219
Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 219-225
However, it is interesting to note that at moderate acid strength, melt having low viscosity attains higher conversion than that with pure ZnC12in hydroliquefaction of brown coal without vehicle oil. Conclusion The hydroliquefaction of three lithotypes of Yallourn brown coal was investigated over molten salt catalysts (ZnC1, and/or SnC1, and/or KC1) in a batch autoclave at 400 “C and a cold pressure of 100 kg/cm2 for 3 h. Conversions of pale, medium light, and medium dark decrease in this order: 66.2-77.1%, 50.4-65.0%, and 41.2-65.690, respectively. It was found that hexane-soluble yields from three lithotypes catalyzed by pure ZnCl, are lower than those from the runs catalyzed by the other five molten salt systems. The acidity (evaluated by i-C4/n-C4ratio of gas produced) suggests that moderate acidity is effective in attaining high conversion and high yield of hexane solubles. However, this view has some limitation because ZnC1,SnC12 give high yields of HS in spite of its acidity comparable to pure ZnCIP. Viscosities (pure ZnC12,240 cP at 400 “C and other melts, 2.7 cP at 400 “C) seem to be correlated with degrees of hydroliquefaction. Therefore, both moderate acidity and reduced viscosity of the melts are supposed as major factors improving the hydroliquefaction of brown coal in the absence of organic solvents. Acknowledgment We are grateful to Dr. Paul Philp of CSIRO, Division of Fossil Fuels, Australia, for valuable discussion on brown
coal liquefaction using molten salt catalyst, and the State Electricity Commission of Victoria for providing the samples. Registry No. SnC12,7772-99-8; ZnC12,7646-85-7; KC1, 744740-7.
Literature Cited Allardlce, D. J.; George, A. M.; Hausser, D.; Neubert, K. H.; Smith, 0. C. “The Variation of Latrobe Valley Brown Coal Properties with Lithotype”. Victoria, State Electricity Commisslon, Brown Coal Res. Div. Rep. (1977), No. 342. Biackburn, D. T. “Floristic, Environmental, and Lithotype Correlations in the Yaiiourn Formation, Victoria”; paper presented at the Cainozoic Evolution of Continental South-Eastern Australia Conf., Canberra, Nov 1980. Camier, R. J. Ph.D. Thesis, Department of Chemical Engineering, University of Melbourne, Melbourne, Australia 1979. Ida, T.; Nomura, M.; Nakatsuji, Y.; Kikkawa, S. Fuel 1979, 58, 361. Martin, F. Fuel 1975, 54. 236. Morlta, 2.; IMa, T. Nlppon Klnzoku Gekkalshl 1980, 79, 655. Nakatsuji, Y.; Kubo, T.; Nomura, M.; Kikkawa, S. Bull. Chem. SOC. Jpn. 1978, 57, 618. Nakatsuji, Y.; Shigeta, K.; Nomura, M.; Kikkawa, S. Sekku Gakkaishl 1980, 2 3 , 420. Nomura, M.; Miyake, M.; Sakashita, H.; Kikkawa, S. Fuel 1982, 67, 18. Nomura, M.; Kimura, K.; Kikkawa, S. Fuel 1982, 6 2 , 1119. Sawatzky, H.; George, A. E.; Smiiey, G. T.; Montgomery, D. S. Fuel 1978, 55, 16. Verheyer, T. V.; John, R. B. Geochlm. Cosmochlm. Acta 1981, 45, 1899. YoshMa, M.; Ohnuki, T. I n “Shinjikken Kagaku Koza, Voi. I”; Japanese Chemical Society, Maruzen, 1975; p 436. Zieike, C. W.; Struck, R. T.; Evans, J. M.; Costanza, C. P.; Gorin, E. Ind. Eng. Chem. Process Des. D e v . 1968, 5, 158.
Received for review September 27, 1982 Revised manuscript received April 11, 1983 Accepted October 21, 1983
Decomposition of Alcohols over Zirconium and Titanium Phosphatest Soofln Cheng, Guang Zhl Peng, and Abraham Clearfleld’ Department of Chemistry, Texas A&M Unlverslv, College Station, Texas 77843
The catalytic properties of the layered compounds-zirconium phosphate, titanium phosphate, mixed zirconium titanium phosphate, and their organically pillared derivatives-toward methanol conversion have been examined. Dimethyl ether was found to be the precursor for hydrocarbon formation. However, strong acidities of the phosphate compounds seem to enhance methane formation. The phenyl-group bridged derivatives were found to have remarkably higher Catalytic activities and selectivities for the conversion of methanol to low molecular weight hydrocarbons. These pillared compounds were considered to provide pore structure for shapsselectiie products. Moreover, the phenyl pillars were suggested to be capable of stabilizing reaction intermediates.
