LA-MODIFIED SBA-15 / H 2 O 2 SYSTEMS FOR THE MICROWAVE ASSISTED OXIDATION OF ORGANOSOLV BEECH WOOD LIGNIN

In this manuscript, the in uence of organosolv beech wood lignin (LOB) on its oxidative conversion to high added-value phenolic aldehydes is discussed. Environmental friendly and low-cost H2O2 was used as the oxygen atom donor. e catalyst was prepared by immobilizing Lanthanum compounds onto the periodic mesoporous channels of siliceous SBA-15. e activity of the La/SBA-15 was investigated towards oxidation of LOB in the presence of hydrogen peroxide as oxidant with microwave irradiation. Considering the characteristics of LOB, an unexpected low syringaldehyde concentration at 10min of reaction time (1.47 g/L, corresponding to 15.66% yield) was obtained; the other major product was vanillin at 25min (0.78 g/L, i.e., 9.94% yield). e high reactivity of syringyl nuclei may be pointed out as the main reason for the faster production and degradation of syringaldehyde in oxidation. Other low molecular weight phenolic products were found: vanillic acid, syringic acid and minor quantities of aceto-derivatives. e pro le of products concentration with the reaction time of catalytic oxidation with microwave irradiation are shown and discussed with reference to the investigated lignin features. e mechanism of the microwave catalytic oxidation for LOB under alkaline conditions was proposed.


INTRODUCTION
Lignin is an extremely complex three-dimensional macromolecule with irregular structure, which is mainly formed by the polymerization of three phenyl propane monomers, i.e. guaiacyl, syringyl and p-hydroxylphenyl units (Caballero et al. 1996a, Caballero et al. 1996b, Li et al. 2002, Wool and Sun 2005).Generally, lignin from hardwood contains higher methoxyl content due to the presence of both guaiacyl and syringyl units.Because of the di culty for the separation of lignin from biomass, the organosolv process takes the advantage of high e ciency and overcomes the pollution caused by alkali or sulphur containing inorganic compounds (Willauer et al. 2000).Currently, the vast majority of lignin is burned or discarded without any industrial utilization for the complexity of lignin and the di culty in extraction.Consequently, increasing research e orts have been made over the past few years to develop environmentally friendly processes, for example, by the use of oxygen, hydrogen peroxide (H 2 O 2 ), and ozone as primary oxidants.
Lignin o ers perspectives for higher added value applications and could be converted to several pro table commodities (Lora andGlasser 2002, Zakzeski et al. 2010).It was worthy to be noticed that aromatic aldehyde such as vanillin, syringaldehyde and p-hydroxybenzaldehyde can be obtained from the oxidation process of lignin.ese aldehydes have wide applications such as avoring, chemical intermediates for pharmaceutical drugs and agricultural defensives (Wu et al. 1994, Sridhar et al. 2005, Sales et al. 2006, Sales et al. 2007).e chemical composition of lignin and its content Maderas.
in wood vary greatly within a plant, depending on its species, age, morphological location and growth environment.It is recognized that the lignins from so woods are predominantly based on structural units derived from guaiacyl units in major quantities, which are the precursors for vanillin yield (Kirk et al. 1980, Adachi et al. 1992).On the other hand, hardwood lignins present structures with much broader chemical compositions, consisting in varying proportion ratios of guaiacyl and syringil groups.Lignin from di erent sources can be oxidized to obtain phenolic compounds and the substrate (lignin) properties has certain e ect on the yield of oxidation products such as vanillin and syringaldehyde.For instance, it has been shown that lignin with low molecular weight derived from so woods are preferred to obtain a better vanillin yield than high-molecular weight lignin (José et al. 2010).
Industrially, the routine way to convert lignin into aromatic aldehydes selects so wood including isolated lignosulphonates and Kra lignins as lignin resources, in alkaline media, by oxygen or air to produce vanillin (Bjorsvik 1999, Mathias andRodrigues 1995).However, with the formation of radical intermediates, such as hydroxyl radicals during the degradation of lignin, the conventional chemical oxidation of lignin leads to a poor selectivity and nal product yield (Tarabanko and Petrukhov 2003).And the chemical oxidation of lignin with oxygen is a reaction in which typical operating conditions involve high pH values (close to 14), high temperatures (up to 150 ºC) and high total pressure (e.g., 10 bar) (Mathias and Rodrigues 1995).Microwave assisted reactions can be highly e cient and polluteless, allowing a reduction in reaction time and energy consumption together with an increase in yields and selectivity in some cases (James et al. 2006, Conesa et al. 2007, Kappe 2008).erefore, 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.In a word, the use of organosolv lignin as raw material, instead of the conventional application of lignosulphonates or Kra lignin, is lacking presently.Our aim ts in the organosolv hardwood lignin application, but instead of using so wood lignin, to explore the possibility of LOB catalytic oxidation to manufacture aldehydes with microwave irradiation.
Recently, studies of SBA-15 or SBA-15 modi ed with some noble metals as catalyst in the lignin oxidation process have received signi cant attention (Badamali et al. 2009, Gu et al. 2010a) because of their regular pore array with uniform pore diameter (2.0-8.0 nm), high surface area and pore volume (Kresge 1992).Moreover, SBA-15 and Metal/SBA-15 catalyst are known to show interesting activities in the 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).Our research team has successfully investigated the oxidation of lignin model compounds with microwave catalysis and now we are applying the system to a real lignin sample.In the present work, we report the application of Lanthanum-containing SBA-15 mesoporous molecular sieves into lignin catalytic oxidation with hydrogen peroxide as oxidant.A typical hardwood lignin, organosolv beech wood lignin (LOB), was chosen as target molecule and its oxidation was investigated with a heterogeneous La/SBA-15 catalyst and microwave irradiation.To the best of our knowledge, few studies dealing with the catalytic conversion of organosolv lignin and the use of immobilized La-containing SBA-15 for the oxidation of LOB have been reported.

