 1.3, (○) 2.4, and (▼) 3.6 bar; T = 160 °C; gas flow rate = 4.5 × 10−3 m3 min−1.

amount consumed during hydrolysis and (2) the vanillin concentration being close to equilibrium (i.e., higher vanillin concentration leading to faster degradation to vanillic acid). At 180 °C, the concentration of vanillin reached a maximum of 13.8 g L−1 after approximately 100 min and then decreased, probably as a result of oxidation to vanillic acid. At 140 and 160 °C, the vanillin concentration reached slightly higher levels of 14.5 and 15.2 g L−1, respectively, after approximately the same time, but no reduction in concentration was observed, probably because the vanillin oxidation rate was too low at these temperatures. Whereas the concentration of vanillin increased from time zero by 60−65%, the concentration of vanillic acid increased by approximately a factor of 10. The increase in vanillic acid concentration was greater with decreasing temperature, and at the highest temperature, the concentration also showed a weak maximum. Effects of Oxygen Partial Pressure at 1.3, 2.4, and 3.6 bar. By sparging with air, 40% O2/60% N2, or 60% O2/40% N2 at a total pressure of 11.5 bar, at a constant temperature of 160 °C and a constant flow rate of 4.5 × 10−3 m3 min−1, the effects of the oxygen partial pressure were investigated. The results are summarized in Figure 10. As can be seen, the effects of the oxygen partial pressure on the rates of vanillin and vanillic acid production were much stronger than the effects of temperature, and the concentrations of the two compounds passed through a maximum at the same time. Again, these observations throw doubt on the suggestion in the literature that the oxidation of vanillin to vanillic acid is the main kinetic route by which the latter is produced. In general, the concentration profiles in the flow-through reactor are rather similar to the profiles measured in the dead-end reactor (shown in Figure 8). In each case, the vanillin concentration passed through a maximum of 15 g L−1 when

sufficient oxygen was present in the reactor, and the maximum value was independent of the oxygen partial pressure. The oxidation rate of NaLS, however, was found to depend strongly on the oxygen partial pressure, and the rate of vanillin production decreased from approximately 0.36 g L−1 min−1 at pO2 = 3.6 bar to 0.08 g L−1 min−1 at pO2 = 1.3 bar. The time−concentration profile of vanillic acid closely followed that of vanillin, which also gives some support to the hypothesis that vanillic acid was produced by oxidation of vanillin,10,18 but on the other hand, the fact that the maximum concentrations of both vanillin and vanillic acid were detected at the same time (during both hydrolysis and oxidation) implies that the production of vanillic acid did not occur only sequentially and that there is probably also a direct route from NaLS. The maximum concentrations of vanillin and vanillic acid were similar in both the batch and flow-through reactors. Whenever vanillin was produced, so was carbon dioxide. As a result, the pH fell (Figure 11a) as the CO2 reacted with NaOH, and at higher O2 partial pressures, at a certain time, the fall was rapid from pH ∼13 to pH ∼9, allowing desorption of CO2 into the gas phase. This fall is important because, as pointed out earlier, pH is a key parameter in the reaction of NaLS to produce vanillin. It is necessary to control pH at a value higher than 12 if the reaction is to proceed at a satisfactory rate, and in this case, it was reduced below this value. From Figures 10 and 11, it is clear that the time at which the reduction of pH from approximately 13 to 9−10 at the two higher oxygen partial pressures corresponds to that at which the maximum in the vanillin concentration profile was observed. Up to this time, a certain amount of NaOH was consumed (Figure 11b). Overall, the maximum vanillin concentration in each case was ∼15 g L−1, and the amount of NaOH consumed to reach this G



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Figure 11. Effects of oxygen partial pressure on (a) pH and CO2 concentration in the gas phase and (b) specific NaOH consumption until the time when the concentration of vanillin reaches a maximum. Conditions: T = 160 °C; pO2 = (●) 1.3, (○) 2.4, and (▼) 3.6 bar; gas flow rate = 4.5 × 10−3 m3 min−1.

Figure 12. (a) Specific oxygen consumption and (b) specific vanillin yield at different oxygen partial pressures up to maximum vanillin yield. T = 160 °C, gas flow rate 4.5 × 10−3 m3 min−1.

