- Email: [email protected]
Characterization of hydro-cracking catalysts by acidity measurement
Applied Elsevier
Catalysis, 47 (1989) 45-57 Science Publishers B.V., Amsterdam
45 -
Printed
in The Netherlands
Characterization of Hydro-Cracking Acidity Measurement S.C. THOMPSON*
and J.F.
MATHEWS
Dept. of Chemical Engineering, (This paper was presented
Catalysts by
Monash
C’niversity,
at the Bicentenary
Clayton,
Catalysis
Victoria 3168 (Australia)
Conference
in Sydney,
1-2 September
1988)
ABSTRACT The number and strength S-653) and one Johnson three different methods: methylene
of acid sites on three catalysts,
two Shell Ni/W catalysts
Matthey Pt/A120tI washcoat on monolith catalyst, amine titration, ammonia temperature-programmed
blue adsorption.
The hydro-isomerization
and -cracking
(S-454
and
were measured by desorption and
characteristics
of the same
three catalysts were evaluated, using cetane as the feed. The catalysts had varying acidity which correlated well with their activity. The Shell S-653 catalyst had the greatest number of acid sites with both strong and weak acid sites present, weak acid sites present,
but with a flatter
next is the Shell S-454 distribution
catalyst
also with strong and
than the Shell S-653
catalyst,
and then
finally the Johnson Matthey catalyst with only very weak acid sites present. The Shell catalysts, with the metals (Ni/W) on the surface, almost the same number of acid sites present, but with varying acid strength,
produced
mainly mono-and
from the Johnson
Matthey
catalyst
acid sites consisted
of predominantly
multi-branched
with platinum n-paraffins
paraffins,
on the surface
whereas the products
and with substantially
with some mono- and multi-branched
fewer
paraffins.
INTRODUCTION
The overall objective of this study is to establish the role of the number and strength of acid sites on the support of a hydro-cracking catalyst used for the conversion of Fischer-Tropsch wax to a diesel range product. Hydro-cracking
catalysts
Hydro-cracking processes generally used in the petroleum refining industry for the cracking of heavy residua consist of two stages, a hydrotreatment stage using, for example, Ni/Mo or CO/MO on alumina catalysts (sulphided) to remove heteroatoms and then a hydro-cracking stage using, for example, Ni/W on silica-alumina catalysts (sulphided). Crystalline silica-alumina (zeolite) catalysts are now replacing the traditional non-crystalline (amorphous) catalysts.
46 TABLE
1
Catalyst composition Chemical
composition
Catalyst S-653
Nickel
(NiO)
Tungsten Alumina
4.2
6.5
nil
30.0
nil
25.1
63.5
99.0
67.5 nil
nil nil
nil 1.0
(wt.-%)
(A120i3) (wt.-%)
Silica (SiO?) (wt.-% ) Platinum (Pt) (wt.-%)
JM
3.2
(wt.-%)
(WO,,)
S-454
Catalysts used in this study
Three commercially available catalysts, two Shell Ni/W catalysts (S-454 and S-653) and one Johnson Matthey Pt/A1203 washcoat on a monolith catalyst, were tested in this study. All three catalysts tested were in the unsulphided form. The two Shell catalysts were the so called second stage hydrocracking catalysts. The Johnson Matthey catalyst is used in the automotive industry to oxidize unconverted products in motor vehicle exhausts to safer compounds, for example, carbon monoxide to carbon dioxide. This catalyst has not been used for hydro-cracking purposes and was only used in this study as a representative of the noble metal group. The chemical composition of each catalyst is given in Table 1. Hydrotreating
trials
Hydrotreating trials were conducted, using cetane as the feed, to evaluate the hydro-isomerization and -cracking characteristics of the three catalysts. Two mechanisms have been proposed for the hydro-isomerization and -cracking of n-paraffins and are discussed in ref. 1. For each catalyst tested the pressure, mass space velocity and hydrogen mass flow-rate were held constant. Temperature was varied to alter the extent of react.ion. Four hydrotreating trials were conducted with the Shell S-653, the fourth being a repeat of the first trial. Only two trials were conducted with both the Shell S-454 and Johnson Matthey catalysts. Catalyst acidity measurement
The acidity of each catalyst was measured by three different methods: amine titration [ 2 1, ammonia temperature-programmed desorption [ 21 and methylene blue adsorption (a technique developed in the Chemistry Department, Monash University [ 3 ] ) . Methylene blue adsorption, which is not a standard
method for acidity measurement, was conducted to establish how an aqueous method compared with the more traditional non-aqueous methods. EXPERIMENTAL
Hydrotreating
trials
The flow diagram for the equipment used in the hydrotreating trials is shown in Fig. 1. Cetane was fed to a down-flow trickle bed reactor by a piston pump. Argon gas regulated to the same pressure as the discharge pressure of the pump was used to provide the necessary force to move the piston. The movement of the piston was controlled by an electric motor geared to the required output speed. Hydrogen gas, from a bottle, was regulated before being fed to the reactor by a mass flow controller. The hydrogen gas was mixed with the cetane prior to entering the reactor. The feed was heated to reaction temperature almost immediately upon entering the reactor. Both the inlet and exit temperature of the reactor were meaHYDROGEN
TO “EN,
GAS
METER
FEED ‘sK$ PISTON PUMP
HEdVY LIOUID PilODUCT
Fig. 1. Flow diagram of the reactor system.
