Texaco Inc. yesterday reported its first-quarter profits rose by an unexpected 9 1/2 percent despite a sharp drop in petroleum sales and attributed the gain to early anticipation of lower oil demand.

But Sun Co.'s first-quarter earnings dropped 52 percent below year-earlier results. Phillips Petroleum Co. had a 5 percent decline, Amerada Hess Corp. recorded a 12.2 percent decrease, and Cities Service Co.'s profits plummeted 66 percent.

Analysts had predicted major refiners would show first-quarter earnings declines of 20 percent to 25 percent below the dramatic results of the 1980 first quarter when pump prices surged by a penny a gallon per week and oil demand remained strong.

But the slump in oil earnings has exceeded most projections, with reduced demand for petroleum products slashing refiners' refining and marketing profits.

Texaco, the third-largest U.S. refiner, earned $658 million ($2.45 a share), up from an operating net of $600.6 million ($2.21) a year earlier. Texaco's sale of its stake in Belridge Oil Co. boosted its final net for the 1980 quarter to $1 billion ($3.69).

Texaco said its U.S. petroleum product sales fell 10.1 percent in the first quarter as higher crude costs and the windfall profits tax raised prices to a level that could not be recovered from the price-resistant consumer.

Sun Co., the 10th-largest refiner, reported its first-quarter profits fell to $120 million (97 cents a share) from $251 million ($2.09) a year earlier because of weak oil demand, higher crude oil costs and reduced prices for its Canadian synthetic crude oil. Revenues were up 29 percent to $4 billion versus $3.1 billion

Phillips, the No. 12 refiner, earned $270.8 million ($178 a share) in the first quarter, down from $284.4 million ($1.85). Revenues rose 25 percent to $4.23 billion from $3.38 billion.

Phillips, which has a $21 million first-quarter loss on domestic refining and marketing operations, said "the recent decline in the price of gasoline at the pump is evidence of the vigorous competition that is taking place in this segment of the industry."

Amerada Hess, ranked No. 15, earned $174.5 million ($2.08 a share), down from $198.9 million ($2.37) in the 1980 first quarter. Revenues climbed 26 percent to $2.9 billion versus $2.3 billion.

Cities Service Co., the No. 18 refiner, said its profits fell to $57.7 million (69 cents a share) from $170.1 million ($2.04), primarily because unusually warm weather and price-induced conservation reduced natural gas sales in the latest period.

American Broadcasting Cos., meanwhile, reported that its first-quarter earnings plummeted 17 percent to $12.7 million (45 cents a share) from $24.3 million (86 cents) in the comparable 1980 period. It cited lower advertising revenue, higher programming costs and effects of last year's actors' strike.

Revenues in the latest quarter totaled $541.2 million, down 1.03 percent from $546.9 million in the first three months on 1980.

ABC Chairman Leonard H. Goldenson and President Elton H. Rule said they had anticipated first-quarter earnings would be adversely affected by lower revenues and a "significant profit decline at the ABC television network."

The decline, they said, resulted mainly from "the weak television advertising marketplace that continued throughout much of the quarter" along with residual effects of the actors' strike and more expensive programming.

Xerox Corp. reported a 7 percent rise in earnings in the first quarter of 1981 to $1.86 a share from $1.74 a share in the first three months of 1980.

Net income for the quarter was $157.4 million on revenues of $2.046 billion, up from last year's net of $146.4 million on revenues of $1.85 billion.

Xerox said in a statement that first-quarter results, particularly revenue, were "adversely impacted by the strengthening dollar." Net income on the other hand, was helped by a lower effective tax rate in international operations.