ARTICLE pubs.acs.org/IECR
Glucose Oxidation in Slurry Reactors and Rotating Foam Reactors Roman Tschentscher,* Tjeerd Alexander Nijhuis, John van der Schaaf, and Jaap C. Schouten Laboratory of Chemical Reactor Engineering, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ABSTRACT: Glucose oxidation using platinum promoted by bismuth was studied as a three-phase model reaction. Using catalyst nanoparticles and measuring the oxygen concentration in the bulk liquid, the kinetic parameters and mass transfer characteristics were determined at a temperature of 333 K. The overall reaction rate was studied experimentally using three different support types: slurry catalysts, pellets, and a solid foam stirrer. The glucose conversion rate and the deactivation rate of the catalyst depend strongly on the ratio between mass transfer and reaction rate. At low catalyst concentrations, the glucose oxidation process is liquidsolid mass-transfer-limited. The block stirrer shows a superior performance over the slurry catalysts due to the high liquidsolid volumetric mass transfer coefficient. The bimodal pore size distribution of the catalyst layer further increases the conversion rate. Using slurry catalysts, a loading of 1 wt % in combination with pure oxygen feed is required to achieve acceptable conversion rates. Under these conditions, the gasliquid mass transfer and partially the liquidsolid mass transfer are the rate-limiting steps. The foam block stirrer shows good gasliquid mass transfer rates and high liquidsolid mass transfer rates, which still increase at high power input. Working under external mass transfer control and using this stirrer type, the catalyst loading can be strongly reduced to loadings less than 0.4 wt %, while the conversion rate remains comparable to slurry particles with loadings of 1 wt %. As the catalyst is fixed, attrition and agglomeration in high viscosity liquids are circumvented. There is further no need for filtration, and the catalyst can simply be reused.
’ INTRODUCTION Gluconic acid is an important intermediate used for the production of nutritients and pharmaceuticals and finds applications as a biodegradable chelating agent.1,2 It is commonly produced by enzymatic oxidation. The chemical route to oxidize carbohydrates using noble metal catalysts supported on carbon or metal oxides is, however, an attractive alternative. It is comparably cheap, environmentally friendly using water as solvent, and requires moderate conditions. At temperatures below 363 K, using air high yields and selectivities could be achieved.3 One advantage of using enzymes is their insensitivity to SO2 and other sulfur derivates, whereas noble metals are easily poisoned by sulfur.4 Another argument against the use of noble metals for glucose oxidation is the high catalyst-to-reactant ratio. In this work, we will, however, show that the process conditions can have a strong influence on the reactor performance. By optimizing the gas feed, the mass transfer and the structure of the catalyst support the overall reaction rate can be strongly enhanced. This makes the usage of noble metal catalysts advantageous. ’ GLUCOSE OXIDATION ON HETEROGENEOUS CATALYSTS Glucose Oxidation on Platinum Group Metals. Glucose oxidation has been extensively studied using platinum group metals and even gold catalysts. Platinum and palladium show turnover frequencies comparable to enzymes.4 The selectivity to gluconate is higher than 98%.5 Only traces of glycolaldehyde and glyceraldehyde have been found. The formation of fructose at temperatures below 343 K is negligible. Furthermore, promoters can be used, such as bismuth or ruthenium. They are inactive if used alone, but deposited on the metal surface, they enhance the reaction rate and can even shift the product distribution r 2011 American Chemical Society
remarkably.68 Reaction rates 20 times higher than using pure Pd have been reported.5,9 In oxidizing conditions, bismuth forms bismuth-gluconate and bismuth-glucose complexes.10 Besson and Gallezot11 have extensively studied this catalyst system. They postulate that the bismuth-glucose complex allows a better orientation of the substrate to the noble metal surface and thereby facilitates the glucose dehydration. In addition, the formation of a bismuthgluconate complex can favor the desorption of gluconate and thereby increase the reaction rate. There are, however, several mechanisms of deactivation occurring, such as overoxidation and chemical poisoning, catalyst leaching, and sintering. Chemical poisoning can occur under reducing conditions due to the formation of organic residues on the active sites at low catalyst potential.3,12 This is especially important at the start of a batch process when the glucose concentration is high. Increasing the oxygen concentration or the mass transfer leads to an increase in the catalyst potential and an oxidative removal of these species.12,16 Nevertheless, at moderate pH values between 6 and 9, no formation of high-molecularweight byproducts was observed.10 With increasing pH, the oxidation rate increases, and the reaction products desorb faster from catalyst. At pH values above 10, byproducts can be formed, coking can occur, and catalyst leaching can inhibit the reaction.10 Even at pH values lower than 9 and ambient temperature, catalyst leaching can be an issue because the product gluconic acid is a chelating agent.6 Investigating the catalytic properties of Special Issue: Nigam Issue Received: April 3, 2011 Accepted: June 7, 2011 Revised: June 3, 2011 Published: June 27, 2011 1620
dx.doi.org/10.1021/ie200694z | Ind. Eng. Chem. Res. 2012, 51, 1620–1634
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Elementary Reaction Steps during Glucose Oxidation13,16 no.
