Indian Journal of Chemical Technology Vol. 8, May 2001, pp. 157-161

Control of mesoporosity in alumina

Manoj Kumar*, Babu Lal, Anand Singh, A K Saxena, V S Dangwal, L D Sharma & G Murali Dhar

Catalysis laboratory, Indian Institute of Petro leum, Dehradun 248 005, India

Received 08 May 2000; accepted 05 January 2001

The textural properties of the alumina carrier play a major role in governing the performance of alumina based hetero­geneous catalysts. The regulation of the pore size distribution in the support is of paramount importance in the development of a promising catalyst. The role of peptising agent, combustible additives and binder in regulating the mesoporosity has been investigated using high pressure mercury porosimetry. Control of pore size in 18-50A radii range can be achieved by peptising alumina monohydrate (boehmite) by 1.5% nitric acid. Additives like polyvinyl alcohol and polyethylene glycol have broadened the mesopores while carbon black has drastically increased pores in 50-100 A range. Addition of 10% cal­cined alumina as binder has been found suitable to maintain pores <50 A radii.

An efficient heterogeneous catalyst should essentially have acttvtty, selectivity , stability, mechanical strength and regenerability. These catalysts may be with or without metals but invariably consist of a car­rier with large internal surface and porous texture. Whether the active sites are provided by metals or by acid centres present on the surface of the support, the accessibility of feed molecules to the locations of these active centre in the pores, depends largely on the textural characteristics of the support. The internal porous texture not only regulates the mass transfer and diffusion of feed molecules to the active sites but also affects the stability and mechanical strength of a catalyst.

Pores are classified into three categories according to their size 1

, the micropores ( <20 A diameter), meso­pores (20 - 500 A) and macropores (> 500 A). The surface area of a support is attributed to the major contribution of micropores and mesopores of narrow range. High pore volume on the other hand, would result due to the presence of larger proportions of macropores. The presence of larger proportions of macroporosity deteriorate the mechanical properties of the catalyst. Catalyst for reforming and hydro­treating in petroleum industry normally employ an alumina carrier. A careful optimization of meso and macropore size distribution is required to develop a suitable catalyst for these processes. In catalytic re­forming and hydrodesulphurization of naphtha where the feed molecules are not large enough would require mesoporous alumina support. But hydrotreating of

*For correspondence (Fax: 091-135-671986; E-mail :iipddn @del2.vsnl .net.i n

heavier feed molecules as in kerosene/diesel, catalyst would require large proportions of mesopores. An optimum size of 250 A has been recommended for heavy oil hydroprocessing2

• In hydrodesulphurization of naphtha or gas oil, catalyst with same metal con­centrations of Co and Mo can be used on the same alumina carrier but with different textural characteris­tics . In other words, modification of the support tex­ture i.e. the pore size distribution should be critically controlled to develop a support for a particular petro­leum process.

Y oshi mura3 have reported that small pores and large surface area catalysts have highest hydrogena­tion activity. Song et al. 4 have shown that conversion of larger molecules like asphaltenes on NiMo/ Al20 3

increases when mean pore radius increased upto -300 A and further increase in pore size has no effect on conversion. Alumina finds extensive applications as a support for catalysts used in a various petroleum re­fining processes because of its thermal stability, exis­tence in several allotropic modifications (phases) and a wide range of surface area and porosity.

Pore size is controlled by the method of the prepa­ration of the gel, or by addition of pore regulating agents5

. Mesopores (- I 00 A) are mainly controlled by the size of alumina crystallites as these are created from dimensions and shape of spaces between the particles (intra particle void space) of alumia. The small pores can be created by following different methods like aluminium hydroxide precipitation and its subsequent aging and calcination6 and also by hy­drothermal treatments7

. Large pores generally origi­nate from inter-particle spaces and are introduced to

158 INDIAN J. CHEM. TECHNOL., MAY 2001

Table 1- The effect of peptizing agent on pore size distribution

Sample Amount of SIJET Total pore % Volume in pores of rad ii , A Pore max, MPR: A A nitric acid m2/g vol. mUg/

