In this work, a series of active Au/NiOx catalysts were successful to
prepare by tracing the concentrations of chloride in the
re-dispersed aqueous solutions. By characterizations, we found that the
appropriate amount of residual chloride in Au catalyst
would induce Au nanoparticles (Au NPs) to locate on the edges of NiOx
particles, which resulted in the active Au/NiOx-9 sample.
Fine control of chloride in the aqueous solution provides a new
perspective to push for addressing the controllable preparation of
active heterogeneous catalysts. Keywords: Au catalyst, Preparation, Chloride, Esterification
In recent decades, Au catalysts have received growing attentions
and been widely applied in many important research fields [1], since
good performance of Au catalysts was discovered [2]. However, the
controllable preparation of highly active heterogeneous catalysts is
still a longstanding challenge till now, especially Au catalysts. Many
efforts have been devoted to this problem. The active site, structure
and the quantum size effect of Au catalyst [3], active oxygen species
of the support [4], suitable reducible oxide supports [5],and so on,
have been extensively studied. Additionally, catalyst precursors,
bases, pH value, aging time, and calcinations temperature are also
crucial conditions [2,6]. Nevertheless, the controllable preparation
of highly active Au catalyst is still difficult to realize even strictly
following all above conditions. Chloride (usually as Cl-) is generally
regarded as a poison for Au catalyst, Because of strong interaction
of chloride and Au. We realized the reproducible preparation of
Au/Fe2O3 catalyst for CO oxidation [7]. It is meaningful to explore
whether this method can be applied to other catalysts and reactions
or not. In this work, Methyl esterification of alcohols was chosen as
model reaction. The controllable preparation of highly active Au/
NiOx catalyst was realized by tracing the concentrations of chloride
in the re-dispersed aqueous solutions.
20ml Ni(NO3)36H2O (0.011 M) and 1.05 ml HAuCl4 (0.24M)
were mixed together and were drop wise added into 60 ml Na2CO3
solution (0.31M) under vigorous stirring in 3h. The turbid liquid was
divided into four sections and separation by centrifugation. Each
section of the recovered precipitate was re-dispersed in different
amount of deionised water and ultrasonically washed for 1h. The
chloride concentration in the re-dispersed aqueous solution of each
section was determined by CHI660D electrochemical workstation.
Then, the solid was separated by centrifugation, dried at 80o C for
3h and calcined at 350 oC for 0.5 h to produce the catalyst sample,
which was denoted as Au/NiOx-X, in which X suggested the chloride
concentration in ppm.
Catalyst activity test
1mmol benzyl alcohol, 30 mg catalyst and 2 ml methanol were
added into a glass tube. And then it was exchanged with oxygen and
reacted at 60o C (1 atom, O2 balloon). After reaction, it was cooled
to room temperature. Biphenyl was used as internal standard and a
certain amount of ethanol were added into the reaction mixture up
to 10mL for quantitative analysis by GC-FID (Agilent 7890A).
The catalytic activities of 15 Au/NiOx samples, which were
prepared from the re-dispersed aqueous solutions with chloride
concentrations in the range of 2 to 108 ppm, for esterification of
benzyl alcohol were studied. According to the results shown in
Figure 1, catalytic activity of Au/NiOx varied with the changing of
chloride concentration. The yields of methyl benzoate were lower
than 21% if the catalysts were prepared from aqueous solutions
containing >22ppm chloride. More active catalysts were produced
when the chloride concentrations were going down. The Au/
NiOx catalysts with the highest catalytic activity were prepared
from aqueous solutions containing 8-13ppm chloride, the yield of
methyl benzoate of catalyst Au/NiOx-9 was >99%. Surprisingly, the
catalysts turned less active again when the chloride concentrations
were < 8ppm. Typically, the yield of methyl benzoate was 20% with
catalyst Au/NiOx-3.
Figure 1: The yield of methyl benzoatevs the chlorine
concentration of the aqueous solution from which the
catalyst samples were prepared.
Figure 2: HR-TEM (left) images and size distributions
(right) of Au/NiOx-22 (a), Au/NiOx-9 (c), and Au/NiOx-
3(e).
TEM measurement results of Au/NiOx are shown in Figure 2.