LIGHT LlDUlD PRODUCT
48
sured. The liquid products were collected in a high-pressure vessel. Excess hydrogen and gaseous products were then passed through a condenser into a second high-pressure collection vessel. The pressure drop across the reactor was measured and the pressure in the high pressure vessels was controlled by a back pressure valve in conjunction with a needle valve. The exit gas flowrate was measured by a wet type gas flowmeter. The liquid product was emptied into a collection vessel at atmospheric pressure and the weight recorded. The liquid product was analysed using a HP 5890 gas chromatograph (GC) with a 50-m crosslinked methyl-silicone capillary column (0.2 mm I.D. and film thickness 0.52 pm). After passing through the column the sample was split, part being sent to a FID detector connected to a HP 1000 computer and part to a HP 5970A mass selective detector for peak identification. The gas product was analysed repetitively during each trial via an in-line sampling valve connected to a HP 5890 GC with a 3-m phenyl isocyanate/ porasil C packed column. After passing through the column the sample was sent to an FID detector connected to a HP 3393 integrator. The two Shell catalysts, initially in the oxide form, were reduced in the following manner: hydrogen was first fed to the reactor at 10 l/h, the temperature was slowly increased to 8O’C and kept at that temperature for one hour, the temperature was then slowly increased to 400°C over a period of three hours and maintained at that level for a further two hours. After being reduced the reactor temperature was lowered to the temperature required for the first trial. Even though the platinum on the Johnson-Matthey catalyst was already present as metal and did not require reducing, the same reducing procedure as used for the two Shell catalysts was performed as a precautionary measure. Catalyst acidity measurement The two Shell catalysts were ground to a fine powder using a mortar and pestle. The aluminium washcoat was removed from the monolith by cutting the monolith into small blocks and then hitting them with a hammer. Under these conditions the washcoat fell away from the monolith and was able to be separated using a gravity separation technique. The washcoat did not require grinding. Amine titration involved the use of seven Hammett indicators of varying pK, strength. These indicators were prepared using benzene as the solvent. A small quantity of each catalyst was dried at 200 ‘C for one hour before being divided into eight smaller samples and placed in glass vials and weighed. Benzene was then added to each vial to cover the catalyst. A few drops of each indicat.or was then added to each vial. The vial with no indicator added was used as a colour standard to check the colour change of samples with indicators added. After the colour of each sample was examined and recorded n-butyl amine was added dropwise until the colour of the indicator on the catalyst changed back to its original colour. The number of moles of n-butyl amine
49
added represents the number of acid sites (Bronsted and Lewis) with acid strength of pK, above the indicator. Using the seven indicators enabled an acid strength distribution to be determined. Ammonia was adsorbed onto the surface of each cat.alyst at room temperature using a HP 5880 GC with a TCD detector. The catalyst was placed in a column and then connected in t.he oven of the GC as a normal column. Two gas sampling valves were used, the first controlled the flow of helium to the detector (either through the column loaded with catalyst or bypassing the column with the catalyst), the second had a sample tube of known volume attached to it (in the closed position ammonia flows through the sample tube at known pressure and helium flows to either the catalyst packed column or directly to the detector as controlled by the first valve, in the open position ammonia bypasses the sample tube and helium flows through the sample tube t.o either the catalyst-packed column or directly to the detector as controlled by the first valve). Pulses of ammonia were injected over the catalyst until breakthrough was detected (the number of moles of ammonia adsorbed was calculated). After ammonia was adsorbed onto the catalyst it was desorbed by heat.ing the sample at a controlled temperature heating rate to 400’ C (the number of moles ammonia desorbed was calculated). This technique also enabled both the number of acid sites (Bronsted and Lewis) and acid strength distribution to be determined. Methylene blue was adsorbed onto the surface of each catalyst by contacting a small quantity of catalyst (0.1 g) with a known amount of methylene blue in aqueous solution. After allowing 24 h for the mixture to equilibrate the catalyst was removed by centrifugation. The absorbance of the separated solut.ion at 664 nm was then determined using an IR spectrophotometer. The number of moles of methylene blue adsorbed from solution was then able to be calculated by comparing the absorbance obtained with the absorbance of standard solutions. The number of moles adsorbed is a measure of the number of Bronsted acid sites on the catalyst. RESULTS
AND DISCUSSION
Hydrotreating
trials