Introduction The group 4 acid phosphates have layered structures and behave as ion exchangers (Clearfield and Alberti, 1982). Adjacent layers are arranged in such a way as to produce cavities between the layers (Clearfield and Smith, 1969; R o u p and Clearfield, 1977). A schematic view of a portion of the structure is shown in Figure 1. Exchange of cations apparently takes place by diffusion of the cations parallel to the layers as all the protons lie near the center of the interlayer space outlining the cavities. In the case of a+ The work reported in this paper was presented at the Symposium on New Catalytic Materials at the 186thNational Meeting of the American Chemical Society, Washington, DC, August 1983.
Ol96-432lf84f 1223-O219$O1.5OfO
zirconium phosphate (a-ZrP), Zr(HP0,),-H20, the free space of the passageways leading into cavities is just sufficient to allow an unhydrated K+ to diffuse in unobstructed (Clearfield et al., 1969). Upon deh dration the a-ZrP, which has an interlayer spacing of 7.6 transforms to S.-ZFtP (dm2= 7.4 A), and at higher temperatures, 7-ZrP, interlayer spacing 7.1 A, is obtained (Clearfield and Pack, 1975). This interlayer distance is so small that the dehydration of cyclohexanol over zirconium phosphate was found to be catalyzed only by the surface acid groups and not those in the interior (Clearfield and Thakur, 1980; Thakur and Clearfield, 1981). Recently it was shown that the a-layered compounds could be pillared by organic phosphate and phosphonates (Dines et al., 1982). Produds of high, presumably internal,
l,
0 1984 American Chemical Society
220
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table I. Effect of a-ZrP Crystallinity on Methanol Conversion" catalyst
gel
S A . , m2/g reaction temp, "C reaction time, h conversion, mol % MeOH to light HC light HC distribution, mol % CH, C2H4 C*H,
> 250
c 4
c, C6
MeOH to MeOH to MeOH to MeOH t o a
DME HCHO CO + CO, HBP
Pressure := 1 atm; WHSV = 3.2 h - ] .
425 6.0 89.7 60.1
90 436 6.5 87.0 35.0
700 420 3 84.3 14.0
93.7 2.0 3.1 1.1
86.2 5.6 4.5 3.6
0.75 84.8 5.4
2.5 82.9 0.6
306 8.0 82.9 0.02
6.7 24.2 0.8 39.1 15.8 1.5 11.9 45.3 0.02 1.7 32.3
25.4 63.9
100
10.7
0 0
94.2 1.5 3.4 0.9
82.2
0 0 82.6
8.3
17.7
39.9
0.1 0
0.3 0
2.7 18.6
0.4 33.8
7.4 23.0
0
S.A. of silica gel.
n
0
ZrP gel on silica gel
140
294
c 3
(2.5.48)
(2.5:15)
0
Figure 1. Schematic illustration of a portion of the a-zirconium phosphate structure showing portions of two adjacent layers.
surface area were obtained and it was therefore of interest to see if reactions taking place between the layers were similar to those occurring on the surface. In this paper we report on the decomposition of methyl alcohol catalyzed by both a-zirconium and a-titanium phosphates and their organic derivatives. Experimental Section Catalyst Preparation. a-Zirconium phosphates of three different crystallinities,labeled gel, 2.5:15, and 2.548, were prepared as described previously (Clearfield and Berman, 1981). These designations indicate an unreflwed precipitate, a precipitate refluxed in 2.5 M H3P04for 15 h and for 48 h, respectively. Another catalyst consisted of ZrP gel supported on silica gel of surface area 700 m2/g. It was prepared by wetting silica gel with 1 M ZrOClz solution, filtering, and drying at 100 "C, followed by stirring with 1M H3P04overnight, filtering, washing, and drying at 350 "C. a-Titanium phosphates, (2.548) and (6:100), and mixed Ti-Zr phosphate were prepared as described elsewhere (Clearfield and Frianeza, 1984). Ticrystallites were prepared tanium pyrophosphate (TiP207) by boiling TiP gel with anhydrous H3P04overnight, followed by washing and drying. Organically pillared derivatives were prepared by adding a solution of ZrOCl,. 8H20into a mixture of H3P04and organic diphosphonic acid solution, which was maintained at 60 "C for 3-7 days in order to obtain crystalline materials (Dines et al., 1982). Catalyst samples were characterized by X-ray powder diffraction, infrared absorption spectroscopy, TGA, and BET surface area measurements. The organic compositions of the pillared compounds were determined by carbon/ hydrogen analysis and thermogravimetric analysis. Apparent and Reaction Methods. The reaction was carried out in an ordinary flow-type reactor at atmospheric