0.0055 mol (kg of NaLS)−1 min−1, respectively, and at pO2 = 1.3 bar the corresponding values were 0.072 and 0.0034 mol (kgNaLS)−1 min−1. This increase in the reaction rates with oxygen partial pressure is associated with the increase in the concentration of dissolved oxygen in the aqueous NaOH solution (CO2), which can be expressed as16

maximum was also approximately constant [4.3, 4.9, and 4.8 g of NaOH (g of vanillin)−1 at pO2 = 1.3, 2.4, and 3.6 bar, respectively, equivalent to ∼320 g of NaOH (kg of NaLS)−1. Thus, the consumption of NaOH per unit mass of vanillin was found to be constant. Clearly, such consumption of NaOH is very high, and it not only adds to the cost but might also create environmental problems. However, the mass of NaOH consumed can be reduced as long as the pH throughout the process is kept above 12. At 1.3 bar, the time to reach the maximum vanillin yield was approximately 100 min, practically independent of temperature (Figure 9) However, with increasing oxygen partial pressure (Figure 10), this time decreased first to 60 min and then to 40 min at pO2 = 2.4 bar and pO2 = 3.6 bar, respectively. The mass of oxygen necessary to reach the maximum vanillin yield of 0.069 g of vanillin (g of NaLS)−1 (or 15.2 g L−1 for all of the runs reported in Figures 9 and 10) is practically independent of temperature and only weakly dependent on oxygen partial pressure (Figure 12a), increasing from 7.2 mol (kg of NaLS)−1 at pO2 = 1.3 bar to 8.4 mol (kg of NaLS)−1 at pO2 = 3.6 bar. These values are in good agreement with the literature, which reports a maximum vanillin yield at oxygen consumption rates between 7.8 and 10.9 mol (kg of NaLS)−1.19 Figure 12 shows that there is a close correlation between the specific oxygen consumption rate (which can be considered the overall reaction rate) and the vanillin production rate. At pO2 = 3.6 bar, the overall specific reaction rate is 0.21 mol (kg of NaLS)−1 min−1, and the specific vanillin production rate is 0.008 mol (kgNaLS)−1 min−1. As the oxygen partial pressure was decreased to pO2 = 2.6, the two rates decreased to 0.134 and

⎛ CO2 = ⎜3.559 − 6.659 × 10−3T − 5.606pO 2 ⎝ + 1.594 × 10−5pO T 2 + 1.498 × 103 2

× 10−0.144M

pO ⎞ 2 ⎟ T ⎠ (6)

Equation 6 shows that the concentration of O2 in the liquid phase is linearly dependent on the oxygen partial pressure. The experimental results discussed in the next section (see eq 8) show that the vanillin production rate is first-order with respect to the O2 partial pressure. Therefore, vanillin oxidation is approximately first-order with respect to the oxygen concentration in the liquid phase.

5. ENGINEERING ASPECTS OF OXIDATION OF NALS TO VANILLIN The first step in the scale-up of a chemical reaction from the laboratory to industry is selection of the type of process (batch or continuous) and type of reactor (column or stirred vessel). Continuous processes are typically preferred by industry and can be run both in columns and in stirred vessels. Selection between the two types of reactors depends, among other factors, on the rate and kinetics of the chemical reaction in the liquid phase, the rate of mass transfer between the gas and liquid phases, and the ratio of the gas and liquid flow rates.20 Obviously, investment and H



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running costs are always taken into consideration as well, but those aspects are not discussed here. The overall kinetics (chemistry and mass transfer) of oxidation of NaLS to vanillin and the flow rates of the two phases determine the residence time necessary to reach the maximum yield and the size of the reactor. In terms of residence time, stirred vessels are more flexible than columns, and this is probably why investigations of the oxidation of NaLS to vanillin reported in the open literature have mainly been performed in stirred vessels.16,21,22 A packed column used to investigate the oxidation of Kraft lignin was found to give a maximum yield of vanillin within approximately 60 min.23 In the work reported here, the reaction time necessary to reach the maximum vanillin yield was approximately 100 min (Figure 8), and such a long residence time requires stirred vessels. A detailed discussion of the fluid dynamics of stirred gas−liquid reactors can be found elsewhere.24 Here, a few critical features are discussed. In principle, the recent progress in numerical simulation [i.e., computational fluid dynamics (CFD)] enables the detailed prediction of velocity and concentration fields in stirred reactors, but this approach is possible only when details of the chemical reactions and rates of mass transfer between the phases are known. Even then, the physics of gas−liquid dispersions is not well understood, so that, in fact, such approaches are generally research topics in their own right. In the engineering approach to the scale-up/modeling of stirred reactors, two problems are mainly considered: flow pattern and overall reaction rate. It is commonly assumed that stirred vessels are perfectly mixed [i.e., that all spatial gradients (velocity, concentration, temperature) are equal to zero], and the intensity of mixing is described in terms of specific energy dissipation rate, frequently assumed to be constant throughout the volume of the reactor. In sparged systems, the mean specific energy dissipation rate depends on the agitator type, size, and speed and the gas flow rate and can be estimated from a number of literature correlations. However, at the pressure, temperature, and scale of the process investigated here, the gas flow rate, QG is 0.55 × 10−3 m3 min−1 to give a gas flow number of FlG = QG/ND3 = 2.6 × 10−3. With a Rushton turbine at this flow number, the decrease in power due to sparging would be negligible.24 The power input to the reactor under gassed conditions (Pg) can be estimated from the power number (Po), which is ∼4 for this impeller at this scale13