A summary of process conditions, cat.alyst employed and mass balances for the hydroconversion trials is provided in Table 2. The feed flow-rate of cetane (unable to be measured directly because cet.ane was leaking back past the piston seal in the feed pump, and unable to be obtained directly from the difference between the product flows and hydrogen feed flow because of difficulty maintaining the pressure in the reactor while controlling the hydrogen feed flow-rate) was calculated by making the following assumptions: (i) Cracking occurs via beta scission (methane and ethane were not detected
50 TABLE
2
Summary of process conditions
and flow-rate for hydrotreating
trials
Trial
Process conditions
S-653 1 Pressure (MPa ) Temperature ( ’ C)
4.8 283
Cetane pumping rate (g/h 1 Hydrogen flow-rate (STD l/h)
4.80 13.0 (g/h
Liquid product collection Gas collection rate (STD Space velocity
(g/g/h)
rate (g/h l/h)
1 1
JM
s-454 2
3
4
1
2
1
2
4.8 241
4.8 262
4.8 284
4.8 262
4.8 292
5.0 365
5.0 412
6.66 13.0
2.87 12.0
1.84 12.0
2.77 12.6
5.59 12.6
1.90 12.4
0.89 12.4
1.16
1.16
1.07
1.07
1.13
1.13
1.10
1.10
4.40 16.35
6.61 17.03
2.81 12.41
1.69 13.34
2.75 13.76
5.5 12.51
1.87 11.02
0.86 13.14
3.20
4.44
1.91
1.23
1.90
3.83
0.84
0.45
in the exit gas of any of the trials and both mono-and multi-branched products were being formed). (ii) Secondary cracking of paraffins between C, and Cl2 occurs. The relative amounts of hydrocarbons in the exit gas (mainly C, and C,) as measured by the GC were used to close the mass balances in these calculations. For all trials the temperature difference between the inlet and exit of the reactor was less than 1 'C. Catalyst activity of each catalyst did not appear to decrease over the period of time they were tested. The repeat trial using the Shell S-653 catalyst, conducted for the sole purpose of estimating loss of activity, showed that there was no loss of activity. An overall product distribution is provided by carbon number for each trial in Table 3. Olefins were not present in any of the liquid or the GC analyses. A plot of conversion versus temperature for each catalyst is shown in Fig. 2. This plot shows the temperature range over which the Shell S-653 catalyst can be operated to achieve conversion between 0 and 100 percent. The full conversion range for the Shell S-454 and Johnson Matthey catalysts were not determined. The order of decreasing catalyst activity (with respect to the minimum temperature required to achieve hydroconversion) is S-653, S-454 and Johnson Matthey. It should also be noted that the width of the temperature range corresponding to 0 to 100 percent conversion for each catalyst appears to increase with decreasing catalyst activity. The GC traces for the liquid products from the Shell S-653 trials 1, 2 and 3 are shown in Fig. 3. The cetane peak has been removed and the product distribution normalised using the C-plot capabilities of the HP 1000 computer. At low temperature, trial 2 (241 ‘C), there is a substantial amount of isomerization occurring compared to cracking, As temperature increases in trial 3 (262 aC ) and then’in trial 1 (283 ’ C ), more cracking occurs. From the product distribution in Table 3 it can also be shown that as temperature increases there
51 TABLE
3
Product distribution Carbon number
of hydrotreating
trials
Boiling point range
Cetane feed
Trial
i=ci S-653 1 1 2
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.8 6.3 9.7
0.0 0.2
0.2 1.2
1.5
0.0 0.3
0.3 2.0
1.3
0.0 2.1
0.3 0.3
1.6
0.3 0.3
0.9 1.5
1.4 1.4
2.6 2.0
1.8
3.0
0.3 0.2
2.0 2.1
1.2
1.8 1.3 1.1
-0.5
36 69 98 126
To To To To
0.0 0.0
69 98 126 151
0.0 0.0 0.0 0.0
151 To 174
200
250
Fig. 2. Temperature
300 TEMPERATURE
versus
6.9 5.2
0.4 0.3 0.4
10.6 11.6
1.9
8.3 6.5
0.2
2.1
0.3
1.1
0.3
2.1 2.3
0.7
5.3
0.7
0.8
3.3 0.5
2.0 0.3 0.1
0.6 0.2
0.6 0.3
0.1 1.6
0.3 0.0 0.0 2.2
6.7
0.1 1.3
24.0
95.5
75.6
87.7
4.3 2.i 0.5
0.0
0.2
0.21
0.0 1.9
0.0 0.9
99.11
31.6
96.3
0.0 1.3 83.6
DEC
6.4 10.2
1.5 2.0 2.1 1.7 1.5
0.27 0.18 0.21
235 To 253 253 To 271 2X To 287
9.9 10.5 8.9
0.3 0.3 0.2
0.0 0.01
174 To 196 196 To 216 216To235
16
2
1
0.0
To 36
15
2
0.0 0.0
-0.5
13 14
1
0.0
5 6
12
4
3
-89 -42 -42To
9 10 11
2
JM
-164
3 4
7 8
s-454
1.1
10.3
0.9 0.8
0.1 0.3 83.1
C
conversion.
is a movement of material in higher carbon number ranges towards lower carbon number ranges, indicating that secondary cracking is occurring. Interestingly, it appears from the GC traces that the number of other species present between the n-paraffin peaks and their relative amounts remain constant with increasing temperature. The GC traces for the liquid products from the Shell S-653 trial 3, Shell S454 trial 2 and the Johnson Matthey t.rial2, (trials which have approximately the same conversion) are shown in Fig. 4. Again the cetane peak has been
3
c.3
‘”
nc12
ncll
ncl0
I;
nc9
nc8
IF
Range Normalized
AHPLITUIX/l000
E= nc9
nc8
Range Normalized
AMPLITUDE/l000
ncl0
nc9
nc12
ncll
:I E i3
F8
M '8
AMPLITUDE/i000 Range Normalized
3 , 10
53
-I SHELL S-653 TRIAL 3 TEMP 262’C
0.00 RT in
5.60
minutes
i1.M
i6.W
22.40
26.00
33.60
U.60
39.20
SHELL S-454
RT &?hwtes
3.
6.60
11.20
I. 5.w
f !1.20
.
16.80
22.40
28.00
I, 16.80
I. 22.40
I 26.00
33.60
*
I 33.60
U.80
33.20
.
I 39.20
.
RT k?nin”tes Fig. 4. Liquid product distributions fot the three catalysts at the same conversion.
4
54
removed and the product distribution normalised using C-plot. The product distributions for the two Shell catalysts are identical except for the amount of isomerized material present. Again, from t,he product distribution in Table 3 the Shell S-454 catalyst produces five times as much isomerized C,, as the Shell S-653 catalyst. The Johnson Matthey catalyst has a completely different product. d~st.ribut~onwith mainly n-paraffins present.