pressure. The catalyst, (1g) in powder form, was packed into a U-shaped Pyrex glass reactor. A thermocouple well was fixed at the center of the catalyst bed to register the temperature of the reaction zone. Catalysts were preheated at the reaction temperature overnight and evacuated for at least 30 min prior to the start of the reaction. Methanol was kept in a 3-stage bubbling trap and the flow rate of diluent, He gas, was controlled by a tubular flowmeter (Matheson Gas Inc.). The He gas was passed through the bubbling trap and carried saturated MeOH vapor (ca. 108 torr at 25 "C) into the catalyst bed. The flow rate of methanol and helium mixture was adjusted to nearly 1mL/min. The flow path past the catalyst bed was wound with heating tape and the temperature was kept at ca. 100 "C to avoid product condensation in the line. The products were analyzed by a Hewlett-Packard 5830A gas chromatograph. A Porapak N column was used for separating the product mixture. In order to determine H2 and CO content, samples were removed just downstream from the reactor at selected intervals. These were analyzed with a Carle GC Model 157 controlled by a Hewlett-Packard Data Automation system. The amount of higher boiling-point products (HBP) or coke was determined based on mass balance of carbon atoms in and out of the catalyst bed. Results Methanol conversion was carried out over different catalysts at temperatures ranging from 300 to 440 "C. The results for a series of a-ZrP catalysts are shown in Table I. It should be noted that samples with low crystallinity but high surface area were used (Clearfield and Berman, 1981). Because of its low thermostability, the gel sample was operated at a relatively low temperature, 294 "C, compared with the other samples. Since the product distribution changed somewhat with time, the time after starting is given as the reaction time. Dimethyl ether (DME) was the major product at lower temperatures. As the temperature increased, the amount of low molecular weight hydrocarbons, mainly C1-C4, increased as did unidentified higher boiling products (HBP). This observation coincides with the commonly accepted opinion that DME is the precursor for hydrocarbon formation. Further evidence was found in studying the effect of contact time. The yield of DME was significantly reduced and the hydrocarbon yield increased as contact time increased. In the case of the gel catalyst, 5.4 mol % of MeOH was initially converted to light hydrocarbons, ranging from C1to C6, in addition to 45.3% yield of DME, 32.3% yield of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984 221
Table 11. MeOH Conversion over TiP Compounds catalyst reaction temp, “C react ion time, h conversion, mol % MeOH to light HC light HC distribution, mol % CH, c 2 H4 C,H,
c;
-
MeOH to MeOH to MeOH to MeOH to a
HBP DME HCHO CO + CO,
(2.5:48)TiP 401 421 3 5.5 82.8 86.9 9.0 19.9 95.1 3.1 0.1 1.8 0 73.4 0.2 0.1
(6: 100)TiP 397 3 74.8 7.5
TiP,O, 402 3 82.4 9.5
96.3 1.5 2.2
95.2 2.2 1.8 0.7 48.1 18.6
4.2 63.1
0.3
0.1
94.2 3.2 2.0 0.3 22.2 48.0 1.9 0.8
423 3 87.0 19.9 96.4 1.6 1.8 0.05 26.8 23.7 2.8
ZrmTi0.2 (HPO,), 403 17 87.6 24.2 92.3 1.9 3.7 1.0 38.2 22.0 0.4 2.8
Pressure = 1atm; WHSV = 3.2 h-l.