elementary step
rate or equilibrium equation
Adsorption 1
ROH + *p T ROHad
2
1/2O2 + *chT Och
K1 = θgluc/cglucθp* K2 = θO/(cO2)1/2θch*
Surface Reaction 3
ROHad + H2O T R(OH)2,ad
4
R(OH)2,ad + Och + *ch f
K3 = θgem/θgluccH2O r4 = k4θgemθOθch*
R(OH)Oad + H2O + 2*ch 5
R(OH)Oad + OH T ROO +
K1 = θglacθp*/θglaccOH
H2O + *p Overoxidation 6
Och + *x f *ox + *ch
r6 = k6θOθx*
7 8
*ox + *ch f Och + *x *ox + R(OH)2,ad + *ch f
Reactivation r7 = k7θoxθch* r8 = k8θgemθoxθch*
R(OH)Oad + H2O + 2*ch Balance for Catalytic Sites I
physisorption
θp* + θgluc + θgem + θglac = 1
II
chemisorption
θch* + θO + θox = 1
III
inactive sites
θx* + θox = 1
Bi/Pd catalysts, Karski et al.9 found no leaching of Pd. Therefore, sintering of particles and Ostwald ripening due to redeposition can be excluded. However, leaching of the promoter Bi due to its affinity to oxygen is a problem. Losses of up to 80% have been reported.8,9 Other promoters, such as ruthenium do not leach that strongly; losses of less than 1 wt % per run have been found.7 Washing out of bismuth can be avoided by applying reducing conditions at the end of the batch process or the reactor outlet, as shown by Hanika et al.1315 Reusing the catalyst in this way, a constant activity over six runs has been reported by Besson et al.5 The major process for catalyst deactivation is, however, the overoxidation of the noble metal.13,16 It depends on the relative affinity of the metal to oxygen and to the organic molecule.3 A redox equilibrium with the surface metal, oxygen as oxidant and glucose as reductor results in a competitive adsorption. As the glucose concentration decreases, the oxygen coverage becomes overwhelming. Small particles deactivate faster than large ones due to the higher surface density of low coordinated sites, such as surface steps and edges.3 The catalyst potential can be kept low by using a low oxygen feed concentration or working under oxygen mass transfer limitation.12 Another way would be to expose the catalyst to oxidizing and reducing conditions alternatively by switching between air and nitrogen as feed gas or by using a chemical looping reactor having an oxidation and a reduction zone.1316 A batch process could be easily controlled measuring the catalyst potential, in the case of platinum, using a Pt-wire as electrode.12 A high redox potential of the metal results in less overoxidation. Platinum is, therefore, more stable against overoxidation than palladium. The addition of bismuth as promoter has three effects on the catalyst overoxidation. First, as already discussed, the reaction rate is increased due to the orientation of glucose on the noble metal surface. This reduces the concentration of oxygen at the catalyst surface and the rate of overoxidation.3 Second, bismuth and the product gluconate form a complex. This enhances the
product desorption and further increases the reaction rate.10 The main effect is, however, the oxygen transfer effect. Bismuth is less noble than platinum. The higher affinity to oxygen results in a fast oxidation of bismuth. The activated oxygen is then transferred to the platinum sites to react with the adsorbed glucose.5,11 With time, bismuth is dissolving from the catalyst surface. By exposing the reaction mixture to reducing conditions, not only the noble metal particles are reactivated, but also bismuth is reduced and redeposits onto the platinum surface. Various works have investigated the use of gold catalysts for carbohydrate oxidations.1,17,1921 The reaction rate depends strongly on the size of the gold nanoparticles.5,18 Comparable reaction rates are found only under high pH conditions resulting in low selectivities of