A comparison of the total number of moles adsorbed (n-butyl amine, ammonia and methylene blue) as determined by each of the three methods employed is provided in Table 4. The total number of moles adsorbed using amine titration was based on neutral red as an indicator (all acid sites with strength up to p&=6.8 were determined). Detecting the colour change of the solid was very difficult (neutral red is yellow in basic form and red in acidic form) because the rate of replacement of the neutral red by n-butyl amine was very slow (several days). The Shell S-653 catalyst. has the greatest number of acid sites (1.1. 10e3 mol/ g), next is the Shell S-454 catalyst (7.3. lo-” mol/g) followed by the Johnson Matthey catalyst (1.1. 10Y4 mol/g). As with amine titration the total number of moles of ammonia desorbed is the greatest with the Shell S-653 catalyst (9.0. 10F4 mol/g), then the Shell S454 catalyst (7.4*1W4 mol/g) and finaIly the Johnson Matthey catalyst (8.4*10-” mol/g). These results are very close to the results obtained by amine titration. The ion-exchange mechanism for the adsorption of the methylene blue by the Shell S-454 catalyst, appeared to vary from that of the Shell S-653 and Johnson Matthey catalysts. Both t.he Shell S-653 and Johnson Matthey catalysts adsorbed all t,he methylene blue in solution, within the 24 h allowed for equilibration, until a maximum adsorption was reached (no matter how much more the concentration of methylene blue in solution is increased the catalyst TABLE 4 Comparison of the total amount of acidity for the three catalysts CataIyst
Methyl blue adsorbed (mol/g,,, 1
Amine adsorbed ” (mol/g,,,)
Ammonia adsorbed fmol/g,,,)
S-653 s-454 JM
2.5*1Ow’ 1.3*10-’ 5.8*10-;
11.1.10-J 7.3*10--”
Q.O.lO-” 7.4.10-1 8.4-10 --.%
1.1-X-~”
“Indicator used for amine adsorpt.ion is neutral red (pK, =6.8).
55
only adsorbs the same amount). The Shell S-454 was unable to adsorb all the methylene blue from solution, within the 24 h allowed for equilibration, even though the maximum adsorption had not been reached. Similar to amine titration and ammonia desorption the Shell S-653 catalyst has the greatest number of acid sites (2.5*10P4 mol/g), next is the Shell S-454 catalyst ( 1.3*10W4 mol/g) followedby the Johnson Matthey catalyst (5.8.10P7 mol/g). The acidity in terms of the total number of moles adsorbed for the three catalysts is much less than that obtained by the other two methods and is possibly either due to the difference between the type of acid site present, Brranstedand Lewis, or to the inability of the large methylene blue molecule to adsorb onto all the acid sites. The acid strength distributions for the three catalysts, as determined by TABLE 5 Acid strength by amine titration Indicator
6.8 4.0 3.3 2.8 1.5 -2.4 -5.6 -8.2
Neutral red 4-Phenylazo-l-naphtylamine p-Dimethylaminoazobenzene 4-Aminoazobenzene 4-Phenylazodiphenylamine 4-Nitrodiphenylamine Benzalacetophenone Anthraquinone
+6.6
N-Butylamine adsorbed
PK
+4.0
f3.3
pKo
Fig. 5. Amine titration.
+,.5
+2x RANGE
-2.4
-5.6
S-653
s-454
JM
1.11.10~” 3.1.10-4 3.1.10-* 3.1.10-4 3.1.10-4 7.0.10-” _ -
7.3.1o-1 3.6.10W4 3.1.10-4 3.1.10-4 2.5-10-4 -
1.1.10-4 -
-6.2
56
JM
I
NH3 Dasorbcd
Temperature
a C
Fig. 6. Ammonia TPD plots for (a) S-653, (b) S-454 and (c) Johnson Matthey catalysts. amine titration, are provided in Table 5 and graphically presented in Fig. 5. The Shell S-653 and S-454 catalysts have bimodal distributions indicating the presence of both weak and strong acid sites. For the Shell S-653 catalyst the
two acid strength ranges are 4.0 < pK, < 6.8 and - 5.6
Comparing the results of the acidity measurement of the two Shell catalysts with the results obtained from the hydro-cracking trials it appears that the
57
distribution of acid strengths of the catalyst may not be significant when tailoring a catalyst to produce a particular product distribution. The two Shell catalysts, with the same metals (Ni/W) on the surface, almost the same number of acid sites but with varying acid strength produce a similar product distribution. The number of acid sites, however, may be extremely important. The Johnson Matthey catalyst with platinum on the surface and with substantially fewer acid sites (the acid sites which do exist are of low strength) has higher selectivity for the formation of n-paraffins compared to the two Shell catalysts with (Ni/W) on the surface. ACKNOWLEDGEMENTS
The authors of this paper wish to thank Bruce Barnett (Shell Chemical, Aust.) for providing samples of the two Shell catalysts (S-653 and S-454), Paul Chapman (Johnson Matthey) for supplying the exhaust catalyst (Pt/Alumina Washcoat on Monolith), Judy Baker (BHP, Melbourne Research Laboratories) for the gas chromatography analyses of the liquid product, Sandra Bessel1 and Peter Iliopoulos (BHP, Melbourne Research Laboratories) for their assistance with the ammonia TPD tests and the Workshop Staff (Chemical Engineering Department, Monash University) for their assistance in building and maintaining the experimental equipment. Thanks are also due to BHP Research for allowing one of the authors (S.C. Thompson) to pursue a PhD degree.
REFERENCES 1 S.C. Thompson and J.F. Mathews, in Proceedings of Chemeca 88, Australia National Conf. Publ. No. 88/16, Institution of Engineers, 1988, p. 222. 2 P.A. Jacobs, in F. Delanney (Ed.), Characterization of Heterogeneous Catalysts, Chemical Industries Series, Vol. 15, Marcel Dekker, New York, 1984 pp. 367-404. 3 G.P. Handreck and T.D. Smith, J. Chem. Sot. Faraday Trans. 1, in press.