Materials
In the lignin structure, each of the monomer gives rise to distinct aromatic nuclei: p-hydroxyphenyl (H), guaiacyl (G) and syringyl nuclei (S), respectively.As a typical hardwood lignin, beech wood lignin was supplied by Yunnan, China.Proximate analysis (moisture, ash, volatile content and xed carbon of lignin sample) and ultimate analysis of the combustible fraction (in weight-by-weight e proximate analysis (Table 1) was developed according to ISO standards for moisture (100 ºC in air), volatile matter (900 ºC in nitrogen atmosphere) and ash (gravimetrically quanti ed a er incineration at 525 ºC for 4 h).Table 2 shows the elementary composition and methoxyl content obtained for LOB.Elemental analysis (carbon, hydrogen and nitrogen contents) was performed in a Perkin Elmer 2400 Series II CHN Elemental Analyzer and the percentage of oxygen was calculated by subtracting the C, H and N contents from 100%.e methoxyl group was determined as follows: lignin (0.15 g) was treated with re uxing concentrated sulfuric acid (10 mL) for 10 min, the reaction mixture was cooled, 70 mL of distilled water was added, and the methanol produced in the reaction was distilled o under vacuum and quanti ed by gas chromatography (Balogh et al. 1992, Vázquez et al. 1997, Vázquez et al. 1999).percentage) are shown in table 1 and table 2. Lignin was treated with oxygen-acetone (150 ºC, 4.5 h) to prepare organosolv beech wood lignin (LOB), according to the procedure described in literature (Evtugvin and Andreolety 1998).A er being dried in an oven at 105 ºC for 3 h to constant weight, and the sample lignin had a high total lignin content (above 94%).e original materials were crushed and pulverized to a size of <0.2 mm before they were analyzed.e carbohydrates on lignin were quanti ed by gas chromatography with ame ionization detection (GC-FID) as trimethylsilyl derivatives a er hydrolysis (Browning 1967, Coimbra et al. 1994).
La/SBA-15 was prepared and characterized according to documented procedures (Gu et al. 2010b).Triblock poly(ethylene oxide) 20 -poly(propylene oxide) 70 -poly(ethylene oxide) 20 (P123, average molecular mass about 5800, Aldrich) and lanthanum nitrate were dissolved in a mixture (pH≈1) under stirring and added tetraethyl orthosilicate (TEOS) to this solution.e mixture was kept under continuous agitation at 40 ºC for 24 h.en the gel was transferred to autoclave and aged at 100 ºC for 24 h.e solid product was recovered by ltration and repeated washing with deionised water, followed by drying at 50 ºC overnight.e P123 template was removed by calcining at 550 ºC for 8 h in air.

Methods
e low molecular weight phenolic compounds (Figure 1) produced by lignin oxidation with La-SBA-15/H 2 O 2 were extracted by solid phase extraction and quanti ed by HPLC-UV, as described in literature (Pinto et al. 2010).
Reversed Phase High Pressure Liquid Chromatography analyses were carried out on Agilent 1100 HPLC system, equipped with a Hypersil ODS column (100 mm × 2.1 mm × 5 μm lm thickness) using methanol/NaH 2 PO 4 solution (water adjust 5 mmol/L ) (70/30, v/v) as mobile phase, ow rate 0.4mL/min, UV-DAD detection wavelength 280 nm, the column temperature 30 ºC, the injection volume 5 μL, calculating the reaction products concentration with external standard method (Herrmann and Stockli 1982).Mw (weight-average, g/mol) 1732 Mn (number-average, g/mol) 1170 Ps (Mw/Mn) 1.5 estimation of molar mass of ppu (the phenylpropane units of lignin).e oxygen atom per ppu (in range 2.0-3.1)belongs to aliphatic hydroxyl, carbonyl and carboxyl groups of ppu, as well as ether linkages.For each lignin type (so wood and hardwood), an increase in oxygen is accompanied by an increase of hydrogen content.Methoxyl frequency per ppu expresses the contribution of the two methoxyl groups from S units and one methoxyl group from G units for hardwood lignin.e Mn of LOB lignin reveals some degree of depolymerization, corresponding to the deligni cation process.e valued obtained (1170 g/mol) is similar to the value (1210 g/mol) reported in literature (Lange and Schweers 1980).