higher boiling-point produds (HBP), small amounts of CO and C 0 2 , and trace amounts of formaldehyde. The selectivity for hydrocarbon formation diminished to 0.6 mol ?’ & in 2.5 h, and DME was the dominant product. In the case of the more crystalline samples, (2.5:15) and (2.548), DME was nearly the only product accompanying H 2 0 at lower temperatures (ca. 300 “C). However, methane became the predominant product at higher temperaures. Increasing S.A. of the catalyst by supporting ZrP on silica gel did not improve the hydrocarbon yield but did slightly depress methane formation. The results in Table I imply that DME is the precursor for hydrocarbon formation. However, a competitive reaction path may lead to methane formation rather than other hydrocarbons. Moreover, porous structures seem to be required in order for DME to be subsequently converted to hydrocarbons before it desorbs from the catalysts’ surface. This requirement probably explains why fairly significant amounts of C2 to C6species were formed over ZrP gel in the beginning of the reaction period. It is thought that gel samples might acquire some porosity through phosphate condensation during the pretreatment procedure, but the porous structure was apparently not stable under the present reaction conditions and therefore selectivity for hydrocarbon formation decreased sharply. Similar results were obtained using TiP in place of ZrP, as shown in Table 11. However, the higher acidity of TiP seems to facilitate more methane formation, as is illustrated by the sample of mixed Ti-ZrP in which 80% of Ti was replaced by Zr. The selectivity for methane was indeed reduced. One fact which must be kept in mind when comparing the results of ZrP and TiP is that the TiP used had a much higher crystallinity than the ZrP, and as a result, the S.A. of the former is much lower. Hence, the hydrocarbon yields were usually less with TiP than with ZrP. Also, highly crystalline TiP207was employed as a catalyst in order to examine the effect of Lewis acid sites (TiP207has only Lewis acid sites). Surprisingly, similar activities and selectivities were observed with TiP20, and 2.5:48) a-ZrP. This indicates that both Bronsted and Lewis acid sites on phosphate compounds are active for methanol conversion. In order to determine the effect of pillaring the layers on the reaction, a series of compounds of the type shown in Figure 2 were used as catalysts. In these compounds, the layers of the ZrPO, framework are bridged by phenyl groups which increase the interlamellar spaces. Inside the cavities, there are OH groups attached to the inorganic orthophosphate segments. The size of cavities can be expanded either by replacing phenyl groups with longer organic compounds or by increasing the ratio of inorganic orthophosphate to organic phosphonate groups. As a result
0
Zr
*
c
o
p
0
0
(a)
(b)
Figure 2. Idealized structures of mixed-component pillared zirconium phosphate derivatives: (a) p-phenylenediphosphonate/phosphate and (b) 4,4‘-biphenyldiphosphonate/phosphate.
of the latter operation, the distances between every two organic chains is lengthened and more OH groups reside in between. Table I11 lists the pillared compounds along with a-ZrP used as catalysts for methanol conversion. Other samples are named P-1-1,PP-1-2, ...,etc. The first term designates the organic groups which bridge the layers, i.e., P for phenyl, PP for diphenyl, MPM for p-dimethyl phenyl, R for NNN”’N’’N”-hexamethyl rosaniline group, and XP is for a mixed Ti-Zr phosphonate bridged by phenyl groups. The two Arabic numbers represent the ratio of inorganic orthophosphate/organic phosphonate in the mixed component samples. All samples, except “R-0-l”, were obtained as crystalline materials and were stable to temperatures of around 400 “C. Thermal gravimetric analysis indicated that organic bridging groups in sample “R-0-1”decomposed at ca. 310 “C. The far right column in Table I11 lists the interlayer distances of these pillared derivatives. X-ray examination of the catalysts after use in the methanol reaction showed that they maintained their interlayer spacings although the intensities of the reflections were reduced. The catalytic activities and selectivities of these organically pillared compounds are tabulated in Table IV. The reaction temperature was 350 “C. Even at this temperature a significant MeOH conversion was achieved. The yield of light hydrocarbons, other than methane, was significantly increased. However, the extent of the increase varies with different organic bridging groups. Samples bridged by phenyl groups give the lowest yield of methane, and the main products are ethylene and C3 species (propylene was found to be the predominant component). In the case of XP-1-1, which was prepared from a mixed Ti(1V)-Zr(1V) solution, the methane formation was again increased. Samples bridged by diphenyl groups give higher methane yield than monophenyl-bridged compounds. However, more significant amounts of C4and C5were also detected over these catalysts. The results imply that
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 2, 1984
222
Table 111. Organically Pillared ZrP R
(HPO,): (O,PRPO,)
cat.
d spacing, A
a-ZrP
l:o
7.6
P-1-1
1:l
9.6
P-1-3
1:2.8
9.6
P-1-5
1:4.9
9.6
PP-0-1
0:l
13.6
PP-1-2
1:2.3
13.6
PP-1-4
1:4.0
13.6
MPM-1-1
1:l
10.8
R-0-1
0:l
amorphous
XP-1-1 (80%Ti, 20%Zr)
1:l
60 -
60
L E80
"
~
.
!ieht HC
HBP
~
an
""I r
6C
40
r
1 4c
+/*-• L'
400
35c
R e a c t i o n Tenperafure
('Cc)
4OC
6C
EO
20
350
' i
c3
350
+ T
I
t
o -.