Their TEM images were similar and seemed amorphous. For the
sample of Au/NiOx-22, the lattice of gold could be observed and
wrapped in NiOx particle. For active Au/NiOx-9, the most of Au
NPs connected with the edges of NiOx particles or the junctions
of several NiOx particles [8]. In consideration of the best catalytic
performance of this sample, this observation strongly supported
the former results about active site in Au catalyst, i.e. the interface
between Au and iron oxide [3]. It suggested that the appropriate
amount of chloride might act as the linkage between Au NPs and
the edges of NiOx particles to gain the active Au catalyst, For Au/
NiOx-22 and Au/NiOx-3, too much or less chloride was presented,
the interaction of Au NPs and NiOx like Au/NiOx-9 decreased
significantly. Accordingly, the catalytic activity lost sharply. By
metering more than 150Au NPs, the mean diameters of Au NPs
in samples Au/NiOx-3, Au/NiOx-9 and Au/NiOx-22were 4.1, 3.8
and 6.6 nm with 1.91, 1.84 and 3.06 standard deviations. The size
distributions of Au NPs in Au/NiOx-3 and Au/NiOx-9 samples were
extremely similar. The marked difference of catalytic activities of
these two catalysts did not come from the size effect of Au particles,
but the contact way of Au NPs and NiOx supports.
At present, there is still not sufficient evidence to explain the real
role of chloride in the formation of Au catalysts. However, according
to the known evidence, we can make some reasonable conjectures.
Firstly, as pH value of the mother aqueous solution rises, chlorine in
chloroauric acid is substituted by the hydroxyl. Au-Cl bond breaks
and then small Au NPs form. Finally, chloride is adsorbed on the
support NiOx as well as Au NPs. Due to the stronger interaction
of chlorideon the edges than on planes of NiOx crystallites, after
the ultrasonication and washing operations, chloride located on
the edges of NiOx crystallites remains. As shown in Figure 3, it is
this kind of residual chloride that induces Au NPs to anchor on the
edges of NiOx crystallites.
In summary, by tracing the chloride concentrations in the
re-dispersed aqueous solution, we successfully prepared active
Au/NiOx catalyst for catalytic methyl esterification of alcohols. If
the chloride concentration was not in the range of 8-13ppm, the
catalytic activity dropped dramatically. These results indicated
that the presence of appropriate amounts of residual chloride was
beneficial to obtain highly active heterogeneous catalysts. This work
can offer a new perspective to realize the controllable preparation
of active heterogeneous catalysts.
In India energy security and exploring the production of valuable chemicals , which are otherwise mostly imported have become imperative necessity The advantage of abundant availability of paddy and its straws, specifically the rice straw in India are taken as an example to establish a perfect Bio-refinery . In these paper possibilities of production of chemicals and energy by Chemical, Biochemical and Thermo chemical platforms are explored. Possible alternatives on the problems of stubble burning in some stares of India are put forward. However, studies on the optimal design, and economic viability of each routes remain to be evaluated.
Biorefineries similar to petroleum refineries is a facility developed, engineered and designed optimally where renewable energy (heat & power) and multiples of chemical products and can be profitably manufactured from biomass with best known environmentally benign process technologies through biochemical and thermo chemical platforms. The energy production from bipomass is also Greenhouse neutral. Cellulosic biomass, because of its massive availability, can be a truly biorefinery representing a feedstock for biofuels and valuable chemicals. Agricultural residues such as straws are ideal candidates for establishing a biorefinery in India. Presently major quantity of straws is used as domestic fuels in rural areas. Rice Straw is produced from Rice Paddy. The various products and by-products are shown in the schematic diagram (Figure 1). On an average, there is 20% husks, 10% bran, 3% polishings, 1-17% broken rice and 50-66% polished rice. Generally Rice Paddy by-products is on an average 30% weight of paddy3).
Figure 1: Products and Bye-products from Rice Paddy (2).
The residual wastes (stubbles) are usually burnt in the field which leads to severe air pollution problems due to discharge of gaseous pollutants including CO, Ozone , N2O, NOx, SO2 , CH4, particulate matters ,smokes and smogs, hydrocarbons. Open burning of crop stubble also results in the emissions of harmful chemicals like polychlorinated dibenzo-p-dioxins, polycyclic aromatic hydrocarbons (PAH’s) and polychlorinated dibenzofurans (PCDFs) referred as as dioxins, besides loss of nutritional values of soil intermas of organic carbon, nitrogen, phosphorous and potassium in many states of India, especially Punjab, Haryana, Rajasthan and Uttar Pradesh. This is not that extent in other part of India. The main reasons are: larger length of the stubbles remains after harvesting in those states which cannot be economically covered under soil to enhance the fertility of the land and attempts to burn these long projected straws over the agricultural land. In the following paragraphs some alternatives to straw stubble burning are suggested [1].