Figure 13. Transient concentration of vanillin at different (a) initial concentrations of NaLS [(●) 170, (○) 220, and (▼) 270 g L−1] and (b) initial reaction rate at pO2 = 1.3 bar and T = 160 °C.

a straight line with slope equal to −0.1 (r2 = 0.9909) was obtained; therefore, j is essentially zero; that is, the oxidation of NaLS to vanillin is of zero reaction order with respect to CNaLS. In a similar way, from the relationship between the vanillin production rate and the initial oxygen partial pressure shown in Figure 10a, the reaction order with respect to the initial oxygen partial pressure was found to be m = 1.08 (r2 = 0.9996); that is, the oxidation of NaLS to vanillin is a first-order reaction with respect to the oxygen partial pressure (and, therefore, the oxygen concentration in the liquid phase) and the reaction rate takes the form rVO = kOpO

PoρN3D5 P = = V V V Pg

2

(7)

The latter value is in reasonable agreement with the literature, where values between 0.6 and 1.75 have been reported. For example, Tarabanko et al.25 reported m = 0.6 for the oxidation of aspen wood at pH 11.6 and 80−110 °C, Santos et al.26 reported m = 1 for the oxidation of eucalyptus lignosulfonate at temperatures between 130−150 °C; and Fargues et al.16 reported m = 1.75 for the oxidation of Kraft lignin at pH > 11.5. Equation 8a implies that hydrodynamic conditions have no effect on the reaction rate. However, this conclusion contradicts the experimental data shown in Figure 14, which clearly show that both the oxygen consumption and the vanillin production rate (but not yield) depend on impeller speed. In such cases, the overall oxidation rate accounting for the mass-transfer resistances in both the gas and liquid phases and the kinetics of the chemical reaction should be used. Assuming that the catalyst is completely dissolved, so that the reaction occurs in the liquid phase, the overall reaction rate of the oxygen (−rO2) can be calculated from20

where V is the liquid volume and ρ is the liquid density. Assuming that all resistances to mass transfer are negligible, the kinetics of the oxidation of vanillin can be analyzed using a simple equation relating the reaction rate to the initial (t = 0) concentrations of lignosulfonate and oxygen at the time it is introduced into the reactor at operating temperature

rVO = kOpO m C NaLS j

(8)

2

rOV

(8a)

O

where is the rate of NaLS oxidation to vanillin; k is the oxidation rate constant; and m and j are the reaction orders with respect to oxygen and NaLS, respectively. The transient concentrations of vanillin measured at different initial concentrations of lignosulfonate are shown in Figure 13a and were used to estimate reaction order j and reaction constant kO in eq 8 by plotting the initial vanillin production rate as a function of the initial NaLS concentration. In log−log coordinates (Figure 13b), I



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percentage of the liquid-phase resistance in the film and in the bulk. At 500 rpm and pO2 = 1.3 bar, the overall oxygen reaction rate (rO2) calculated from the experimental results shown in Figure 12a is approximately 0.285 mol of O2 s−1 m−3. The specific liquid mass-transfer coefficient, kLa from eq 10 is on the order of 0.08− 0.09 s−1. The oxygen partial pressure in the reactor varies with time between 0.01 bar at time zero and 0.5 bar at 100 min, so the average is approximately 0.2 bar. With these values, the resistance might vary between 30% and 70% in the liquid film and between 70% and 30% in the bulk of the liquid, which shows that neither of these resistances can be neglected during the design of the reactor. Obviously, the detailed kinetics of oxygen consumption is necessary, but these can be determined much more easily than the kinetics of NaLS oxidation to vanillin. From Figure 14, it is clear that the oxygen consumption rate and vanillin yield are not affected by the impeller speed during the first 20 min of the oxidation. During that time, the measured oxygen concentration in the gas phase at the outlet of the reactor is very low (on the order of 0.01 bar). These results indicate that, initially, the oxidation is very fast and occurs very close to the interface (or at the interface), therefore, it is insensitive to the liquid-phase mass-transfer coefficient. After that time, both oxygen consumption and vanillin production are significantly lower at the lowest speed (kLa = 0.043 s−1), which implies that, at this speed, the oxidation becomes mass-transfer-controlled. As the impeller speed is increased to 500 and 650 rpm (kLa = 0.085 and 0.146 s−1, respectively), differences between the oxygen consumption and vanillin production rates become less noticeable, implying that the oxidation is now controlled by the chemical kinetics. This also means that the reaction zone moves deeper into the liquid film and into the bulk of the liquid. When the oxygen supply was reduced by lowering the air flow rate, which also led to a reduction of the hold-up (data not shown), the changes in oxygen consumption rate and vanillin yield concentration were similar to those discussed previously.

