LA-CONTAINING SBA-15 / H 2 O 2 SYSTEMS FOR THE MICROWAVE ASSISTED OXIDATION OF A LIGNIN MODEL PHENOLIC MONOMER

A convenient and efficient application of heterogeneous Lanthanum-containing SBA-15 systems for the microwave assisted oxidation of a lignin model phenolic monomer, 3-methoxy4-hydroxybenzyl alcohol, is reported. Environmental friendly and low-cost H 2 O 2 was used as the oxygen atom donor. The catalyst was prepared by immobilizing Lanthanum species on to the periodic mesoporous channels of siliceous SBA-15. Powder X-ray diffraction data and Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) revealed that the host retains its hexagonal mesoporous structure after immobilization and most of the lanthanum species are better dispersed in the calcined materials. The surface area and pore size of La/SBA-15 was considerably decreased indicating the intrapore confinement of the Lanthanum species. The activity of the La/SBA-15 was investigated in the oxidation of 3-methoxy-4-hydroxybenzyl alcohol in the presence of hydrogen peroxide as oxidant. 68% conversion of 3-methoxy-4-hydroxybenzyl alcohol to vanillin or other undetectable by-products was obtained after 30 min of reaction under 200W microwave irradiation, compared to a poor 25% degradation after 24 h under conventional heating. The possibility of recycling the catalyst was studied.


INTRODUCTION
As a three dimensional amorphous plant poly-phenol, lignin is the second most abundant biopolymer on earth (Wool and Sun 2005).Currently, the vast majority of lignin is burned without any industrial application.Increasing research efforts have been made over the past few years to develop environment-friendly processes, for example, by the use of oxygen, hydrogen peroxide (H 2 O 2 ), and ozone as primary oxidants.Due to the formation of radical intermediates, the conventional chemical oxidation of lignin leads to a poor selectivity and final product yield.Microwave assisted reactions can be highly efficient and polluteless, allowing a reduction in reaction time and energy consumption together with an increase in yields and selectivity in some cases (Conesa et al. 2007;Kappe 2008).Therefore, microwave assisted selective catalytic process based on a concerted oxygen atom transferred from H 2 O 2 might solve these problems with water as the only by-product.
Since 1990s, the discovery of a new family of ordered mesoporous silica materials has sparked considerable interest because of their regular pore array with uniform pore diameter (2.0-8.0 nm), high surface area and pore volume (Kresge et al. 1992).In the family of mesoporous molecular sieves, SBA-15 exhibits larger pore sizes and thicker pore walls compared with other materials (Zhao et al. 1998a).Highly ordered SBA-15 or SBA-15 modified with some noble metals has become in recent years an important catalyst for a variety of synthetic transformations, such as selective oxidation of styrene, cyclohexane and total oxidation of toluene (Reddy et al. 2009;Bendahou et al. 2008;Zhang et al. 2007).
As an important promoter, the rare-earth elements have been used widely in the catalysts (Cui et al. 2006;Ma et al. 2006;Jia et al. 2008).Amongst the rare-earth elements, Lanthanum is the most common and important element to be used as catalysis material.
In the present work, we report the synthesis of Lanthanum-containing SBA-15 mesoporous molecular sieves by a direct synthesis method in an acid medium.The properties of La/SBA-15 were characterized by powder X-ray diffraction (XRD) and N 2 adsorption-desorption analysis.The most commonly studied lignin model phenolic monomer, 3-methoxy-4-hydroxybenzyl alcohol, was chosen as target molecule and its oxidation was investigated with a heterogenised La/SBA-15 catalyst and microwave irradiation.The use of model compounds enables optimum (typically catalytic) conditions to be more easily determined on more complex lignin derived substrates.To the best of our knowledge, few studies dealing with the use of immobilized La-containing SBA-15 for the oxidation of lignin model compounds have been reported (Badamali et al. 2009).

Preparation of La/SBA-15.
La/SBA-15 was prepared using a direct synthesis procedure according to the procedure reported by Zhao et al. (1998a) with minor modifications.
Two g triblock poly(ethylene oxide) 20 -poly(propylene oxide) 70 -poly(ethylene oxide) 20 (P123, average molecular mass about 5800, Aldrich) and 0.5 g lanthanum nitrate (La(NO 3 ) 3 •6H 2 O) were dissolved in a mixture of 35 mL 2 mol•L -1 HCl and 15 mL deionized water (pH≈1) under stirring.Then 4 g of tetraethyl orthosilicate (TEOS) were added to this solution (the molar ratio of La/Si = 1/20).The mixture was kept under continuous agitation at 40 ºC for 24 h.Then the gel was transferred to a Teflon-lined stainless steel autoclave and aged at 100 ºC for 24 h.The solid product was recovered by filtration and repeated washing with deionised water, followed by drying at 50 ºC overnight.The P123 template was removed by calcining at 550 ºC for 8 h in air.The SBA-15 material without lanthanum species was also prepared as reference according to Zhao et al. (1998a) for comparison.
The X-ray diffraction (XRD) analysis were performed on a D5000 Siemens powder diffractometer equipped with CuKα radiation (40KV and 30mA).The scattering intensities were measured over an angel range of 0°<2θ<4° with a step size ∆(2θ)=0.02°and a step time of 8 s.
The N 2 adsorption/desorption isotherms were measured on a Micromeritics ASAP2010 at liquid N 2 temperature.Specific surface areas of the samples were calculated from the adsorption isotherms by the BET (Brunauer -Emmett -Teller) method (Brunauer et al. 1938) and pore size distribution from the desorption isotherms by the Barrett-Joyner-Halenda (BJH) method (Barrett et al. 1951).
The ICP-AES was used to determine the content of La in the synthesized samples, which was performed on an Optima 4300DV.Before any measurements were taken, the solid sample was dissolved in 12 mL 0.1 mol•L -1 HCl solution mixed with 4 mL 1 mol•L -1 HF solution.