% >1000 >500 >250 >100 >50 18-50

AI20,-A 0.5 240 0.55 0.7 1 0.88 1.25 2.06 34.41 65.59 43 68 AI, O,-B 1.0 215 0.59 0.62 0.89 1.56 3.29 13.17 86.83 44 61 AI20.1-C 1.5 200 0.69 0.52 0.70 1.13 2.78 13.27 86.73 44 67 AI 20 3- D 1.65 182 0.52 0 0.48 1. 15 2.2 1 5.68 94.32 42

Table 2-The e ffect of additives on pore size distribution

Sample Amount of addi­tive (10%)

AI 20 .1-C Al 20 .1-E Polyv iny l alcoho l Al 20 rF Polyethylene

glycol (mo l. wt. 400)

Al 20 r G Polyethylene g lycol (mol. w t. 600)

Al20 ,-H Carbon black

200 188 218

230

185

Total pore vo l.

mL/g/

0.69 0.69 0.63

0.60

0.71

>1000

0.52 3.42 0.40

0.73

1.29

the support by the addition of combustible organic materials like activated carbon, graphite, saw dust , and po lymers8

·9 added to the aluminium hydroxide

during precipitation or pellet/ extrudate forming stage. Precipitation involves the formation of particles which may agglomerate. The size and shape of the partic les and the agglomerates will influence the po­ros ity. If smal l particles are only present then close p:.1cki ng and low void space is obtained . If large parti­cles or agglomerates are present this would result in large void space and macro porosi ty . Control of small pore size can be achieved by controlling the particles/ agg lomerate size and shape. This could be done by making use of proper precipitation agents like ammo­nia to obtain hydroxide intermediate of the required size and shape.

Peptization in which the dispersion of colloid is achieved by changing the composition of dispersion medium6 is used to treat boehmite before the material is formed or calcined '0· ''. It is stated that peptization with ac id has removed larger pores without affecting the pores of 100 - 300 A radii . Addition of acid as peptizing agent was found to favour the rupture of intramicelle bonds leading to smaller particles which pack together more closely to eliminate larger pores". The amount of additive is also dictated by the nature

% Volume in pores of radii , A Pore MPR, max , A A

>500 >250 >100 >50 18-50

0.70 1.13 2.78 13.27 86.73 44 67 4.72 6.97 10.94 23.37 76.63 46 218 0.79 1.41 4.94 20.35 79.65 44 62

0.97 1.31 2.93 12.70 87.30 41 124

1.62 2.05 4.82 72.36 27.64 55 106

of the intermediate and size of agglomerate for exam­ple bayerite is diffcult to peptise than boehmite 12

.

The use of additive has also been reported to con­trol the pore size. The additive may contain a par­ticu lar functional group which may react with alumina or aluminum hydroxide or it may possess bulky func­tional groups which change pore s ize by merel y sepa­rati ng the particles by space fillin g. Water soluble polymer aclditives have such effect. It was observed that polymer stabi lize the colloidal suspension 11

.

Larger pores are to be controlled as they have marked effect on mass transfer and mechanical strength. Ad­ditives are mixed with aluminium hydroxide/ oxyhy­droxide or wi th alum ina powder. These are then pel­leted/ extrudated and calcined. Duri ng pellet forming process additives like graphite, po lyvinyl alcohol , polyethylene glycols are incorporated which are later on removed by control led combustion to avoid high temperatures and sintering. Additive acts as a filler and meso/ macro porosity is created by space filling. It may be stated that although the use of fillers largely affects the macroporosity, the control of macropo­rosity is linked to the small pores also and small pores may be affected by the same additive.

Binders are used to increase the strength of pellet by forming interparticle bonds 14

. Pellet forming is

KUMAR et al.: CONTROL OF MESOPOROSITY IN ALUMINA 159

Table 3- Effect of binder on pore size distribution

Sample

Al 20 r i AI 20 r J AI 20 r K AI 20 r L

Binder, calcined

alumina o/o

10 20 30 100

*MPR- Mean Pore Radius

235 235 205 250

Total pore vol. mL/g

0.60 0.64 0.73 0.60

>1000

1.94 5.36 29.2 1.54

usually done by extrusion or pelletizing in a press. The extrusion process is done at low pressure which may require more interparticle bonding by use of binder or by peptization.