400
-
\
*+*
350
" 3
"I" - A
2:
-
400
R e a c t i o n Temperature ( ' C )
Figure 3. Temperature effect on reactivity in methanol conversion over (0) (2.5:15)a-ZrP;(0)(2.5:48)a-TiP;and (e) P-1-5.
Figure 4. Temperature effect on light HC selectivity in methanol conversion over (0)(2.5:15)a-ZrP; (0)(2.5:48)TiP;and (e) P-1-5.
methanol conversion is a shape-selective reaction. On the other hand, interesting results were obtained while using sample "PP-0-l", a diphenyl-bridged catalyst which contains no orthophosphate groups. It gave a higher hydrocarbon yield with lower methane production as compared to the other diphenyl-bridged catalysts. Finally, the only also demonstrated fairly good amorphous sample "R-0-1" activity and selectivity for hydrocarbon formation even though the TGA indicated that all organic bridging groups should have decomposed in the pretreatment stage. Its catalytic activity indicates that the sample may still retain pore structure after organic groups decompose and leave the framework. The effect of contact time on the catalytic activities of the original a-ZrP and a pillared compound is illustrated in Table V. Here, contact time is defined as the reciprocal
of the weight hour space velocity (WHSV) of MeOH. Variation of the contact time shows similar results on both kinds of Catalysts although their product distributions are quite different. Generally speaking, DME is the dominant product when the contact time is small. Increasing contact time reduced DME yield and favors hydrocarbon formation as well as coke buildup. Figure 3 shows another comparison based on the effect of temperature on the catalytic activities of a-ZrP, a-Tip, and phenyl-bridged zirconium phosphate samples. The pillared sample is obviously active for hydrocarbon formation at much lower temperatures than a-TiP and a-ZrP. In addition, Figure 4 shows how different the light hydrocarbon product distribution is over these three catalysts. Methane formation is markedly depressed and C2-C4 species are the favored products over the pillared sample.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 2, 1984 223
Table IV. MeOH Conversion over Pillared ZrP at 350 Tu catalyst
P-1-1
P-1-3
P-1-5
crystallinity reaction time, h conversion, mol % MeOH t o light HC light HC distribution, mol % CH4
VH 4.0 59.6 10.0
H 4.0 70.2 14.4
H
L
VL
5.5 100 48.1
4.0 89.2 19.3
4.0 89.4 23.9
37.2 33.1 3.3 26.4
29.4 32.0 3.3 35.3
31.0 30.6 3.4 31.8 3.2
51.4 24.7 2.2 21.6
13.6 34.1
19.0 32.4
51.6
51.1 18.3
65.9 8.5 5.9 9.4 8.6 1.7 41.0 14.4
1.9
4.3
0.3
0.6
0.1
'ZH4
C2H6
c3 c 4
cs
MeOH t o MeOH t o MeOH t o MeOH t o
HBP DME HCHO CO + CO,
XP-1-1 PP-1-1
PP-1-2
PP-1-4
F 5.0
F
R-0-1
PP-0-1 MPM-1-1 H 5.5 96.5 51.8
F 5.0
93.6 42.0
5.0 87.8 44.2
82.8 10.0
amorphous 6.0 85.8 13.7
62.0 12.0 3.6 12.7 9.7
59.8 10.4 2.6 16.6 10.5
44.5 18.4 3.0 26.6 7.5
60.0 20.4 5.1 14.4
45.1 26.3 4.7 23.8
43.5 8.1
32.8 10.6
42.9
21.9 50.5
25.6 43.1 13.7 3.4
0.01
0.2
1.8 0.07
0.4
" Pressure = 1 atm; reaction temperature = 350 "C; WHSV = 3.2 h-l. Table V. Effect of Contact Time" catalyst contact time, h reaction temp, "C reaction time, h conversion, mol % MeOH t o light HC light HC distribution, mol % CH4 c 2 H4 C2H6 c3 c 4
cs
C6 MeOH t o MeOH t o MeOH t o MeOH t o
HBP DME HCHO CO t CO,
(2.5.15). -ZrP
0.15
0.055
P-1-5
0.030
0.016
0.13
425
0.028 370
6.0 90.8 36.8
9.5 81.3 18.0
6.5 79.6 9.8
8.0 80.3 5.0
4.0 94.7 49.3
4.0 98.0 31.3
92.2 2.4 3.7 1.7 0 0 0 46.5 8.1
93.6 1.8 3.6 0.9 0 0 0 16.4 46.7
93.3 1.8 4.9
92.8 1.7 5.6 0 0 0 1.9 73.4
48.7 22.5 2.7 24.5 1.5
0.4 42.7 2.6
17.6 45.1
0.4
0.2
0 0 0 0 69.4 0.4 0.06
30.0 29.3 2.0 31.6 6.6
0.1
0.04
" Pressure = 1 atm; contact time = l/WHSV. A series of experiments were carried out employing ethanol as reactant over a-ZrP and two pillared samples in order to have a better understanding of the reaction mechanism of methanol conversion. The results are shown in Table VI. Extremely high conversions were observed in these reactions. Nevertheless, ethylene was almost the only product detected by GC, accompanied by a large amount of water regardless of whether a-ZrP or pillared samples were the catalysts.