Figure 14. Effects of impeller speed on (a) oxygen consumption until the time that the maximum vanillin yield was reached and (b) corresponding vanillin yield. Conditions: T = 160 °C, pO2 = 1.3 bar, air flow rate = 4.5 × 10−3 m3 min−1 .

−rO2 =

1 1 k Ga

+

H kLae

+

H kC NaLS f

pO

2

(9)

where kGa and kLa are the specific mass-transfer coefficients of gas and liquid, respectively; H is the Henry’s constant; k is the reaction rate constant; and e and f are enhancement factors. At a sufficiently high mean specific energy dissipation rate, because the oxygen is sparingly soluble, the resistance to oxygen mass transfer in the gas phase (1/kGa) in sparged stirred reactors is typically negligible.25 To estimate the resistance in the liquid film (H/kLae) and in the bulk of the liquid (H/kCNaLS f), the specific liquid mass-transfer coefficient (kLa), Henry’s constant (H), and reaction rate constant (k) are necessary. The specific liquid masstransfer coefficient for oxygen in low-viscosity liquids has been determined many times for different low-viscosity systems including water and electrolyte solutions. It has been shown to depend strongly on the precise composition of the liquid, so an accurate value for the present solution is not available. However, a reasonable estimate can be made from a generic correlation developed for electrolyte solutions27 ⎛ Pg ⎞0.7 kLa = 2.0 × 10−3⎜ ⎟ vs 0.2 ⎝V ⎠

6. CONCLUSIONS Oxidation of NaLS to vanillin is extremely complex, and details of its chemical kinetics are far from clear. It is commonly accepted in the literature that there are two types of reaction that dominate: hydrolysis, caused by a high temperature and strongly alkaline conditions, and catalytic oxidation. These processes have been investigated using air and different O2/N2 mixtures as oxidizing agents in such a way that the two reactions occurred simultaneously, and the concept of overall effective kinetic parameters was employed to describe the vanillin yield. In this work (for the first time in the open literature), the two reactions have been partially separated, while all process parameters were still maintained in the ranges typical for industrial applications. This approach enabled the determination of the effects of selected process (engineering) parameters on the vanillin yield and selectivity of each reaction. Initially, NaLS solution was stirred at an elevated constant temperature in a batch reactor in the presence of nitrogen, which showed that vanillin could be produced by hydrolysis only. Except at the lowest temperature, the vanillin concentration reached a plateau at a rate that increased with increasing temperature until hydrolysis was complete. However, neither the presence of catalyst nor the mixing intensity had any effect. Hydrolysis started at just above 100 °C, and it produced not only vanillin but also vanilic acid, traces of acetovanillone, and

(10)

where vs is the superficial gas velocity. Henry’s constant for oxygen in 3 M solution of NaOH at 160 °C is not available in the literature, but it can be estimated from the solubility of oxygen in 1.5 M solution of NaOH28 as H = 3.2 × 104 Pa m3/mol. Detailed calculations of the resistances in the liquid film and in the bulk require the oxidation rate constant, which is very difficult to estimate with reasonable accuracy. Therefore, eqs 9 and 10 together with experimental data were used to estimate the J



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multiple other compounds. These observations contradict the literature in a number of ways: first, vanilic acid is not a product of only the oxidation of vanillin, and second, hydrolysis starts at much lower temperatures (120 °C) than those reported in the literature (150−170 °C). It has also been found that hydrogen is produced during hydrolysis, but the mechanism is far from clear. During oxidation, the oxygen partial pressure was the critical parameter determining reaction rate or, in other words, the residence/reaction time necessary to reach maximum vanillin yield. As the partial pressure increased, the oxidation rate of NaLS to vanillin also increased, but the maximum vanillin yield remained constant. However, the increase in oxygen partial pressure also led to an increased vanillin oxidation rate after the maximum vanillin concentration was reached. This increase means that, at higher oxygen partial pressures, the vanillin yield becomes more sensitive to the residence time and the control of the process might be more difficult. The effects of temperature and maximum vanillin yield were practically negligible. Overall, approximately 55% of the vanillin was produced by hydrolysis and 45% by oxidation. In subsequent experiments, air or mixtures of nitrogen and oxygen to give higher partial pressures of O2 were introduced to the reactor so that both hydrolysis and oxidation occurred. Both dead-end and gas flow-through conditions were employed. The effect of impeller speed on vanillin yield indicated that, during oxidation, mass transfer controls the overall reaction rate but this effect diminished when hydrolysis and oxidation occurred simultaneously. These results also indicate that, during modeling/scale-up, resistance to mass transfer in the liquid phase has to be considered.