The aim of present work is to investigate the effect of peptizing agents, additives and binder on the meso pore size distribution of alumina powder using high pressure mercury porosimetry.

Experimental Procedure Commercial alumina monohydrate (boehmite)

powder was used to prepare the samples. Alumina paste suitable for extrusion was prepared by peptizing and kneading the alumina powder with the peptizing agent nitric acid. Alumina hydrate powder with an appropriate quantity of peptizing agent (nitric acid) was added at a constant rate with continuous mixing and kneading. The cake was then extruded through screw type extruder using multiple hole die, dried at l00°C for l6h and finally calcined at 550°C for 4h . The peptized alumina hydrate with varying amounts, 0.5, l, 1.5 and 1.65 % of nitric acid after drying and calcination yielded Al 20 3 -A , B, C and D respec­tively.

Alumina hydrate peptized with 1.5% mtnc acid was chosen to study the effect of additives and bind­ers. Thus on alumina hydrate 1.5% nitric acid was added first for peptization and then 10% of additive was added, kneaded thoroughly, extruded, dried at ll0°C and calcined at 550°C. The samples A\ 20 3-E, F, G and H contained 10% polyvinyl alcohol , poly­ethylene glycol (mol. wt. 400), polyethylene glycol (mol. wt. 600) and carbon black respectively.

Similarly, on peptized monohydrate with 1.5% HN03, calcined alumina was mixed, kneaded ex­truded, dried and finally calcined at 550°C. Samples Ah03-I, J and H contained 10, 20 and 30% calcined alumina respectively. AI 20 3-L, is calcined alumina

o/o Volume in pores of radii, A

>500 >250 >100 >50

2.25 2.7I 4.04 13.34 5.80 6.23 7.59 53.46 29.3 30.1 32.1 50.4 2.21 2.54 6.67 34.16

Pore max , A

18-50

86.66 45 46.54 51 49.6 65.84 48

MPR, A

I II 322

88

prepared from calcination of boehmite at 550°C for 10 h in air.

The surface area was determined in a volumeteric glass adsorption unit by studying N2 adsorption iso­therm at 77K. Total pore volume and pore size distri­bution were determined using a high pressure mercury porosimeter (4000 kG/cm2

) Autoscan 60, Quan­tachrome, USA.

Results and Discussion The data on the effect of nitric acid as peptizing

agent on the textural properties of alumina monohy­drate is displayed in Table 1. By increasing the con­centration of nitric acid from 0.5 to 1.5% the surface area has decreased and a corresponding increase in pore volume is observed . The peptizing action of ni­tric acid is more marked in the mesopore range of 18-50 A radii. On increasing the acid amount from 0.5 to I%, the % volume in pores of radii 18-50 A increased from 65.59 to 86.83 % and does not show any increase in this pore range on further increasing the acid amount to I .5%. The percent volume in pores be­tween 50-100 A has decreased, while in pores >100 A radii do not seem to be affected much on increasing the acid concentration. On further increasing the acid amount from 1.5 to 1.65%, the pores in the range 50-I 00 A decrease and <50 A go upto 94%. It appears that the pores in the range of 18-50 A radii are main­tained upto the acid concentration of 1.5%. The pores affected by peptization are of < 100 radii and no marked effect on pores >I 00 A was observed. Opti­mum 1.5% HN03 was found to be suitable for pepti­zation to control pore size below 100 A radii meso pore range.