Discussion The mechanisms proposed for the acid-catalyzed hydrocarbon formation from methanol have been summarized in the review article by Chang (1983). The conversion of methanol to low molecular weight hydrocarbons involves two key steps: ether formation and C-C bond formation. However, our results have shown that methane formation is a considerable side-reactionwith zirconium and titanium phosphates as catalysts. Ether formation from methanol is actually a dehydration reaction. It is not unexpected to observe that zirconium and titanium phosphates are very active in dehydration reactions (Clearfield and Thakur, 1980; Clearfield and Frianeza, 1984; Gryaznova et al., 1982). It was also reported before that both Bronsted and Lewis acid sites were active in these reactions (Clearfield and Thakur, 1980; Clearfield and Frianeza, 1984; Gryaznova et al., 1982; Hattori et al., 1977). Hattori et al. (1977) carried out propanol dehydration over a-ZrP calcined at various temperatures between 300 and 800 OC. Although TGA curves
Table VI. Ethanol Conversion over ZrP Compoundsa catalyst reaction time, h conversion, mol % EtOH to light HC light HC distribution, mol % CH, Z ' H4 C, EtOH>toHBP EtOH t o CH,CHO EtOH t o CO+ CO,
(2.5: 15)a-ZrP
P-1-1
PP-1-4
4 100 62.5
4 96.0 84.7
4 100 66.0
0.1 99.1 0.8 35.2 2.1 0.2
0.1 99.9
100 10.3
32.0 0 2.0
1.0
a Pressure = 1 atm; reaction temp = 350 "C;WHSV = 3.2 h-'.
showed that their orthophosphate samples decomposed at ca. 600 OC, they observed that similar catalytic activities were retained up to a calcination temperature of 600 OC. Previous studies in this laboratory also revealed that both Bronsted and Lewis acid sites of a-ZrPand a-TiP were responsible for cyclohexanol dehydration (Clearfield and Frianeza, 1984). The initial step for methanol etherification over the orthophosphate surface obviously involves water condensation at the Bronsted acid sites and formation of methoxy groups on the surface CH30H
\
+ ---OH /
\
€A+
-P /P - O C H ~
+
H20
(1)
224
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
This methoxy group is considered to be highly polar due to the strong electronegativity of the phosphate group. On the other hand, the breakup of P-0-P bonds resulting in the formation of methoxy groups and Bronsted acid sites probably accounts for the catalytic activities observed on pyrophosphate samples
trapped and further react to form hydrocarbons. A mechansim proposed by van den Berg et al. (1980) is reasonable for elucidating C-C bond formation. This mechanism involves oxonium ions as intermediates. The processes that would occur on the phosphate surface are CH3OCH3
-
\ /
+
-P-OH
LP-O-LCH36CH31 / H
(7)
The methoxy species can then react with methanol through a four-member ring electron transfer process to form dimethyl ether as shown in eq 3. This process, however, is
H~C-O-CHZCH~
I
I
/7\ t CH4 t HCHO
higher H C ' s
(3)
H
/
+ C H 3 0 H (9)
.I t CbOCH3
probably in competition with a six-member ring electron transfer process (eq 4) which leads to the formation of
lP-OH
C2H4
1 I
/i\ \ /P-OH
+
(4)
The length of hydrocarbon chains obtained in eq 9 depends on the contact time as well as the pore size of the catalysts. Furthermore, Table IV shows that the most active catalysts for hydrocarbon formation are those pillared samples containing large amounts of phenyl or diphenyl bridging groups. Since the benzene ring usually serves as a source of electrons, or as a base, it should be reasonable to believe that the organic bridging compounds not only open the layers up but also stabilize the oxonium ion intermediates. Ethanol dehydration probably proceeds by a mechanism different from that of methanol. The existence of a methyl group near the hydroxyl functional group in ethanol may lead to p-elimination U