Catalytic activity studies
In a typical reaction, 15 g of LOB (Mn=1170 g/mol) and 35% aqueous H 2 O 2 (10 mL) was dissolved in 150 mL of alkaline solution (20 g of NaOH).A er complete dissolution, 5 g catalyst was added in the resulting mixture and diluted until a nal volume of 250 mL (pH=13-14).en the reaction mixture were placed in a 300 mL microwave tube and irradiated at 200 W on a CEM discover microwave reactor for the time speci ed in table 4. e same mixture in a round-bottomed ask was also reacted under conventional heating at 60 ºC for 24 h for comparative purposes.Products yield were calculated as follows: F i,in and F i,out are the molar ow rate of the i species for reactant and product at the inlet and at the outlet of the reactor, respectively.e low molecular weight phenolic compounds (Figure 1) produced by lignin oxidation with La-SBA-15/H 2 O 2 were extracted by solid phase extraction and quanti ed by HPLC-UV, as described in literature (Pinto et al. 2010).
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 lm thickness) using nitrogen as carrier gas.e 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 º C /min and then 25 ºC /min to 290 ºC keeping for 15 min.Substrate conversion and product selectivity were determined using external standard method, with n-decane as external standard

RESULTS AND DISCUSSION
LOB is a hardwood lignin and, accordingly, G and S (Figure 1) products were found.e maximum yield of V was 0.78 g/L (9.94% molar yield) and the yield of Sy was 1.47 g/L (15.66% molar yield) at di erent reaction times.Secondary products were also found, but below 6% yield based on initial molar amount of lignin (see Table 4).
La -Modifi ed SBA-15/H 2 O 2 ... Gu et al.Table 4 includes results of the oxidation of LOB using La/SBA-15 at di erent times with changeable temperature (<423K) under microwave irradiation.e changes of product distribution of LOB oxidation with time are shown in gure 2. e plot shows that the amounts of V and Sy increase signi cantly during the initial 10 min.In the same period of time, the amount of VA and VO increased slowly.With progressive reaction time, the yield of V reaches a plateau of 9.94% at 25 min; the yield of Sy decreases a er it passes a maximum yield of about 17% at 7 min; the yield of VA slightly increase (max at 12 min) and then decreases to a constant yield of 2.5%; and similarly, the yield of VO slightly increases (max at 5 min) and then decreases to zero a er 20 min.During the whole reaction time, negligible amounts of SA and SO was detected because the S units have higher reactivity than guaiacyl counterparts in alkaline systems (Tsutsumi et al. 1995) and under conditions of O 2 or H 2 O 2 oxidation in alkaline medium (Wu et al. 1994;Sultanov and Wallis 1991).We suggest the Sy is produced at higher yield, but it is promptly degraded; in the case of S units, this e ect is probably more evident due to the above mentioned reactivity of S nuclei.VA and SA were also formed, showing similar trend of the pro le of V and Sy, respectively.e total molar yield of products was up to 32% and the selectivity of phenolic aldehydes was about 25% when the reaction time was 10 minutes.Compared with the present report of lignin oxidation with O 2 as oxidant (Pinto et al. 2011), our microwave catalytic oxidation with H 2 O 2 as oxidant method has advantages of e cient activity with higher aldehydes yield and reduction in reaction time, as well as friendly environmental e ects with milder condition and lower environmental compared with conventional chemical oxidation.e aromatic aldehyde, the oxidative products from lignin in the catalytic process, can be further oxidized to aromatic acids, and even to high molecular weight compounds including phenolic dimers and quinines (Crestini et al. 2005).
It was also shown that rate of substrate conversion and yield of each oxidative products were signi cantly higher with the catalyst La/SBA-15 than that without the catalyst.In the absence of a catalyst, the total product yield was only 2% at 24 h (Table 4, entry 7), in comparison with 32% for product molar yield with La/SBA-15 as catalyst at 10 min (Table 4, entry 2).For example, the yield of V and Sy were 9.56% and 15.66% in 10 minutes with the La/SBA-15 catalyst, 9.18% and 15.14% higher than that of the non-catalytic process, whose maximum yield was only 0.38% and 0.52%.e maximum yield of VA and SA were 5.19% (10 min) and 2.45% (5 min) in the La/SBA-15 catalytic process, 8.95 and 3.45 times those obtained in the non-catalytic process, respectively.
Aceto derivatives (VO and SO) were detected at low concentration because of the competing addition of OH-to α-position (leading to aceto derivatives) and to γ-position (leading to phenolic aldehydes) of quinonemethide (Villar et al. 2001, Tarabanko et al. 2004).e concentration of aceto derivatives is comparatively low, e.g., VO concentration increased during the reaction and it did not exceed 0.1 g/L while the concentration of SO in LOB oxidation mixture was only residual.
As a hardwood lignin for LOB, G and S (Figure 2) products were found.In details, the maximum yield of V was 0.78 g/L (9.94% molar yield) and the yield of Sy was 1.47 g/L (15.66% molar yield) at di erent reaction times, comparative evaluation with those reported in the literature (Pinto et al. 2011).Over-oxidized products were also found, but below 5.2% in product molar yield (Table 4).Taking into consideration of LOB high proportion of S units, a high yield in alkaline catalytic oxidation was expected.However, the yield of products for the S relative yield (about 15%) was even lower than the G yield (18%), which is in agreement with results published by other authors (Pinto et al. 2011).Based on the results of this work and previous studies concerning the reaction between hydrogen peroxide and lignin compounds (Kempf 1975, Xiang andLee 2000), we propose a possible reaction mechanism for oxidation of LOB by hydrogen peroxide under alkaline conditions with microwave catalysis (Figure 3).Under alkaline conditions, hydrogen peroxide is quickly decomposed to molecular oxygen and other radical species.An extremely strong nucleophile (HOO-), produced by hydrogen peroxide, is considered to be the active species for alkaline hydrogen peroxide oxidation (Eq.1).Some perhydroxyl anions (HOO-) and hydroxyl anions (•OH) produced by HOO-(Eq.2) attack the aryl ether bonds and other linkages under these conditions (Eq.3), and others decomposed to O 2 and H 2 O (Eq. 4-5), which in uence oxidation e ciency for hydrogen peroxide.Lignin-derived products of commercial interest such as vanillin and syringaldehyde are produced by side-chain reaction through this pathway.