Due to continuous depletion of non-renewable energy resources and high cost of chemicals due to import, at present there is a worldwide attention towards development of renewable resources of energy and chemicals for sustainable development for the welfare of mankind. For example: Economic production of bioethanol from lignocellusic biomass. Conversion of lignocellulosic plant materials to biochemicals is also regarded as one of the most promising alternatives to fossil fuels. Most abundantly available biomass in the countries like India and China are straws (rice, wheat, oats, rye, barleys, Zea Mays, corn stalks etc.), out of which rice straw occupies the first position and followed by wheat straw in terms of availability in Eastern, and North Eastern Indian states (West Bengal, Bihar, Assam, Orissa, Manipur etc.) whereas reverse is true for Northern India (U.P. Haryana, Punjab, Rajasthan etc.).
b) Manuring/Composting with cowdung and others etc.
c) Briquettes
d) mats
e) Mushroom cultivation(as growth substrate)
f) Vegetables Cultivation
g) Animal Bedding material
h) Poultry Litter & Mulch
i) Feed for ruminants/Animal feed
j) Packaging goods for transporting goods &machineries
k) Frost prevention in horticulture
l) Strawberries (preventing damage to the fruit)
m) Thatching
n) Rope making
o) Traditional building materials, fibre boards,Particle board, insulation material
p) Energy (heat, power, fuels)
q) An intergrated solid state fermentation approach for production of enzymes from agro-wastes including straws
Lignocellulosic biomass could thus be utilized for both production of biofuels as well as biochemical’s due to its nature of renewability, low price, widespread availability and containing high content of pentose and hexose sugar polymers. These are detailed elsewhere [2-6]. Straw and Stubbles can be used for various Chemicals, valuable products and energy. The most notable products which can economically manufactured are: Pulp & Paper, Particle Board, Pulp and Paper Board, Straw board, board of rice husk.
Energy technologies and thermal combustion consists of Non- Conventional uses of straws. Valuable chemicals include Cellulose, High Alpha cellulose, Plastics, Fuels and Energy,Bio-gas and in situ, Bio-oil, Nanocellulose and nano composites, Pentosans, Xylose, Xylitol, α-Cellulose, Glucose , Fructose, Hydroxy methyl Furan, Ethanol and host of many other chemicals [7-10]. These are shown in Figure 2.
Refineries based on Cellulose, Ethanol, Sucrose, Glucose, Lignin have been proposed and given elsewhere various Unit Operations and Processes involved to produce a biorefinery are as under:
a) Pulping
b) Gasification
c) Pyrolysis
d) Destructive distillation
e) Plasma Treatment
f) Chemical Treatment
g) Electron Irradiation
Chemical platform
a) Activated carbon
b) Chemical transformation through Catalyst(Sn-beta zeolites)
c) Synthetic Fuel using Solar Furnace
d) Cellulose nano crystals and nanocomposites:
Cellulose nanocrystals have been largely applied as reinforcing fillers in the preparation of nano composites materials with improved mechanical and barrier properties.
Figure 2: Products and Bye-products from Rice Paddy (2).
Bio-chemical platform
Bio-chemical platform
a) Renewable fuels: Ethanol, Biodiesel,Butanol, Hydrogen
b) Chemicals: Acetone,Furfurol,Propanediol, Ketones etc.
c) Organic Acids:Acetic, Lactic , Succinic, Gluconic, Butyric etc.
Polyphenols: glycerol, Arabitol, Xylitol, Sorbitol Lactones such as 3-hydroxy butyrolactone
Phenolics: p-hydroxy benzoic acids and vanillin However,aldopentose xylose (20-40% of the total carbohydrates are normally found in agricultural residues.
Reaction Schemes (4,9,21,29,30)
Chemical reaction consists of series, parallel and combination of series-parallel reactions
Figure 3 Reaction Scheme and Kinetic models are developed (Pentosan( both xylan and arabinan) is hydrolyzed to both aldopentoses which are converted into two or more steps into furfural. Loss of furfural takes place due to side reactions which leads to condensation and to the formation of resins. Both levulinic acid and furfural can be produced from straw or any biomass & levulinic acid can be converted to succinic acid and formic acid [13].
Figure 3: Products and Bye-products from Rice Paddy (2).