Catalysis, 47 (1989) 45-57 Science Publishers B.V., Amsterdam
45 -
Printed
in The Netherlands
Characterization of Hydro-Cracking Acidity Measurement S.C. THOMPSON*
and J.F.
MATHEWS
Dept. of Chemical Engineering, (This paper was presented
Catalysts by
Monash
C’niversity,
at the Bicentenary
Clayton,
Catalysis
Victoria 3168 (Australia)
Conference
in Sydney,
1-2 September
1988)
ABSTRACT The number and strength S-653) and one Johnson three different methods: methylene
of acid sites on three catalysts,
two Shell Ni/W catalysts
Matthey Pt/A120tI washcoat on monolith catalyst, amine titration, ammonia temperature-programmed
blue adsorption.
The hydro-isomerization
and -cracking
(S-454
and
were measured by desorption and
characteristics
of the same
three catalysts were evaluated, using cetane as the feed. The catalysts had varying acidity which correlated well with their activity. The Shell S-653 catalyst had the greatest number of acid sites with both strong and weak acid sites present, weak acid sites present,
but with a flatter
next is the Shell S-454 distribution
catalyst
also with strong and
than the Shell S-653
catalyst,
and then
finally the Johnson Matthey catalyst with only very weak acid sites present. The Shell catalysts, with the metals (Ni/W) on the surface, almost the same number of acid sites present, but with varying acid strength,
produced
mainly mono-and
from the Johnson
Matthey
catalyst
acid sites consisted
of predominantly
multi-branched
with platinum n-paraffins
paraffins,
on the surface
whereas the products
and with substantially
with some mono- and multi-branched
fewer
paraffins.
INTRODUCTION
The overall objective of this study is to establish the role of the number and strength of acid sites on the support of a hydro-cracking catalyst used for the conversion of Fischer-Tropsch wax to a diesel range product. Hydro-cracking
catalysts
Hydro-cracking processes generally used in the petroleum refining industry for the cracking of heavy residua consist of two stages, a hydrotreatment stage using, for example, Ni/Mo or CO/MO on alumina catalysts (sulphided) to remove heteroatoms and then a hydro-cracking stage using, for example, Ni/W on silica-alumina catalysts (sulphided). Crystalline silica-alumina (zeolite) catalysts are now replacing the traditional non-crystalline (amorphous) catalysts.
46 TABLE
1
Catalyst composition Chemical
composition
Catalyst S-653
Nickel
(NiO)
Tungsten Alumina
4.2
6.5
nil
30.0
nil
25.1
63.5
99.0
67.5 nil
nil nil
nil 1.0
(wt.-%)
(A120i3) (wt.-%)
Silica (SiO?) (wt.-% ) Platinum (Pt) (wt.-%)
JM
3.2
(wt.-%)
(WO,,)
S-454
Catalysts used in this study
Three commercially available catalysts, two Shell Ni/W catalysts (S-454 and S-653) and one Johnson Matthey Pt/A1203 washcoat on a monolith catalyst, were tested in this study. All three catalysts tested were in the unsulphided form. The two Shell catalysts were the so called second stage hydrocracking catalysts. The Johnson Matthey catalyst is used in the automotive industry to oxidize unconverted products in motor vehicle exhausts to safer compounds, for example, carbon monoxide to carbon dioxide. This catalyst has not been used for hydro-cracking purposes and was only used in this study as a representative of the noble metal group. The chemical composition of each catalyst is given in Table 1. Hydrotreating
trials
Hydrotreating trials were conducted, using cetane as the feed, to evaluate the hydro-isomerization and -cracking characteristics of the three catalysts. Two mechanisms have been proposed for the hydro-isomerization and -cracking of n-paraffins and are discussed in ref. 1. For each catalyst tested the pressure, mass space velocity and hydrogen mass flow-rate were held constant. Temperature was varied to alter the extent of react.ion. Four hydrotreating trials were conducted with the Shell S-653, the fourth being a repeat of the first trial. Only two trials were conducted with both the Shell S-454 and Johnson Matthey catalysts. Catalyst acidity measurement
The acidity of each catalyst was measured by three different methods: amine titration [ 2 1, ammonia temperature-programmed desorption [ 21 and methylene blue adsorption (a technique developed in the Chemistry Department, Monash University [ 3 ] ) . Methylene blue adsorption, which is not a standard
method for acidity measurement, was conducted to establish how an aqueous method compared with the more traditional non-aqueous methods. EXPERIMENTAL
Hydrotreating
trials
The flow diagram for the equipment used in the hydrotreating trials is shown in Fig. 1. Cetane was fed to a down-flow trickle bed reactor by a piston pump. Argon gas regulated to the same pressure as the discharge pressure of the pump was used to provide the necessary force to move the piston. The movement of the piston was controlled by an electric motor geared to the required output speed. Hydrogen gas, from a bottle, was regulated before being fed to the reactor by a mass flow controller. The hydrogen gas was mixed with the cetane prior to entering the reactor. The feed was heated to reaction temperature almost immediately upon entering the reactor. Both the inlet and exit temperature of the reactor were meaHYDROGEN
TO “EN,
GAS
METER
FEED ‘sK$ PISTON PUMP
HEdVY LIOUID PilODUCT
Fig. 1. Flow diagram of the reactor system.