methane and formaldehyde. The latter product is considered to be the precursor in the formation of H2, CO, and coke as shown in eq 5 and 6 (Wu et al., 1971). Supporting HCHO HCHO
-
-+
Hz+ CO
(5)
+ HzO
(6)
C
evidence for process (4)is that small amounts of HCHO, H2, and CO were always detected in this study. A perceptible difference between processes (3) and (4) is that methanol etherification is a reversible process, but methane formation is apparently irreversible. Results obtained with ZrP and TiP can be reasonably explained according to this mechanism. As the contact time increases, DME has a higher probability of reversible decomposition through path (3) and this facilitates methane formation via path (4).Similar results are expected when the reaction temperature increases. Moreover, the intramolecular hydride transfer in eq 4 should proceed more easily as high temperatures labilize the bonds. This process is also enhanced when the acidity of the phosphate group increases and the methyl group bonded with phosphate becomes more ionic. This accounts for the observation that more methane was obtained when titanium atoms took the place of zirconium in the catalysts. On the other hand, silicate and aluminate usually give only trace amounts of methane in this reaction because they have relatively low acidities. Organically pillared derivatives of zirconium and titanium phosphates provide porous structures so that the DME obtained from methanol dehydration can become
'..,>
0-H'
I In the present study, ethylene formed in the above process desorbs from the catalyst surface promptly before it has a chance to react further. Moreover, ethylene itself is apparently not the precursor for hydrocarbon formation over zirconium and titanium phosphates. Acknowledgment This work was supported by the Texas A&M Center for Energy and Mineral Resources and the National Science Foundation under Grant No. CHE81-14613, for which grateful acknowledgment is made. We also wish to thank Dr. Deepak Thakur,Internorth Corporation, for suggesting and encouraging this study. Registry No. Zr(HPO,),, 13772-29-7; TiP,07, 13470-09-2; CH,OH, 67-56-1; CHI, 74-82-8;CzH4,74-85-1;EtOH, 64-17-5;P, 71-43-2; PP, 92-52-4; MPM, 106-42-3; R, 63758-89-4; DME, 115-10-6; a-titanium phosphate, 13765-94-1.
Literature Cited
Chang, C. D. Cera/. Rev. - Sci. Eng. 1983, 25(1), 1. Clearfield, A,; Albertl, G. In "Inorganic Ion Exchange Materials"; Clearfield, A., Ed.; CRC Press: Boca Raton, FL, 1982; Chapters 1 and 2. Clearfield, A,; Berman, J. R. J . Inorg. Nucl. Chem. 1961, 4 3 , 2141. Clearfield, A.; Duax. W. L.; Medlna. A. S.;Smith, G. D.; Thomas, J. R. J . Phys . Chem. 1969, 73, 3424. ClearfleM, A,; Frianeza, T. N. J . Cstal. 1984, 85, 398. Clearfieid, A,; Pack, S.P. J . Inorg. Nucl. Chem. 1975, 37, 283.
Ind. Eng. Chem. Prod. Res. Dev. 1904, 23, 225-229 Ciearfieid, A.; Smith, G. D. Inorg. Chem. 1069, 8 , 431. Clearfield, A.; Thakur, D. S. J . Catal. 1980, 65, 185. Dines, M. G.; DiGiacomo, P. M.; Cailahan, K. P.; Griffith, P. C.; Lane, R. H.; Cooksey, R. E. I n "Catalytically Modified Surfaces in Catalysis and Eiectrocatalysis";Miller, J. S., Ed.; ACS Symposium Series 192, Washington, DC, 1982. Gryaznova, Z. V.; Ponomareva, N. N.; Nefedova. A. R.; Yeshchenko, L. S.; Dvoshina, R. N.; Yakovenko, Z. I . React. Kinet. Catal. Lett. 1082, 19(3-4), 393. Hattori, T.; Ishiguro, A.; Murakami, Y. Nippon Kagaku Kaishi 1077, 6 , 761.
225
Thakur, D. S.;Ciearflekl, A. J . Catal. 1081, 69, 230. Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311. van den Berg, J. P.; Wolthuzen, J. P.; van Hooff, J. H. C. Proc. Vth Conf. Zeolites, Naples, Italy, 1980; p 649. Wu, E. L.; Kuhi, G. H.; Whyte, T. E., Jr.; Venuto, P. B. Adv. Chem. Ser. 1071, No. 101, 490.