Furan derivatives such as Furfural (2-furaldehyde), HMF (5-hydroxymethyl-2-furaldehyde), 2,5 furan dicarboxylic acids(33-35) are most important chemicals. HMF is produced industrially on a modest scale as a carbon-neutral feedstock for the production of fuels and other chemicals such as levulinic acid, gamma-valerolactone, or other byproducts. HMF itself has few applications and it is primarily produced in order to be converted into other more useful compounds [14-20]. Of these the most important is 2,5-furandicarboxylic acid, which has been proposed as a replacement for terephthalic acid in the production of polyesters. HMF can be converted to 2,5-dimethylfuran (DMF), a liquid that is a potential biofuel with a greater energy content than bioethanol. Hydrogenation gives 2,5-bis(hydroxymethyl) furan. Acid-catalysed hydrolysis converts HMF into gamma-valerolactone, with loss of formic acid. HMF is practically absent in fresh food, but it is naturally generated in sugar-containing food during heat-treatments like drying or cooking. Along with many other flavor- and color-related substances, HMF is formed in the Maillard reaction as well as during caramelization. In these foods it is also slowly generated during storage. Acid conditions favour generation of HMF. HMF is a well known component of baked goods. Upon toasting bread, the amount increases from 14.8 (5 min.) to 2024.8 mg/kg (60 min). It is a good wine storage time−temperature marker, especially in sweet wines such as Madeira and those sweetened with grape concentrate arrope [21-24].
Sachharomyces cerevisiae, Zymomonous Mobilis, Clostidium thermocellum, Ruminococcus albus- a bacterium are generally used for conversion of cellulose to ethanol. Theoretically 1kg of sucrose on inversion, gives 1.053 kg of invert sugar, glucose and fructose combined together. Further,one tone of invert sugar yields 644.8 litres of absolute alcohol(ethanol of 100% ) or 678.7 litres of rectified spirit.The net CO2 emission of burning a biofuel like ethanol is zero since the cO2 emitted on combustion is equal to that aabsorbed from the atmosphere by photosynthesis during growth of the plant(sugarcane) used to manufacture ethanol.
Inversion: C12H22O11+H2O→C6H12O6+C6H12O6
Sucrose + Water →Invertase→Glucose + Fructose (Invert Sugars)
Biobutanol is produced by microbial fermentation, similar to bioethanol, and can be made from cellulosic feedstocks such as straws. The most commonly used microorganisms are strains of Clostridium acetobutylicum and Clostridium beijerinckii, C. Saccharoperbutylacetonicum and C. saccharobutylicum. In addition to butanol, these organisms also produce acetone and ethanol, so the process is often referred to as the “ABE (acetone-butanolethanol) fermentation [25-30]. Production of lactic acid from straw derived cellulose,cellulase production with Tricoderma citriviridae on solid bed, use of acid hydrolysates for lactic acid production using various strains such as Lactobacillus delbrueckii or lactobacillus pentosus can be explored.A number of products can be produced from sucrose as shown in Figure 4.
Figure 4: Products and Bye-products from Rice Paddy (2).
Anaerobic Digestion
Biogas production (25,31-32), Anaerobic conversion of carbohydrate /cellulosics, especially of agricultural residues , has been considered for biogas ( methane) production which is typically, CH4=50-65%, CO2=35-50%, H2O=30-160g/m3, H2S=1.5-12.5g/ m3 inn presence of methane producing bacteria. Typically some of the methane forming microorganisms likes Methanaomonas, Methanococcous mazei n.sp. methannobacterium sohn genii n.sp. etc. are employed. The two best described pathways involve the use of acetic acid or inorganic carbon dioxide as terminal electron acceptors:
CO2+4h2→CH4+2H2O
CH3COOH→CH4+2CO2
During anaerobic respiration of carbohydrates, H2 and acetate are formed in a ratio of 2:1 or lower, so H2 contributes only ca. 33% to methanogenesis [31], with acetate contributing the greater proportion. In some circumstances, for instance in the rumen, where acetate is largely absorbed into the bloodstream of the host, the contribution of H2 to methanogenesis is greater.
The basic principles of Gasification technology are as under:
a. Steam Reforming of Straws:
Superheated steam reacts endothermally (consumes heat) with the carbonaceous components of straws to produce hydrogen and carbon monoxide fuel gases (synthesis gas or syngas)
b. Steam Reforming reaction: H2 O +C + Heat → H2 +CO
Water –gas shift reactions also occur simultaneously with the steam reforming reactions to yield additional hydrogen and carbon dioxide.
India being an agriculture based country with plenty of biomass renewable resources can produce potential bio-products and bio energy at a cheaper rate compared to other renewable sources. Being carbon neutral these resources is eco friendly, yields much less green house gaseous emissions compared to fossil fuels [32- 37]. In this present paper various alternatives for straw utilization, specifically the plausible solutions of current problems of straw stubble burnings in a few Indian states are highlighted. However detailed optimum design of process and plant with economic feasibility need to work out.