LIGHT LlDUlD PRODUCT
48
sured. The liquid products were collected in a high-pressure vessel. Excess hydrogen and gaseous products were then passed through a condenser into a second high-pressure collection vessel. The pressure drop across the reactor was measured and the pressure in the high pressure vessels was controlled by a back pressure valve in conjunction with a needle valve. The exit gas flowrate was measured by a wet type gas flowmeter. The liquid product was emptied into a collection vessel at atmospheric pressure and the weight recorded. The liquid product was analysed using a HP 5890 gas chromatograph (GC) with a 50-m crosslinked methyl-silicone capillary column (0.2 mm I.D. and film thickness 0.52 pm). After passing through the column the sample was split, part being sent to a FID detector connected to a HP 1000 computer and part to a HP 5970A mass selective detector for peak identification. The gas product was analysed repetitively during each trial via an in-line sampling valve connected to a HP 5890 GC with a 3-m phenyl isocyanate/ porasil C packed column. After passing through the column the sample was sent to an FID detector connected to a HP 3393 integrator. The two Shell catalysts, initially in the oxide form, were reduced in the following manner: hydrogen was first fed to the reactor at 10 l/h, the temperature was slowly increased to 8O’C and kept at that temperature for one hour, the temperature was then slowly increased to 400°C over a period of three hours and maintained at that level for a further two hours. After being reduced the reactor temperature was lowered to the temperature required for the first trial. Even though the platinum on the Johnson-Matthey catalyst was already present as metal and did not require reducing, the same reducing procedure as used for the two Shell catalysts was performed as a precautionary measure. Catalyst acidity measurement The two Shell catalysts were ground to a fine powder using a mortar and pestle. The aluminium washcoat was removed from the monolith by cutting the monolith into small blocks and then hitting them with a hammer. Under these conditions the washcoat fell away from the monolith and was able to be separated using a gravity separation technique. The washcoat did not require grinding. Amine titration involved the use of seven Hammett indicators of varying pK, strength. These indicators were prepared using benzene as the solvent. A small quantity of each catalyst was dried at 200 ‘C for one hour before being divided into eight smaller samples and placed in glass vials and weighed. Benzene was then added to each vial to cover the catalyst. A few drops of each indicat.or was then added to each vial. The vial with no indicator added was used as a colour standard to check the colour change of samples with indicators added. After the colour of each sample was examined and recorded n-butyl amine was added dropwise until the colour of the indicator on the catalyst changed back to its original colour. The number of moles of n-butyl amine
49
added represents the number of acid sites (Bronsted and Lewis) with acid strength of pK, above the indicator. Using the seven indicators enabled an acid strength distribution to be determined. Ammonia was adsorbed onto the surface of each cat.alyst at room temperature using a HP 5880 GC with a TCD detector. The catalyst was placed in a column and then connected in t.he oven of the GC as a normal column. Two gas sampling valves were used, the first controlled the flow of helium to the detector (either through the column loaded with catalyst or bypassing the column with the catalyst), the second had a sample tube of known volume attached to it (in the closed position ammonia flows through the sample tube at known pressure and helium flows to either the catalyst packed column or directly to the detector as controlled by the first valve, in the open position ammonia bypasses the sample tube and helium flows through the sample tube t.o either the catalyst-packed column or directly to the detector as controlled by the first valve). Pulses of ammonia were injected over the catalyst until breakthrough was detected (the number of moles of ammonia adsorbed was calculated). After ammonia was adsorbed onto the catalyst it was desorbed by heat.ing the sample at a controlled temperature heating rate to 400’ C (the number of moles ammonia desorbed was calculated). This technique also enabled both the number of acid sites (Bronsted and Lewis) and acid strength distribution to be determined. Methylene blue was adsorbed onto the surface of each catalyst by contacting a small quantity of catalyst (0.1 g) with a known amount of methylene blue in aqueous solution. After allowing 24 h for the mixture to equilibrate the catalyst was removed by centrifugation. The absorbance of the separated solut.ion at 664 nm was then determined using an IR spectrophotometer. The number of moles of methylene blue adsorbed from solution was then able to be calculated by comparing the absorbance obtained with the absorbance of standard solutions. The number of moles adsorbed is a measure of the number of Bronsted acid sites on the catalyst. RESULTS
AND DISCUSSION
Hydrotreating
trials