CONCLUSION
In this work, microwave assisted catalytic oxidation of a typical organosolv lignin over Lanthanum modi ed SBA-15/H 2 O 2 system was investigated.e following conclusions were reached from this systematic study.
A typical hardwood organosolv beech wood lignin (LOB) was prepared and characterized by GPC and the empirical formula was assessed, which was selected to proceed for catalytic oxidation with hydrogen peroxide under microwave irradiation.
For the rst time we have demonstrated that mesoporous La/SBA-15 acts as an e cient catalyst for oxidation of LOB. e active La species seemed to be stabilized within the mesoporous host, rendering unusual oxidative ability and excellent selectivity for aldehydes.e catalytic oxidation with hydrogen peroxider had lead to di erent yields of aldehydes, and the respective acids and aceto derivatives.e highest yield on V was 0.78 g/L (9.94% yield) and the yield of Sy was 1.47 g/L (15.66% yield).e balance of the reactions leading to aldehydes production, aldehydes degradation and competing reactions determines the maximum yield and selectivity to be attained.9.56% yield of vanillin and 15.66% yield of syringaldehyde were obtained a er 10 min of reaction under 200W microwave irradiation, compared to a poor 0.38% and 0.52% yield of corresponding aldehydes.e result is attributed to the presence of isolated hydroxyl groups and the meso-micro pore architecture, which provides an ideal environment for the reaction.Microwave assisted reactions were found to be e cient and selective as compared to the thermal reactions.e mechanism of the microwave catalytic oxidation for LOB under alkaline conditions was proposed.e reaction patterns of the oxidation involve the cleavage of lignin ring, aryl ether bond, or other linkages within lignin.By using the ndings of this investigation and those of previous work, we have illustrated the reaction pathway for oxidation of LOB.

Figure 1 .
Figure 1.Product distribution in the oxidation of LOB substrate under microwave irradiation

Figure 3 .
Figure 3. Proposed reaction sequence for catalytic oxidation of LOB by hydrogen peroxide with microwave irradiation under alkaline condition

Table 1 .
Analysis of moisture, volatile matter, ash content, lignin content and carbohydrate in LOB (% (w/w) on dry matter)

Table 3 .
C9 formula and results of Mw, Mn and polydispersity (Ps) of LOB

Table 4 .
Products concentration and yield obtained from LOB oxidation with di erent reaction time using La/SBA-15 catalysis a a Microwave reaction conditions: 15 g LOB, 5 g catalyst and 10 mL 35% aqueous H 2 O 2 in a nal volume of 250 mL alkaline solution (pH=13-14), microwave irradiation 200 W b -Not detectable c 60 ºC for 24 h, without microwave irradiation and without catalyst