Catalytic activity studies
In a typical reaction, 3-methoxy-4-hydroxybenzyl alcohol (1.0 mmol, 154 mg), acetonitrile (5.0 mL), the catalyst (100 mg) and 35% aqueous H 2 O 2 (0.34 mL, 3.0 mmol) were placed in a microwave tube and irradiated at 200W on a CEM discover microwave reactor for the time specified in Table 2.The same mixture in a round-bottomed flask was also reacted under conventional heating at 60 ºC for 24 h for comparative purposes.Reactants conversion and products yield were calculated as follows: F i,in and F i,out are the molar flow rate of the i species for reactant and product at the inlet and at the outlet of the reactor, respectively.
GC analyses were carried out on Agilent 6890 GC system, equipped with a DB-17MS capillary column (30 m × 0.25 mm × 0.25 µm film thickness) using nitrogen as carrier gas.The initial column injector was set to 300 ºC with an initial column temperature of 60 ºC, raised to 150 ºC with a ramp rate of 15 ºC/min and then 25 ºC/min to 290 ºC keeping for 15min.Substrate conversion and product selectivity were determined using external standard method, with n-decane as external standard.Figure 1b shows the XRD La-containing SBA-15 and the reflections are marginally shifted toward 2θ values which confirmed the immobilization of the La complex within the ordered SBA-15 structure (Kureshy et al. 2006).For the N 2 adsorption-desorption isotherms for siliceous SBA-15 and La/SBA-15 (Figure 2-3), typical irreversible type IV adsorption isotherms with a H1 hysteresis loop as defined by IUPAC (Sing et al. 1985), were observed.This H1-type hysteresis loop suggests that the material has regular mesoporous channels with narrow Gaussian pore size distribution centred at 7.2 nm for siliceous SBA-15, at 6.3 nm for La/SBA-15 (Table 1).In fact, the main pore diameter decreased as the percentage of lanthanum species increased, which is in agreement with those published by other authors (Bendahou et al. 2008;Groen et al. 2003).The composition of solid products determined by ICP-AES is also listed in Table 1.The results also show that the content of La in the solid sample measured by ICP-AES (La/Si≈1/100) is obviously lower than of the initial gel mixture (La/Si=1/20), indicating a very small quantity of the solubility of lanthanum nitrate in the strong acidic medium.This indicates that the lanthanum species in the gel cannot be introduced completely into SBA-15 under some acidic conditions.
Table 2 includes results of the oxidation of 3-methoxy-4-hydroxybenzyl alcohol using La/ SBA-15 at different times with changeable temperature (<423K) under microwave irradiation.Only traces of detectable low molecular weight products such as vanillin were observed at short times of reaction (<5min).About 68% of substrate conversion was observed with an optimum yield to vanillin within 30 min of reaction.Complete oxidation of 3-methoxy-4-hydroxybenzyl alcohol was observed after 40 min with high molecular weight compounds perhaps including phenolic dimers and quinones, which was also mentioned in previous reported results (Crestini et al. 2005).
Blank microwave runs of 1 (without catalyst and without H 2 O 2 ; Table 2 entry 10) gave no conversion after 15 min and only 8% of substrate conversion was observed in the presence of H 2 O 2 without catalyst (Table 2 entry 11).This clearly indicates that La/SBA-15 material catalyses the reaction.Interestingly, the support SBA-15 alone gave 34% of substrate conversion after 30 min, whereas 68% conversion was obtained using immobilized La/SBA-15 catalyst, compared to a poor 25% conversion after 24 h under conventional heating without microwave irradiation (  The reusability of the catalyst was studied after isolation and subsequent activation of the catalyst after reaction completion (Table 2, Figure 5).The used catalyst in the first cycle of the reaction was separated by filtration, washed three times with ethanol, dried in an oven at 100 ºC for 24 h, and activated at 300 ºC for 4 h in air.The first recycling run provided similar activities of the catalyst with complete 3-methoxy-4-hydroxybenzyl alcohol oxidation after 30 min.Subsequent reuses gave very similar results with the active La/SBA-15 preserving most of its initial activity after 6 runs.It can be concluded that the catalyst can be reused at least 6 times and there is no appreciate loss in catalytic activity.