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Pg = power input to the reactor under gassed conditions, W p0 = water vapor pressure pNaOH = water partial pressure above NaOH, bar pO2 = oxygen partial pressure, bar pT = total pressure in the reactor, bar Po = power number QG = gas flow rate, m3 s−1 R = universal gas constant rHV = rate of NaLS hydrolysis to vanillin, g s−1 L−1 rOV = rate of NaLS oxidation to vanillin, g s−1 L−1 T = temperature, °C or K as appropriate t = time, min or s as appropriate V = volume of liquid, m3 vs = superficial gas velocity, m s1 yO2 = oxygen mole fraction in the gas ρ = liquid density, kg m−3

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the EPSRC for financial support from Grant EP/G011133/1, Borregaard for the supply of NaLS, and the other members of the CentaCAT team at Queen’s University Belfast and the Castech project for help with and discussions related to this work.

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NOMENCLATURE A = constant in the Arrhenius equation CNaLS = concentration of lignosulfonate, g L−1 D = impeller diameter, m E = activation energy, J mol−1 e = enhancement factor in eq 9 f = volume fraction of liquid in the reactor FlG = gas flow number H = Henry’s constant, Pa m3 mol−1 j = reaction order in eq 8 kGa = specific gas mass-transfer coefficient, s−1 kH = hydrolysis rate constant, (g L−1)1−n s−1 kLa = specific liquid mass-transfer coefficient, s−1 kO = oxidation rate constant (g L−1)1−n s−1 M = concentration of NaOH mol L−1 m = reaction order in eq 8 N = impeller speed, s−1 n = reaction order in eqs 1 and 2 K



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(17) Balej, J. Water-Vapor Partial Pressures and Water Activities in Potassium and Sodium Hydroxide Solutions over Wide Concentration and Temperature Ranges. Int. J. Hydrogen Energy 1985, 10, 233. (18) Fargues, C.; Mathias, A.; Silva, J.; Rodrigues, A. Kinetics of vanillin oxidation. Chem. Eng. Technol. 1996, 19, 127. (19) Salvesen, J. R.; Brink, D. L.; Diddams, D. G. Process for making vanillin. U.S. Patent 2,434,626, 1948. (20) Levenspiel, O. Chemical Reaction Engineering, 3rded.; John Wiley & Sons: New York. 1999. (21) Araujo, J. D. P.; Grande, C. A.; Rodrigues, A. E. Vanillin production from lignin oxidation in a batch reactor. Chem. Eng. Res. Des. 2010, 88, 1024. (22) Araujo, J. D. P.; Grande, C. A.; Rodrigues, A. E. Structured packed bubble column reactor for continuous production of vanillin from Kraft lignin oxidation. Catal. Today 2009, 147, S330. (23) Sridhar, P.; Araujo, J. D.; Rodrigues, A. E. Modeling of vanillin production in a structured bubble column reactor. Catal. Today 2005, 105, 574. (24) Nienow, A. W. Hydrodynamics of Stirred Bioreactors. Appl. Mech. Rev. 1998, 51, 3. (25) Tarabanko, V. E.; Pervishina, E. P.; Hendogina, Y. V. Kinetics of aspen wood oxidation by oxygen in alkaline media. React. Kinet. Catal. Lett. 2001, 72, 153. (26) Santos, S. G.; Marques, A. P.; Lima, D. L. D.; Evtuguin, D. V.; Esteves, V. I. Kinetics of Eucalypt Lignosulfonate Oxidation to Aromatic Aldehydes by Oxygen in Alkaline Medium. Ind. Eng. Chem. Res. 2011, 50, 291. (27) Van’t Riet, K. Review of Measuring Methods and Results in Nonviscous Gas−Liquid Mass Transfer in Stirred Vessels. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 357. (28) Tromans, D. Modeling oxygen solubility in water and electrolyte solutions. Ind. Eng. Chem. Res. 2000, 39, 805.

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Catalytic Conversion of Sodium Lignosulfonate to Vanillin

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