Received for review November 4,1983 Accepted January 30,1984
Catalyst for Steam Gasification of Wood to Methanol Synthesis Gas Yoshlnorl Tanaka, Tsuyoshl Yamaguchl, Kenjl Yamasakl, AkHuml Ueno, and Yoshlhlde Kolera Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan
A catalyst was developed for steam gasification of wood to generate methanol synthesis gas. Upon screening various metal oxides supported on alumina catalysts, the most favorablecatalyst was NiO/AI,O, since it supplied a gas mainly composed of hydrogen and carbon monoxide with an Hp/CO ratio in volume being close to 2.0. It was proved that a composition and yield of a gas from wood strongly depends upon metal oxides employed and a mean micropore size of alumina support. The effects of gasification temperature and amount of water added upon a composition and yield of a gas generated were also studied.
Introduction Methanol, as well as hydrogen, has been desired to be a clean fuel because of less emissions of NO, and SO, during combustion. Methanol is produced from hydrogen and carbon monoxide, supplied from steam reformation of methane and/or higher hydrocarbons, with a ratio of H,/CO controlled to be 2.0 by a shift reactor. Coal and heavy oil residues are considered to be qualified resources for synthesis gas and their gasification techniques have undergone great progress by Tarrer et al. (1979) and Harney and Mills (1980). In this decade much attention has been paid to wood gasification probably because of the fact that wood is renewable and contains lesser amounts of sulfur compounds than coal and heavy oil residues. Swaaij (1980) and Coffman (1981) studied pyrolysis of wood in the presence of steam and oxygen. Wood can be gasified at 300 "C and produces a gas preferably composed of carbon dioxide, while at 600 "C hydrogen, carbon monoxide, and methane are mainly produced with considerable amounts of higher hydrocarbons, alcohols, and organic acids. The yield of a gas generated by wood pyrolysis at 600 "C is, however, as low as 40% in weight of wood employed, since large amounts of char and tarlike fragments are also produced. Accordingly, an application of a proper catalyst has been desired to improve a yield and composition of a gas generated. In the present communication, a catalyst suitable for the generation of methanol synthesis gas from wood is reported. The effects of metal oxides supported on alumina and a mean pore size of alumina support upon a yield and compositiopl of a gas generated are also discussed. Finally, a life test of the catalyst for wood gasification is briefly mentioned. Experimental Section For the gasification experiments, sawdust of hemlock and spruce (supplied from Ebara Seisakusho Co.) was used. The average size of the sawdust is about 1.0 mm in diameter and the composition was analyzed by conventional chemical techniques; for hydrogen, carbon, and nitrogen a combustion analysis was applied using a CHN corder 0196-4321/84/1223-0225$01.50/0
(Carbon-Hydrogen-Nitrogen Analyser , Y anagimoto Seisakusho Co.), and for sulfur and chlorine gravimetric and a colorimetric analyses were carried out using aqueous solutions of barium chloride and mercury(I1) thiocyanate, respectively. The amount of ash was estimated by weighing the residues after combustion of a certain amount of the sawdust at 600 "C for 2 h. Catalysts employed were prepared by a conventional impregnation method using aqueous solutions of metal nitrates and alumina spheres. For Vz05and Moo3 supported catalysts, aqueous solutions of vanadium oxalate and hexaammonium molybdate were employed, respectively. The loadings of metals were all 20 wt 5%. Several kinds of alumina (7-A1203, supplied from Sumitomo Aluminum Co.) were employed, their micropore sizes being varied from 60 to 2000 8. The catalysts thus prepared were dried at 110 "C and then calcined at 700 "C for 4 h in air. The surface areas of catalysts were measured by the BET method using nitrogen at its liquid temperature. The surface areas of metals in catalysts were measured by hydrogen chemisorption after the reduction of the catalysts at 700 "C in hydrogen stream for 4 h. Two types of reactors were used; one is a batch-feed reactor and the other is a continuous feed reactor equipped with a screw feeder. The batch-type reactor was used for catalysts screening. A catalyst calcined at 700 "C was mixed with the sawdust with a weight ratio of 3:2, and 5 g of this mixture was placed on a bed in the batch reactor. The gasification was carried out at temperatures between 500 and 800 "C in the presence of water vapor. Argon gas was used as an inert gas with a flow rate of 200 mL/min. The argon gas passed through a water reservoir heated at 50 "C before introduction to the reactor. Thus, the argon gas includes ca. 13.7 vol % water vapor. The fraction of water vapor can be varied by changing the temperature of the water reservoir. A gas generated passed through a cold trap to eliminate an excess amount of water vapor and tarlike fragments and then the gas was subjected to gas chromatography using columns packed with molecular sieve 13X and Porapak Q. The gasification reaction was carried out for 1 h and then the catalyst was separated 0 1984 American Chemical Society