Some additives like polyvinyl alcohol, polyethyl­ene glycol and carbon black have been tried on alu­mina hydrate, peptized with optimum 1.5% HN03 in order to see the effect of these additives on the meso pores of relatively large size. The results are shown in

160 INDIAN J. CHEM. TECHNOL., MAY 2001

Table 2. Addition of 10% polyvinyl alcohol has de­creased the volume in pores of <50 A radii to the ex­tent of 10% and an increase of 8% in pores >100 A (compare samples C and E). Meso pores has been broadened and macroporosity has also been devel­oped on the addition of polyvinyl alcohol as also indi­cated by a drastic increase in mean pore radius (MPR) from 67 to 218 A polyethylene glycol (mol. wt. 400) has increased the pores > I 00 A and decreased pores between 18-50 A. But the increase in pores >I 00 A is to the lesser extent as compared to polyvinyl alcohol. No change in pore size range between 100 - 250 A has been observed (sample C and F). Polyethylene glycol (mol. wt. 600) has not shown any significant change in the entire pore size range probably due to bulky molecule not able to enter the pores of alumina properly.

Carbon black has shown a sharp decrease in vol­ume of pore in 18-50 A range, about 60% decrease was observed (compare sample C and H) and a corre­sponding increase in pores >50 A which has reflected in a decrease in surface area also. A marginal increase in pore volume may be due to some increase in pore between 100 to 1000 A. But maximum increase is reflected in pores between 50-100 A (67% volume in these pores) .

Calcined alumina has been used as a binder to in­crease the strength and also to modify the larger pores. The pores between 18-50 A are not affected on addition of I 0% calcined alumina to peptized mono­hydrate with 1.5% HN03 (sample C and I) Table 3. But percent volume in pores > 100 A has increased and also the MPR has increased . Both meso and macro porosity has been created. But total pore vol­ume has shown a decrease probably due to the occu­pation of some activated alumina in seats of some large pores. Data on y-Ab03 obtained from calcina­tion of boehmite has also been included in the table as reference. On increasing the amount of calcined alu­mina to 20%, the pores <50 A has decreased but >250 A as well > 1000 A have increased substantially. This has reflected in drastic increase in MPR (322 A, sam­ple J). This indicates that besides meso pores, macro pores have also been developed. On further increasing the ac tivated alumina to 30% (sample K) pore greater than 250 A radii have increased enormously without affecting the pore range 18-50 A further as compared to sample J. Large macro porosity has been developed which could affect adversely the mechanical strength of the catalyst.

On increasing the amount of calcined alumina, continuous increase in percent volume in pores >250 A and particularly pores > 1000 A was observed . The total pore volume also show an increasing trend ac­cordingly. The addition of 10% calcined alumina, the pores <50 A are maintained (compare sample C and I) but has increased both meso and macro pores. On the other hand with 20% alumina or above the meso and macro porosity has increased considerably but the pore size in 18-50 A is greatly influenced and re­duced.

It may be stated that 10% addition of calcined alu­mina is suitable for supports which has controlled meso porosity of narrow range <50 A radii with some meso and macro porosity also. While 20% alumina and above has created large pore meso porosi ty and macro porosity but has diminished pores in the range <50 A radii.

At the end, it may be stated that an alumina support of desired textural characteristics and controlled pore size distribution can be tailor m<1de by a careful selec­tion of alumina hydrate and its modification by pep­tizing agent, additives and binders.

Conclusions Peptization of alumina mono hydrate (boehmite)

with nitric acid as peptizing agent can be used to con­trol the narrow meso pore size distri bution below 100 A radii. The control of pore size in 18-50 A radii range can be achieved by using 1.5% nitric acid. The pores > 100 A radii are not affected with the peptizing agent. Addition of combustible add itives like polyvi­nyl alcohol and polyethylene glycol on peptized mono hydrate has broadened the meso pores and has also created some macro porosity . Carbon black has dras­tically decreased the pores < 50 A radii and increased pores in 50-100 A range. Macro pores are not affected significantly. Addition of binder like calcined alumina from 10 to 30%, continuously increase percent vol­ume is pores >250 A and also in pores > 1000 A cre­ating macro porosity. Addition of 10% alumina has improved !both meso and macro pores slightly while maintaining pore <50 A, but addition of 20% and above has created meso and macro porosity at the cost of decreasing pores of <50 A.

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