A summary of process conditions, cat.alyst employed and mass balances for the hydroconversion trials is provided in Table 2. The feed flow-rate of cetane (unable to be measured directly because cet.ane was leaking back past the piston seal in the feed pump, and unable to be obtained directly from the difference between the product flows and hydrogen feed flow because of difficulty maintaining the pressure in the reactor while controlling the hydrogen feed flow-rate) was calculated by making the following assumptions: (i) Cracking occurs via beta scission (methane and ethane were not detected
50 TABLE
2
Summary of process conditions
and flow-rate for hydrotreating
trials
Trial
Process conditions
S-653 1 Pressure (MPa ) Temperature ( ’ C)
4.8 283
Cetane pumping rate (g/h 1 Hydrogen flow-rate (STD l/h)
4.80 13.0 (g/h
Liquid product collection Gas collection rate (STD Space velocity
(g/g/h)
rate (g/h l/h)
1 1
JM
s-454 2
3
4
1
2
1
2
4.8 241
4.8 262
4.8 284
4.8 262
4.8 292
5.0 365
5.0 412
6.66 13.0
2.87 12.0
1.84 12.0
2.77 12.6
5.59 12.6
1.90 12.4
0.89 12.4
1.16
1.16
1.07
1.07
1.13
1.13
1.10
1.10
4.40 16.35
6.61 17.03
2.81 12.41
1.69 13.34
2.75 13.76
5.5 12.51
1.87 11.02
0.86 13.14
3.20
4.44
1.91
1.23
1.90
3.83
0.84
0.45
in the exit gas of any of the trials and both mono-and multi-branched products were being formed). (ii) Secondary cracking of paraffins between C, and Cl2 occurs. The relative amounts of hydrocarbons in the exit gas (mainly C, and C,) as measured by the GC were used to close the mass balances in these calculations. For all trials the temperature difference between the inlet and exit of the reactor was less than 1 'C. Catalyst activity of each catalyst did not appear to decrease over the period of time they were tested. The repeat trial using the Shell S-653 catalyst, conducted for the sole purpose of estimating loss of activity, showed that there was no loss of activity. An overall product distribution is provided by carbon number for each trial in Table 3. Olefins were not present in any of the liquid or the GC analyses. A plot of conversion versus temperature for each catalyst is shown in Fig. 2. This plot shows the temperature range over which the Shell S-653 catalyst can be operated to achieve conversion between 0 and 100 percent. The full conversion range for the Shell S-454 and Johnson Matthey catalysts were not determined. The order of decreasing catalyst activity (with respect to the minimum temperature required to achieve hydroconversion) is S-653, S-454 and Johnson Matthey. It should also be noted that the width of the temperature range corresponding to 0 to 100 percent conversion for each catalyst appears to increase with decreasing catalyst activity. The GC traces for the liquid products from the Shell S-653 trials 1, 2 and 3 are shown in Fig. 3. The cetane peak has been removed and the product distribution normalised using the C-plot capabilities of the HP 1000 computer. At low temperature, trial 2 (241 ‘C), there is a substantial amount of isomerization occurring compared to cracking, As temperature increases in trial 3 (262 aC ) and then’in trial 1 (283 ’ C ), more cracking occurs. From the product distribution in Table 3 it can also be shown that as temperature increases there
51 TABLE
3
Product distribution Carbon number
of hydrotreating
trials
Boiling point range
Cetane feed
Trial
i=ci S-653 1 1 2
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.8 6.3 9.7
0.0 0.2
0.2 1.2
1.5
0.0 0.3
0.3 2.0
1.3
0.0 2.1
0.3 0.3
1.6
0.3 0.3
0.9 1.5
1.4 1.4
2.6 2.0
1.8
3.0
0.3 0.2
2.0 2.1
1.2
1.8 1.3 1.1
-0.5
36 69 98 126
To To To To
0.0 0.0
69 98 126 151
0.0 0.0 0.0 0.0
151 To 174
200
250
Fig. 2. Temperature
300 TEMPERATURE
versus
6.9 5.2
0.4 0.3 0.4
10.6 11.6
1.9
8.3 6.5
0.2
2.1
0.3
1.1
0.3
2.1 2.3
0.7
5.3
0.7
0.8
3.3 0.5
2.0 0.3 0.1
0.6 0.2
0.6 0.3
0.1 1.6
0.3 0.0 0.0 2.2
6.7
0.1 1.3
24.0
95.5
75.6
87.7
4.3 2.i 0.5
0.0
0.2
0.21
0.0 1.9
0.0 0.9
99.11
31.6
96.3
0.0 1.3 83.6
DEC
6.4 10.2
1.5 2.0 2.1 1.7 1.5
0.27 0.18 0.21
235 To 253 253 To 271 2X To 287
9.9 10.5 8.9
0.3 0.3 0.2
0.0 0.01
174 To 196 196 To 216 216To235
16
2
1
0.0
To 36
15
2
0.0 0.0
-0.5
13 14
1
0.0
5 6
12
4
3
-89 -42 -42To
9 10 11
2
JM
-164
3 4
7 8
s-454
1.1
10.3
0.9 0.8
0.1 0.3 83.1
C
conversion.
is a movement of material in higher carbon number ranges towards lower carbon number ranges, indicating that secondary cracking is occurring. Interestingly, it appears from the GC traces that the number of other species present between the n-paraffin peaks and their relative amounts remain constant with increasing temperature. The GC traces for the liquid products from the Shell S-653 trial 3, Shell S454 trial 2 and the Johnson Matthey t.rial2, (trials which have approximately the same conversion) are shown in Fig. 4. Again the cetane peak has been
3
c.3
‘”
nc12
ncll
ncl0
I;
nc9
nc8
IF
Range Normalized
AHPLITUIX/l000
E= nc9
nc8
Range Normalized
AMPLITUDE/l000
ncl0
nc9
nc12
ncll
:I E i3
F8
M '8
AMPLITUDE/i000 Range Normalized
3 , 10
53
-I SHELL S-653 TRIAL 3 TEMP 262’C
0.00 RT in
5.60
minutes
i1.M
i6.W
22.40
26.00
33.60
U.60
39.20
SHELL S-454
RT &?hwtes
3.
6.60
11.20
I. 5.w
f !1.20
.
16.80
22.40
28.00
I, 16.80
I. 22.40
I 26.00
33.60
*
I 33.60
U.80
33.20
.
I 39.20
.
RT k?nin”tes Fig. 4. Liquid product distributions fot the three catalysts at the same conversion.
4
54
removed and the product distribution normalised using C-plot. The product distributions for the two Shell catalysts are identical except for the amount of isomerized material present. Again, from t,he product distribution in Table 3 the Shell S-454 catalyst produces five times as much isomerized C,, as the Shell S-653 catalyst. The Johnson Matthey catalyst has a completely different product. d~st.ribut~onwith mainly n-paraffins present.
A comparison of the total number of moles adsorbed (n-butyl amine, ammonia and methylene blue) as determined by each of the three methods employed is provided in Table 4. The total number of moles adsorbed using amine titration was based on neutral red as an indicator (all acid sites with strength up to p&=6.8 were determined). Detecting the colour change of the solid was very difficult (neutral red is yellow in basic form and red in acidic form) because the rate of replacement of the neutral red by n-butyl amine was very slow (several days). The Shell S-653 catalyst. has the greatest number of acid sites (1.1. 10e3 mol/ g), next is the Shell S-454 catalyst (7.3. lo-” mol/g) followed by the Johnson Matthey catalyst (1.1. 10Y4 mol/g). As with amine titration the total number of moles of ammonia desorbed is the greatest with the Shell S-653 catalyst (9.0. 10F4 mol/g), then the Shell S454 catalyst (7.4*1W4 mol/g) and finaIly the Johnson Matthey catalyst (8.4*10-” mol/g). These results are very close to the results obtained by amine titration. The ion-exchange mechanism for the adsorption of the methylene blue by the Shell S-454 catalyst, appeared to vary from that of the Shell S-653 and Johnson Matthey catalysts. Both t.he Shell S-653 and Johnson Matthey catalysts adsorbed all t,he methylene blue in solution, within the 24 h allowed for equilibration, until a maximum adsorption was reached (no matter how much more the concentration of methylene blue in solution is increased the catalyst TABLE 4 Comparison of the total amount of acidity for the three catalysts CataIyst
Methyl blue adsorbed (mol/g,,, 1
Amine adsorbed ” (mol/g,,,)
Ammonia adsorbed fmol/g,,,)
S-653 s-454 JM
2.5*1Ow’ 1.3*10-’ 5.8*10-;
11.1.10-J 7.3*10--”
Q.O.lO-” 7.4.10-1 8.4-10 --.%
1.1-X-~”
“Indicator used for amine adsorpt.ion is neutral red (pK, =6.8).
55
only adsorbs the same amount). The Shell S-454 was unable to adsorb all the methylene blue from solution, within the 24 h allowed for equilibration, even though the maximum adsorption had not been reached. Similar to amine titration and ammonia desorption the Shell S-653 catalyst has the greatest number of acid sites (2.5*10P4 mol/g), next is the Shell S-454 catalyst ( 1.3*10W4 mol/g) followedby the Johnson Matthey catalyst (5.8.10P7 mol/g). The acidity in terms of the total number of moles adsorbed for the three catalysts is much less than that obtained by the other two methods and is possibly either due to the difference between the type of acid site present, Brranstedand Lewis, or to the inability of the large methylene blue molecule to adsorb onto all the acid sites. The acid strength distributions for the three catalysts, as determined by TABLE 5 Acid strength by amine titration Indicator
6.8 4.0 3.3 2.8 1.5 -2.4 -5.6 -8.2
Neutral red 4-Phenylazo-l-naphtylamine p-Dimethylaminoazobenzene 4-Aminoazobenzene 4-Phenylazodiphenylamine 4-Nitrodiphenylamine Benzalacetophenone Anthraquinone
+6.6
N-Butylamine adsorbed
PK
+4.0
f3.3
pKo
Fig. 5. Amine titration.
+,.5
+2x RANGE
-2.4
-5.6
S-653
s-454
JM
1.11.10~” 3.1.10-4 3.1.10-* 3.1.10-4 3.1.10-4 7.0.10-” _ -
7.3.1o-1 3.6.10W4 3.1.10-4 3.1.10-4 2.5-10-4 -
1.1.10-4 -
-6.2
56
JM
I
NH3 Dasorbcd
Temperature
a C
Fig. 6. Ammonia TPD plots for (a) S-653, (b) S-454 and (c) Johnson Matthey catalysts. amine titration, are provided in Table 5 and graphically presented in Fig. 5. The Shell S-653 and S-454 catalysts have bimodal distributions indicating the presence of both weak and strong acid sites. For the Shell S-653 catalyst the
two acid strength ranges are 4.0 < pK, < 6.8 and - 5.6
Comparing the results of the acidity measurement of the two Shell catalysts with the results obtained from the hydro-cracking trials it appears that the
57
distribution of acid strengths of the catalyst may not be significant when tailoring a catalyst to produce a particular product distribution. The two Shell catalysts, with the same metals (Ni/W) on the surface, almost the same number of acid sites but with varying acid strength produce a similar product distribution. The number of acid sites, however, may be extremely important. The Johnson Matthey catalyst with platinum on the surface and with substantially fewer acid sites (the acid sites which do exist are of low strength) has higher selectivity for the formation of n-paraffins compared to the two Shell catalysts with (Ni/W) on the surface. ACKNOWLEDGEMENTS
The authors of this paper wish to thank Bruce Barnett (Shell Chemical, Aust.) for providing samples of the two Shell catalysts (S-653 and S-454), Paul Chapman (Johnson Matthey) for supplying the exhaust catalyst (Pt/Alumina Washcoat on Monolith), Judy Baker (BHP, Melbourne Research Laboratories) for the gas chromatography analyses of the liquid product, Sandra Bessel1 and Peter Iliopoulos (BHP, Melbourne Research Laboratories) for their assistance with the ammonia TPD tests and the Workshop Staff (Chemical Engineering Department, Monash University) for their assistance in building and maintaining the experimental equipment. Thanks are also due to BHP Research for allowing one of the authors (S.C. Thompson) to pursue a PhD degree.
REFERENCES 1 S.C. Thompson and J.F. Mathews, in Proceedings of Chemeca 88, Australia National Conf. Publ. No. 88/16, Institution of Engineers, 1988, p. 222. 2 P.A. Jacobs, in F. Delanney (Ed.), Characterization of Heterogeneous Catalysts, Chemical Industries Series, Vol. 15, Marcel Dekker, New York, 1984 pp. 367-404. 3 G.P. Handreck and T.D. Smith, J. Chem. Sot. Faraday Trans. 1, in press.